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| Knowledge Base |
| Click on a topics to reveal/hide the details. |
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| 1. AC Adapter |
A linear or switched-mode power supply (or in some cases just a transformer) that is built into the top of a plug is known as a "plug pack", "plug-in adapter", "adapter block", "domestic mains adapter" or just "power adapter". Slang terms include "wall wart" and "power brick". They are even more diverse than their names; often with either the same kind of DC plug offering different voltage or polarity, or a different plug offering the same voltage. "Universal" adapters attempt to replace missing or damaged ones, using multiple plugs and selectors for different voltages and polarities. Replacement power supplies must match the voltage of, and supply at least as much current as, the original power supply.
The least expensive AC units consist solely of a small transformer, while DC adapters include a few additional diodes. Whether or not a load is connected to the power adapter, the transformer has a magnetic field continuously present and normally cannot be completely turned off unless unplugged.
Because they consume standby power, they are sometimes known as "electricity vampires" and may be plugged into a power strip to allow turning them off. Expensive switched-mode power supplies can cut off leaky electrolyte-capacitors, use powerless MOSFETs, and reduce their working frequency to get a gulp of energy once in a while to power, for example, a clock, which would otherwise need a battery. |
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| 2. AC/ DC Supply |
| In the past, mains electricity was supplied as DC in some regions, AC in others. A simple, cheap unregulated power supply would run directly from either AC or DC mains, often without using a transformer. The power supply consisted of a rectifier and a filter capacitor. The rectifier was essentially a conductor, having no sudden effect when operating from DC. |
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| 3. Active Regulators |
Active regulators employ at least one active (amplifying) component such as a transistor or operational amplifier. Shunt regulators are often (but not always) passive and simple, but always inefficient because they (essentially) dump the excess current not needed by the load. When more power must be supplied, more sophisticated circuits are used. In general, these active regulators can be divided into several classes:
- Linear series regulators
- Switching regulators
- SCR regulators
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| 4. Alarm Power Supply Unit |
The alarm power supply unit (APSU) is used as a smart device for various DSL, ONT, 2G-3G-4G Wireless, WiFi, Microwave Satellite router/modems, providing backup power, battery monitoring and alarm integration functions over a broadband communication link. The APSU is separate class of power supply and can be quickly recognised by a built-in web server that communicates in open standard protocol (such as CSV/XML IP ALARM) directly to a remote monitoring server. The APSU has automatic network supervision algorithms designed to work with legacy systems which depend on a reliable communications path and 24/7 access to the public or private network.
It is often required by service companies providing broadband services via the Internet. The APSU is a remotely monitored device that is ideal for FTTH power supply installations. The APSU can be either a UPS or a PSU and can be built to operate on any voltage/wattage capacity including power over Ethernet (PoE) wiring configuarations and operate within a network as a power sourcing equipment (PSE). |
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| 5. Ambient temperature |
Room temperature implies a temperature inside a temperature-controlled building. Ambient temperature simply means "the temperature of the surroundings" and will be the same as room temperature indoors.
Room temperature is a common term to denote a certain temperature within enclosed space to which humans are accustomed. Room temperature is thus often indicated by general human comfort, with the common range of 20 °C (68 °F) to 25 °C (77 °F), though people may become acclimatized to higher or lower temperatures.
For scientific calculations, room temperature is usually taken to be 20 or 25 degrees Celsius, (293 or 298 kelvin (K), 68 or 77 degrees Fahrenheit). For numerical convenience, 300.00 K (26.85 °C, 80.33 °F) is often used. However, room temperature is not a uniformly defined scientific term as opposed to Standard Temperature and Pressure, or STP, which has several, slightly different definitions.
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| 6. Attitude |
| The attitude of equipment is its orientation with respect to a defined frame of reference.
Attitude dynamics is the modeling of the changing position and orientation of an equipment, due to external forces acting on the body. Attitude control is the purposeful manipulation of controllable external forces to establish a desired attitude, whereas attitude determination is the utilization of sensors to ascertain the current equipment attitude. Mathematical and physical treatment of the basic aspects of these topics is well-developed, but the field is quite active with respect to advanced topics and applications.
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| 7. Battery Power Supply |
A battery is a type of linear power supply that offers benefits that traditional line-operated power supplies lack: mobility, portability and reliability. A battery consists of multiple electrochemical cells connected to provide the voltage desired.
The most commonly used dry-cell battery is the carbon-zinc dry cell battery. Dry-cell batteries are made by stacking a carbon plate, a layer of electrolyte paste, and a zinc plate alternately until the desired total voltage is achieved. The most common dry-cell batteries have one of the following voltages: 1.5, 3, 6, 9, 22.5, 45, and 90. During the discharge of a carbon-zinc battery, the zinc metal is converted to a zinc salt in the electrolyte, and magnesium dioxide is reduced at the carbon electrode. These actions establish a voltage of approximately 1.5 V.
The lead-acid storage battery may be used. This battery is rechargeable; it consists of lead and lead/dioxide electrodes which are immersed in sulfuric acid. When fully charged, this type of battery has a 2.06-2.14 V potential(A 12 volt car battery uses 6 cells in series). During discharge, the lead is converted to lead sulfate and the sulfuric acid is converted to water. When the battery is charging, the lead sulfate is converted back to lead and lead dioxide.
A nickel-cadmium battery has become more popular in recent years. This battery cell is completely sealed and rechargeable. The electrolyte is not involved in the electrode reaction, making the voltage constant over the span of the batteries long service life. During the charging process, nickel oxide is oxidized to its higher oxidation state and cadmium oxide is reduced. The nickel-cadmium batteries have many benefits. They can be stored both charged and uncharged. They have a long service life, high current availabilities, constant voltage, and the ability to be recharged. |
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| 8. Burst Immunity |
| Bursts of high voltage pulses are applied to the power lines to simulate events such as repeating voltage spikes from a motor. |
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| 9. Circuit Breakers |
| One benefit of using a circuit breaker as opposed to a fuse is that it can simply be reset instead of having to replace the blown fuse. A circuit breaker contains an element that heats, bends and triggers a spring which shuts the circuit down. Once the element cools, and the problem is identified the breaker can be reset and the power restored. |
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| 10. Combination (Hybrid) Regulators |
| Many power supplies use more than one regulating method in series. For example, the output from a switching regulator can be further regulated by a linear regulator. The switching regulator accepts a wide range of input voltages and efficiently generates a (somewhat noisy) voltage slightly above the ultimately desired output. That is followed by a linear regulator that generates exactly the desired voltage and eliminates nearly all the noise generated by the switching regulator. Other designs may use an SCR regulator as the "pre-regulator", followed by another type of regulator. An efficient way of creating a variable-voltage, accurate output power supply is to combine a multi-tapped transformer with an adjustable linear post-regulator. |
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| 11. Common Tests |
| Electromagnetic Compatibility and Electrical Safety - Generic Criteria for Network Telecommunications Equipment |
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| 12. Comparing Linear Vs. Switching Regulators |
The two types of regulators have their different advantages:
- Linear regulators are best when low output noise (and low RFI radiated noise) is required
- Linear regulators are best when a fast response to input and output disturbances is required.
- At low levels of power, linear regulators are cheaper and occupy less printed circuit board space.
- Switching regulators are best when power efficiency is critical (such as in portable computers), except linear regulators are more efficient in a small number of cases (such as a 5V microprocessor often in "sleep" mode fed from a 6V battery, if the complexity of the switching circuit and the junction capacitance charging current means a high quiescent current in the switching regulator).
- Switching regulators are required when the only power supply is a DC voltage, and a higher output voltage is required.
- At high levels of power (above a few watts), switching regulators are cheaper (for example, the cost of removing heat generated is less).
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| 13. Conducted Emissions |
| Similar to radiated emissions, except the signals are measured at the power lines with a filter device. |
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| 14. Conducted Immunity |
| Low frequency signals (usually 10 kHz to 80 MHz) are injected onto the data and power lines of a device. This test is used to simulate the coupling of low frequency signals onto the power and data lines, such as from a local AM radio station. |
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| 15. Current Limiting |
Current limiting is the practice in electrical or electronic circuits of imposing an upper limit on the current that may be delivered to a load with the purpose of protecting the circuit generating or transmitting the current from harmful effects due to a short-circuit or similar problem in the load. This term is also used to describe the ability of an over current protective device (fuse or circuit breaker) to reduce the peak current in a circuit, by opening and clearing the fault in a sub-cycle time frame.
The simplest form of current limiting for mains is a fuse. As the current exceeds the fuse's limits it blows thereby disconnecting the load from the source. This method is most commonly used for protecting the house-hold mains. A circuit breaker is another device for mains current limiting.
Compared to circuit breakers, fuses attain faster current limitation by means of arc quenching. Since fuses are passive elements, they are inherently secure. Their drawback however is the single operation principle: once blown, they need to be replaced or reset.
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| 16. Current Limiting In Electronic Power Circuits |
Active current limiting or short-circuit protection
Electronic circuits like regulated DC power supplies and power amplifiers employ, in addition to fuses, active current limiting since a fuse alone may not be able to protect the internal devices of the circuit in an over-current or short-circuit situation. A fuse generally is too slow in operation and the time it takes to blow may well be enough to destroy the devices. A typical short-circuit/overload protection scheme is shown in the image. The schematic is representative of a simple protection mechanism employed in regulated DC supplies and class-AB power amplifiers‡. Q1 is the pass or output transistor. Rsens is the load current sensing device. Q2 is the protection transistor which turns on as soon as the voltage across Rsens becomes about 0.65 V. This voltage is determined by the value of Rsens and the load current through it (Iload). When Q2 turns on, it removes base current from Q1 thereby reducing the collector current of Q1. Neglecting the base currents of Q1 and Q2, the collector current of Q1 is also the load current. Thus, Rsens fixes the maximum current to a value given by 0.65/Rsens, for any given output voltage and load resistance. For example, if Rsens = 0.33 ?, the current is limited to about 2 A even if Rload becomes a short (and Vo becomes zero). With the absence of Q2, Q1 would attempt to drive a very large current (limited only by Rsens, and dependent on the output voltage Vo if Rload is not zero) and the result would be greater power dissipation in Q1. If Rload is zero the dissipation will be much greater (enough to destroy Q1). With Q2 in place, the current is limited and the maximum power dissipation in Q1 is also limited to a safe value (though this is also dependent on Vcc, Rload and current-limited Vo). Further, this power dissipation will remain as long as the overload exists, which means that the devices must be capable of withstanding it for a substantial period. For example, the pass-transistor in a regulated DC power supply system (corresponding to Q1 in the schematic above) rated for 25 V at 1.5 A (with limiting at 2 A) will normally (i.e. with rated load of 1.5 A) dissipate about 7.5 W for a Vcc of 30 V‡‡ (1). With current limiting, the dissipation will increase to about 60 W if the output is shorted‡‡ (2). Without current limiting the dissipation would be greater than 300 W‡‡ (3) - so limiting does have a benefit, but it turns out that the pass-transistor must now be capable of dissipating at least 60 W. In short, an 80-100 W device will be needed (for an expected overload and limiting) where a 10-20 W device (with no chance of shorted load) would have been sufficient. In this technique, beyond the current limit the output voltage will decrease to a value depending on the current limit and load resistance.
‡ – For class-AB stages, the circuit will be mirrored vertically and complementary devices will be used for Q1 & Q2.
‡‡ – The following conditions are considered for determining the power dissipation in Q1, with Vo = 25 V, Iload = 1.5 A (limit at 2 A), Rsens = 0.33 ? (for limiting at 2A) and Vcc = 30 V —
- Normal operation: Vo = 25 V at a load current of 1 A. So Q1 dissipates a power of (30 - 25) V * 1.5 A = 7.5 W. The transistor used must be a 10-20 W device to account for ambient temperature (i.e., derated) and must be mounted on a heat-sink.
- Output shorted, with limiting at 2A: The dissipation is given by (30 - 0.65) V * 2 A = 58.7 W. The 0.65 V is the drop across Rsens. In practice, if the power supply Vcc is not able to provide the maximum short-circuit current it will collapse thereby reducing dissipation in Q1. However this is dependent on how "stiff" the supply is. A stiffer supply will sustain the voltage for a heavier current draw before collapsing. Further, the transistor used must be a 80-100 W device to account for ambient temperature (i.e., derated) and must be mounted on a heat-sink.
- Output shorted, and no limiting: A shorted load will mean that only Rsens is present as the load. With this, the circuit will attempt to put 25 V across Rsens (0.33 ?) - here the output voltage has to be measured at the emitter of Q1 since Q1 is connected as an emitter-follower and the lower end of Rsens is effectively grounded due to the short. Thus the load current (and collector current of Q1) becomes nearly 76 A, and the dissipation in Q1 becomes (30 - 25) V * 76 A = 380 W. This is a very large power to dissipate, since in normal circumstances Q1 will only be required to dissipate about 7.5 W (60 W at worst with limiting), and even a 100 W transistor will not withstand a 380 W dissipation. Without Rsens (i.e., Q1 emitter is directly connected to the load) the situation is even worse — Q1 becomes a dead short across 30 V and will draw current limited only by its internal resistance. In practice, the dissipation will be less because the supply (Vcc) will collapse under such a condition. However the dissipation will still be enough to destroy Q1.
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| 17. DC Voltage Stabilizers |
Many simple DC power supplies regulate the voltage using a shunt regulator such as a zener diode, avalanche breakdown diode, or voltage regulator tube. Each of these devices begins conducting at a specified voltage and will conduct as much current as required to hold its terminal voltage to that specified voltage. The power supply is designed to only supply a maximum amount of current that is within the safe operating capability of the shunt regulating device (commonly, by using a series resistor). In shunt regulators, the voltage reference is also the regulating device.
If the stabilizer must provide more power, the shunt regulator output is only used to provide the standard voltage reference for the electronic device, known as the voltage stabilizer. The voltage stabilizer is the electronic device, able to deliver much larger currents on demand. |
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| 18. DC-to-DC Converter |
In electronic engineering, a DC to DC converter is an electronic circuit which converts a source of direct current (DC) from one voltage level to another. It is a class of power converter.
DC to DC converters are important in portable electronic devices such as cellular phones and laptop computers, which are supplied with power from batteries primarily. Such electronic devices often contain several sub-circuits, each with its own voltage level requirement different from that supplied by the battery or an external supply (sometimes higher or lower than the supply voltage, and possibly even negative voltage). Additionally, the battery voltage declines as its stored power is drained. Switched DC to DC converters offer a method to increase voltage from a partially lowered battery voltage thereby saving space instead of using multiple batteries to accomplish the same thing.
Most DC to DC converters also regulate the output voltage. Some exceptions include high-efficiency LED power sources, which are a kind of DC to DC converter that regulates the current through the LEDs, and simple charge pumps which double or triple the input voltage. |
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| 19. Direct current |
Direct current (DC) is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also be through semiconductors, insulators, or even through a vacuum as in electron or ion beams. The electric charge flows in a constant direction, distinguishing it from alternating current (AC). A term formerly used for direct current was Galvanic current.
Types of direct current.
Direct current may be obtained from an alternating current supply by use of a current-switching arrangement called a rectifier, which contains electronic elements (usually) or electromechanical elements (historically) that allow current to flow only in one direction. Direct current may be made into alternating current with an inverter or a motor-generator set.
The first commercial electric power transmission (developed by Thomas Edison in the late nineteenth century) used direct current. Because there used to be an advantage of alternating current over direct current in transforming and transmission, electric power distribution till a few years ago were nearly all alternating current. In the mid 1950s, HVDC transmission was developed, which is now replacing the older high voltage alternating current systems. For applications requiring direct current, such as third rail power systems, alternating current is distributed to a substation, which utilizes a rectifier to convert the power to direct current. See War of Currents.
Direct current is used to charge batteries, and in nearly all electronic systems as the power supply. Very large quantities of direct-current power are used in production of aluminum and other electrochemical processes. Direct current is used for some railway propulsion, especially in urban areas. High voltage direct current is used to transmit large amounts of power from remote generation sites or to interconnect alternating current power grids.
Various definitions, within electrical engineering, the term DC is used to refer to power systems that use only one polarity of voltage or current, and to refer to the constant, zero-frequency, or slowly varying local mean value of a voltage or current.[1] For example, the voltage across a DC voltage source is constant as is the current through a DC current source. The DC solution of an electric circuit is the solution where all voltages and currents are constant. It can be shown that any stationary voltage or current waveform can be decomposed into a sum of a DC component and a zero-mean time-varying component; the DC component is defined to be the expected value, or the average value of the voltage or current over all time.
Although DC stands for "Direct Current", DC sometimes refers to "constant polarity." With this definition, DC voltages can vary in time, such as the raw output of a rectifier or the fluctuating voice signal on a telephone line.
Some forms of DC (such as that produced by a voltage regulator) have almost no variations in voltage, but may still have variations in output power and current.
Applications, Direct-current installations usually have different types of sockets, switches, and fixtures, mostly due to the low voltages used, from those suitable for alternating current. It is usually important with a direct-current appliance not to reverse polarity unless the device has a diode bridge to correct for this (most battery-powered devices do not).
This symbol is found on many electronic devices that either require or produce direct current.
Many telephones connect to a twisted pair of wires, and internally separate the AC component of the voltage between the two wires (the audio signal) from the DC component of the voltage between the two wires (used to power the phone).
Telephone exchange communication equipment, such as DSLAM, uses standard -48V DC power supply. The negative polarity is achieved by grounding the positive terminal of power supply system and the battery bank. This is done to prevent electrolysis depositions. An electrified third rail can be used to power both underground (subway) and overground trains. |
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| 20. Efficiency |
The efficiency of an entity (a device, component, or system) in electronics and electrical engineering is defined as useful power output divided by the total electrical power consumed (a fractional expression), typically denoted by the Greek letter small Eta (?).
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| 21. Efficiency Of Typical Electrical Devices |
Efficiency should not be confused with effectiveness: a system that wastes most of its input power but produces exactly what it is meant to is effective but not efficient. The term "efficiency" only makes sense in reference to the wanted effect. So a light bulb might have 2% efficiency at emitting light yet still be 98% efficient at heating a room. (In practice it is nearly 100% efficient at heating a room because the light energy will also be converted to heat eventually, apart from the small fraction that leaves through the windows). An electronic amplifier that delivers 10 watts of power to its load (for example a loudspeaker), while drawing 20 watts of power from a power source is 50% efficient. (10/20 × 100% = 50%)
- Electric kettle: over 90% (comparatively little heat energy is lost during the 2 to 3 minutes a kettle takes to boil water).
- A four-quadrant gate is highly effective, yet it has an electrical efficiency close to 0%.
- An electric fire is 100% efficient in terms of converting electrical energy into the desired result, ie heating.
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| 22. Electrostatic discharge (ESD) Immunity |
| Electrostatic discharges with various properties (rise time, peak voltage, fall time, and half time) are applied to the areas on the device that are likely to be discharged too, such as the faces, near user accessible buttons, etc. Discharges are also applied to a vertical and horizontal ground plane to simulate an ESD event on a nearby surface. Voltages are usually from 2kV to 15kV, but commonly go as high as 25kV or more. |
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| 23. EMC |
Anechoic RF chamber used for EMC testing (radiated emissions and immunity). The furniture has to be made of wood or plastic, and not metal.
Electromagnetic compatibility (EMC) is the branch of electrical sciences which studies the unintentional generation, propagation and reception of electromagnetic energy with reference to the unwanted effects (Electromagnetic interference, or EMI) that such energy may induce. The goal of EMC is the correct operation, in the same electromagnetic environment, of different equipment which use electromagnetic phenomena, and the avoidance of any interference effects.
In order to achieve this, EMC pursues two different kinds of issues. Emission issues are related to the unwanted generation of electromagnetic energy by some source, and to the countermeasures which should be taken in order to reduce such generation and to avoid the escape of any remaining energies into the external environment. Susceptibility or immunity issues, in contrast, refer to the correct operation of electrical equipment, referred to as the victim, in the presence of unplanned electromagnetic disturbances.
Interference, or noise, mitigation and hence electromagnetic compatibility is achieved primarily by addressing both emission and susceptibility issues, i.e., quieting the sources of interference and hardening the potential victims. The coupling path between source and victim may also be separately addressed to increase its attenuation. |
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| 24. EMI |
EMI- Electromagnetic Interference: EMI is unwanted effects in the electrical system due to electromagnetic radiation and electromagnetic conduction. Electromagnetic radiation and electromagnetic conduction are differentiated by the way an EM field propagates. Conducted EMI is caused by the physical contact of the conductors as opposed to radiated EMI which is caused by induction (without physical contact of the conductors). Electromagnetic disturbances in the EM field of a conductor will no longer be confined to the surface of the conductor and will radiate away from it. This persists in all conductors and mutual inductance between two radiated electromagnetic fields will result in EMI.
Due to this EMI, the electromagnetic field around the conductor is no longer evenly distributed and causes skin effect, proximity effect, hysteresis losses, transients, voltage drops, electromagnetic disturbances, EMP/HEMP, eddy current losses, harmonic distortion, and reduction in the permeability of the material.
EMI can be conductive and/ or radiative. Its behavior is dependent on the frequency of operation and cannot be controlled at higher frequencies. For lower frequencies, EMI is caused by conduction and, for higher frequencies, by radiation. For ex: Skin effect is due to the conductive EMI and proximity effect is due to the radiative EMI.
The worst part of a high frequency electromagnetic signal is that it makes every conductor an antenna, in the sense that they can generate and absorb electromagnetic fields. In the case of a PCB (printed circuit board), which consists of capacitors and semiconductor devices which are soldered to the bread board, the capacitors and soldering act like antennas, generating and absorbing electromagnetic fields. The chips on these boards are so close to each other that the chances of conducted and radiated EMI are significant. Bread boards are designed in such a way that the case of the board is connected to the ground and the radiated EMI is diverted to ground. Technological advancements have drastically reduced the size of chipboards and electronics; however, this means they are also much more sensitive to EMI.
The most common solution to EMI is electromagnetic shielding. However, EMI shielding is expensive and has negative consequences. Another method to reduce EMI is to twist wires; however many facilities have tens of thousands of feet of wire, so this is not practical.
A common example of radiated EMI is a cable TV wire and the TV. If you unhook the cable from the TV and place it in front of the plug, video can still be seen on the TV. This is due to electromagnetic signals capable of traveling through the air from cable to TV. |
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| 25. EMI & Electrical Configurations |
When there is no return path of current due to an improper electrical configuration, the chance of generating EMI is significantly increased. When the electrical circuit is not complete (no return path of current), the current doesn't know which way to go. This will certainly cause the electrical wire to generate the electromagnetic field into the air as radiated EMI. EMI through the ground wire is also very common in an electrical facility.
The designers of the distribution system did not have to worry about the compatibility of wire, transformers, heavy machinery, sensitive computers, copy machines, fax machines, server rooms, fluorescent lighting and automated phone systems, all functioning together in one electrical environment. The EM fields of all the various wire and electrical equipment are constantly interfering with each other, degrading performance, degrading efficiency, degrading equipment and degrading wire.
A standard specification is an explicit set of requirements for an item, material, component, system, or service. In the EMC field, there are several organizations that set standards for performance. The largest of these organizations are the Institute of Electrical and Electronics Engineers (IEEE), the International Electrotechnical Commission (IEC), the American National Standards Institute (ANSI), and the US Military (MILSTD). |
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| 26. Fuses |
A fuse is a piece of wire, often in a casing that improves its electrical characteristics. If too much current flows, the wire overheats and melts. This interrupts the power supply, and the equipment stops working until the problem that caused the overload is identified and the fuse is replaced.
There are various types of fuses used in power supplies.
- fast blow fuses cut the power as quick as they can
- slow blow fuses tolerate more short term overload
- wire link fuses are just an open piece of wire, and have poorer overload characteristics than glass and ceramic fuses
- Some power supplies use a very thin wire link soldered in place as a fuse.
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| 27. Ground |
Ground in electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth.
Electrical circuits may be connected to ground (earth) for several reasons. In mains powered equipment, exposed metal parts are connected to ground to prevent contact with a dangerous voltage if electrical insulation fails. Connections to ground limit the build-up of static electricity when handling flammable products or when repairing electronic devices. In some telegraph and power transmission circuits, the earth itself can be used as one conductor of the circuit, saving the cost of installing a separate return conductor.
For measurement purposes, the Earth serves as a (reasonably) constant potential reference against which other potentials can be measured. An electrical ground system should have an appropriate current-carrying capability in order to serve as an adequate zero-voltage reference level. In electronic circuit theory, a "ground" is usually idealized as an infinite source or sink for charge, which can absorb an unlimited amount of current without changing its potential. Where a real ground connection has a significant resistance, the approximation of zero potential is no longer valid. Stray voltages or earth potential rise effects will occur, which may create noise in signals or if large enough will produce an electric shock hazard.
The use of the term ground (or earth) is so common in electrical and electronics applications that circuits in portable electronic devices such as cell phones and media players as well as circuits in vehicles such as ships, aircraft, and spacecraft may be spoken of as having a "ground" connection without any actual connection to the Earth. This is usually a large conductor attached to one side of the power supply (such as the "ground plane" on a printed circuit board) which serves as the common return path for current from many different components in the circuit.
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| 28. High-voltage Power Supply |
High voltage refers to an output on the order of hundreds or thousands of volts. High-voltage supplies use a linear setup to produce an output voltage in this range.
Additional features available on high-voltage supplies can include the ability to reverse the output polarity along with the use of circuit breakers and special connectors intended to minimize arcing and accidental contact with human hands. Some supplies provide analog inputs (i.e. 0-10V) that can be used to control the output voltage, effectively turning them into high-voltage amplifiers albeit with very limited bandwidth.
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| 29. Inrush Current Limiting |
| An inrush current limiter is a device or group of devices used to limit inrush current. Negative temperature coefficient (NTC) thermistors and resistors are two of the simplest options, with cool-down time and power dissipation being their main drawbacks, respectively. More complex solutions can be used when design constraints make simpler options infeasible. |
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| 30. Isolation Voltage |
High Voltage Protection and Isolation of telecommunications circuits and SCADA circuits to and
from Hydro Sub Stations. In order to protect the continuous service of important
telecommunications and control circuits to and from Hydro Sub Stations, protective devices must be applied to prevent equipment failure and protect personnel from serious shock hazards (can be life threatening). The first electronic protection devices were designed and developed by Positron Industries Inc in the early 1970s. This equipment provided high voltage isolation from the possibilities of high voltage faults and lightning damage using transformer copper based isolation techniques. In the late 1980s RLH Industries, Inc. introduced fiber optic high voltage protection equipment (HVPE) to the industry. The concept of using Fiber optics as a HVPE device is based on the theory that fiber does not conduct or carry high voltage from the high voltage source to the through the fiber optic cable and is immune to other interferences that are inherently present with copper communications cables. All communications (wire line services) between a hydro sub station and the telephone company's central office are vulnerable to the effects of Ground Potential Rise (GPR) and lightning surges which can destroy sensitive and important telecommunications circuits and be carried along the copper telephone cable. Both copper and fiber based high voltage protection equipment methods are being implemented today. In addition, any personnel working with this equipment during these situations is at risk of serious or possibly life threatening situations.
The fundamental theory of isolation devices is to isolate any high voltages and currents from propagating from the hydro sub station towards the telephone company's central office. Since the early 1970s, most hydro utilities and telephone operating companies trained their staff to evaluate and determine the safety risks associated with Ground Potential Rise and High Voltage isolation mitigation. Currently most hydro utilities deploy isolation devices, however this is not 100% widespread. To date, only 60% to 70% of all telecommunications wire line facilities are isolation protected with either copper or fiber based HVPE. The balance still rely on carbon block or gas tube shunts to ground protection. This type of protection will not fully protect against the hazards of high voltage faults and lightning strikes. Even more importantly, today's control and data circuits carry more information and control functions than in the past. As a result, these circuits are becoming even more critical and cannot be interrupted at all. With this situation it is little wonder why more emphasis is placed on fail safe wire line facilities.
Historically, many cases of personnel experiencing life threatening situations have been reported and documented. These situations are less severe than in the past, with the implementation of high voltage isolation techniques. In order to keep abreast of the latest telecommunication circuit designs and advancements, vendors who manufacture protection and isolation equipment must keep up with these advances and develop products which will meet the standards established by IEEE, National Electrical Codes (UL/CSA),FCC, and Telcordia in respect to maintaining standardized levels of equipment quality, protection and isolation design standards.
One of the most important protection theory principle is the 300 Volt point. This reference level was established to ensure 100% protection for any craftsperson working on a telecommunications facility. The fundamental basis for the 300 volt point was established in conjunction with Hydro and Telephone companies to ensure the safety of both telephone company and hydro utilitiy employees. This original standard has been accepted by all regulatory bodies in North America and has been well defined in the applicable standards of these bodies. As a very fundamental description, the 300 Volt point is a zone of influence around a hydro sub station which is dependent on the ground resistivity in ohms, the amount of fault current in amperes and will define a boundary a certain distance from the location of the ground grid of the hydro sub station. Each sub station has its own unique zone of influence since the variables explained above are different for each location.
In the past 35 years, the importance of utilizing high voltage isolation equipment has demonstrated the safety and operational performance of implementing this type of protection. It is becoming even more apparent that in the future, this type of protection will even play a greater role owing to the increased amount of information that is being transmitted down the copper wire facilities. |
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| 31. Line Regulation |
| Line regulation is the capability to maintain a constant output voltage level on the output channel of a power supply despite changes to the input voltage level. Line regulation is expressed as percent of change in the output voltage relative to the change in the input line voltage. |
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| 32. Linear Power Supply |
The voltage produced by an unregulated power supply will vary depending on the load and on variations in the AC supply voltage. For critical electronics applications a linear regulator will be used to stabilize and adjust the voltage. This regulator will also greatly reduce the ripple and noise in the output direct current. Linear regulators often provide current limiting, protecting the power supply and attached circuit from over current.
Adjustable linear power supplies are common laboratory and service shop test equipment, allowing the output voltage to be set over a wide range. For example, a bench power supply used by circuit designers may be adjustable up to 30 volts and up to 5 amperes output. Some can be driven by an external signal, for example, for applications requiring a pulsed output. |
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| 33. Linear Regulators |
Linear regulators are based on devices that operate in their linear region (in contrast, a switching regulator is based on a device forced to act as an on/off switch). In the past, one or more vacuum tubes were commonly used as the variable resistance. Modern designs use one or more transistors instead, perhaps within an Integrated Circuit. Linear designs have the advantage of very "clean" output with little noise introduced into their DC output, but are most often much less efficient and unable to step-up or invert the input voltage like switched supplies.
Entire linear regulators are available as integrated circuits. These chips come in either fixed or adjustable voltage types. |
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| 34. Load Regulation |
Load regulation is the capability to maintain a constant voltage (or current) level on the output channel of a power supply despite changes in load
Load Regulation can be defined as a percentage by the equation
Where:
- FullLoad is the load that draws the greatest current (is the lowest specified load resistance - never short circuit)
- MinimumLoad is the load that draws the least current (is the highest specified load resistance - possibly open circuit for some types of linear supplies, usually limited by pass transistor minimum bias levels)
- NominalLoad is the typical specified operating load
For switching power supplies, the primary source of regulation error is switching ripple rather than control loop inefficiency. In such cases Load Regulation is defined without normalizing to Voltage at Nominal Load and then has the units of volts.
LoadRegulation,volts = Voltage(FullLoad) - Voltage(MinimumLoad)
A simple way to manually measure load regulation is to connect three parallel load resistors to the power supply where two of the resistors, R2 and R3 are connected through a switches while the other resistor, R1 is connected directly. The values of the resistors are selected such that R1 gives the minimum load resistance, R1||R2 gives the nominal load resistance and either R1||R2||R3 or R2||R3 (depending on how you choose to switch) given the full load resistance. A voltmeter is then connected in parallel as well and the measured values of resistance for each switch state give the inputs to the load regulation equation. Programmable loads are typically used to automate the measurement of load regulation. |
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| 35. MTBF |
A product's service life is its expected lifetime, or the acceptable period of use in service. It is the time that any manufactured item can be expected to be 'serviceable' or supported by its originating manufacturer.
Expected service life consists of business policy, using tools and calculations from maintainability and reliability analysis. Service life is a unique commitment made by the item's manufacturer and is usually specified as a median. Actual service life is the maximal recorded life of a product.
Service life is different from a predicted life, or MTTF/MTBF (Mean Time to Failure/Mean Time Between Failures)/MFOP(Maintenance-free operating period). Predicted life is useful such that a manufacturer may estimate, by hypothetical modeling and calculation, a general rule for which it will honor warranty claims, or planning for mission fulfillment. The difference between service life and predicted life is most clear when considering mission time and reliability in comparison to MTBF and service life.
For example: A missile system can have a mission time of less than one minute, a service life of 20 years, active MTBF of 20 minutes, dormant MTBF of 50 years and a reliability of .999999.
A consumer item will have different expectations about service and longevity based upon factors such as use, cost, and quality.
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| 36. Operating Temperature |
An operating temperature is the temperature at which an electrical or mechanical device operates. The device will operate effectively within a specified operating temperature range which varies based on the device function and application context, and ranges from the minimum operating temperature to the maximum operating temperature (or peak operating temperature). Outside of this range, the device may fail. Aerospace and military-grade devices generally operate over a broader temperature range than industrial devices; consumer-grade devices generally have the lowest operating temperature range.
Electrical and mechanical devices used in military and aerospace applications must endure greater environmental variability, including temperature range. For example, resistors are manufactured in several grades:
- Commercial grade: 0 °C to 70 °C (sometimes ?25 °C to 70 °C)
- Industrial grade: ?40 °C to 85 °C (sometimes ?25 °C to 85 °C)
- Military grade: ?55 °C to 125 °C (sometimes -65 °C to 275 °C)
These grades ensure that a device is suitable for its application, and may withstand the environmental conditions in which it is used. In the United States, the Department of Defense has defined the United States Military Standard for all products used by the United States armed forces. A product's environmental design and test limits to the conditions that it will experience throughout its service life are specified in MIL-STD-810, the Department of Defense Test Method Standard for Environmental Engineering Considerations and Laboratory Tests.
The MIL-STD-810G standard specifies that the "operating temperature stabilization is attained when the temperature of the functioning part(s) of the test item considered to have the longest thermal lag is changing at a rate of no more than 2.0°C (3.6°F) per hour." It also specifies procedures to assess the performance of materials to extreme temperature loads. |
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| 37. Overload Protection |
| Power supplies should have some type of overload protection. Overload protection is important to protect the electronic equipment hooked up to the power supply and to also prevent overheating, which could potentially lead to an electrical fire. Fuses and circuit breakers are two of the more frequent mechanisms used for overload protection. |
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| 38. Pi Filter |
The capacitor-input filter, also called pi filter due to its shape that looks like the Greek letter pi, is a type of electronic filter. Filter circuits are used to remove unwanted or undesired frequencies from a signal.
A typical capacitor input filter consists of a filter capacitor C1, connected across the rectifier output, an inductor L, in series and another filter capacitor, C2, connected across the load, RL. A filter of this sort is designed for use at a particular frequency, generally fixed by the AC line frequency and rectifier configuration. When used in this service, filter performance is often characterized by its regulation and ripple.
- The capacitor C1 offers low reactance to the AC component of the rectifier output while it offers infinite reactance to the DC component. As a result the capacitor shunts an appreciable amount of the AC component while the DC component continues its journey to the inductor L
- The inductor L offers high reactance to the AC component but it offers almost zero reactance to the DC component. As a result the DC component flows through the inductor while the AC component is blocked.
- The capacitor C2 bypasses the AC component which the inductor had failed to block. As a result only the DC component appears across the load RL.
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| 39. Potting Material |
| Potting, formally called "encapsulation", potting consists of immersing the part or assembly in a liquid resin, and then curing it. Potting can be done in a pre-molded potting shell, or directly in a mold. Today it is most widely used to protect semiconductor components from moisture and mechanical damage, and to serve as a mechanical structure holding the lead frame and the chip together. In earlier times it was often used to discourage reverse engineering of proprietary products built as printed circuit modules. It is also commonly used in high voltage products to allow live parts to be placed much closer together, so that the product can be smaller; also, to keep dirt and conductive contaminants such as impure water out of sensitive areas. Another use is to protect deep-submergence items such as sonar transducers from collapsing under extreme pressure, by filling all voids. Potting can be rigid or soft. When void-free potting is required, it's common practice to place the product in a vacuum chamber while the resin is still liquid, hold a vacuum for several minutes to draw the air out of internal cavities and the resin itself, then release the vacuum. Atmospheric pressure collapses the voids and forces the liquid resin into all internal spaces. Vacuum potting works best with resins that cure by polymerization, rather than solvent evaporation. |
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| 40. Power Conversion |
| The term "power supply" is sometimes restricted to those devices that convert some other form of energy into electricity (such as solar power and fuel cells and generators). A more accurate term for devices that convert one form of electric power into another form (such as transformers and linear regulators) is power converter. The most common conversion is from AC to DC. |
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| 41. Power Factor |
The power factor of an AC electric power system is defined as the ratio of the real power flowing to the load to the apparent power, and is a dimensionless number between 0 and 1 (frequently expressed as a percentage, e.g. 0.5 pf = 50% pf). Real power is the capacity of the circuit for performing work in a particular time. Apparent power is the product of the current and voltage of the circuit. Due to energy stored in the load and returned to the source, or due to a non-linear load that distorts the wave shape of the current drawn from the source, the apparent power will be greater than the real power.
In an electric power system, a load with low power factor draws more current than a load with a high power factor for the same amount of useful power transferred. The higher currents increase the energy lost in the distribution system, and require larger wires and other equipment. Because of the costs of larger equipment and wasted energy, electrical utilities will usually charge a higher cost to industrial or commercial customers where there is a low power factor.
Linear loads with low power factor (such as induction motors) can be corrected with a passive network of capacitors or inductors. Non-linear loads, such as rectifiers, distort the current drawn from the system. In such cases, active or passive power factor correction may be used to counteract the distortion and raise the power factor. The devices for correction of the power factor may be at a central substation, spread out over a distribution system, or built into power-consuming equipment. |
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| 42. Power Line Dip Immunity |
| The line voltage is slowly dropped down then brought back up. |
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| 43. Power Line Surge Immunity |
| A surge is applied to the line voltage. |
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| 44. Power Supply Applications |
A modern computer power supply is a switch with on and off supply designed to convert 110-240 V AC power from the mains supply, to several output both positive (and historically negative) DC voltages in the range + 12V,-12V,+5V,+5VBs and +3.3V. The first generation of computers power supplies were linear devices, but as cost became a driving factor, and weight became important, switched mode supplies are almost universal.
The diverse collection of output voltages also have widely varying current draw requirements, which are difficult to all be supplied from the same switched-mode source. Consequently most modern computer power supplies actually consist of several different switched mode supplies, each producing just one voltage component and each able to vary its output based on component power requirements, and all are linked together to shut down as a group in the event of a fault condition. |
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| 45. Power Supply Types |
| Power supplies for electronic devices can be broadly divided into linear and switching power supplies. The linear supply is a relatively simple design that becomes increasingly bulky and heavy for high current devices; voltage regulation in a linear supply can result in low efficiency. A switched-mode supply of the same rating as a linear supply will be smaller, is usually more efficient, but will be more complex. |
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| 46. Product Strategy |
Manufacturers will commit to very conservative service life, usually 2 to 5 years for most commercial and consumer products (for example computer peripherals and components). However, for large and expensive 'durable' items (airliner, hydroelectric power plants) the items are not 'consumable', and service lives and maintenance activity will factor large in the service life. Again, an airliner might have a mission time of 11 hours, a predicted active MTBF of 10,000 hours with maintenance (or 15,000 hours without maintenance), a reliability of .99999 and a service life of 40 years.
The most common model for item lifetime behavior, in relative failure terms follows a bathtub curve. On the abcissa of this curve is the term 'lambda' or failure rate, which is the inverse of MTBF. The ordinate axis is time. Initially, the bathub shows early life failures, generally not witnessed by the consumer and usually termed as factory defects. The flat middle portion of the bathtub, or 'useful life', is a slightly inclined, exponentially increasing, constant failure rate period where the consumer enjoys the benefit conferred by the item. As the bathtub reaches its far right terminus, it is exponentially increasing, modeling untoward physical effects related to Arrhenius rate effects.
For an individual product, there may be several bathtub curves, related to different aspects of the product. For instance, a tire will have a service life partitioning related to the tread and the casing.
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| 47. Programmable Power Supply |
Programmable power supplies
Programmable power supplies are those in which the output voltage can be varied remotely. One possible option is digital control by a computer interface. Variable properties include voltage, current, and frequency. This type of supply is composed of a processor, voltage/current programming circuits, current shunt, and voltage/current read-back circuits.
Programmable power supplies can furnish DC, AC, or AC with a DC offset. The AC output can be either single-phase or three-phase. Single-phase is generally used for low-voltage, while three-phase is more common for high-voltage power supplies.
When choosing a programmable power supply, several specifications should be considered. For AC supplies, output voltage, voltage accuracy, output frequency, and output current are important attributes. For DC supplies, output voltage, voltage accuracy, current, and power are important characteristics. Many special features are also available, including computer interface, overcurrent protection, overvoltage protection, short circuit protection, and temperature compensation. Programmable power supplies also come in a variety of forms. Some of those are modular, board-mounted, wall-mounted, floor-mounted or bench top.
Programmable power supplies are now used in many applications. Some examples include automated equipment testing, crystal growth monitoring, and differential thermal analysis. |
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| 48. Radiated Emissions |
| One or more antennas are used to measure the amplitude of the electromagnetic waves that a device emits. The amplitude must be under a set limit, with the limit depending on the devices classification. |
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| 49. Radiated Immunity |
| Electromagnetic Compatibility and Electrical Safety - Generic Criteria for Network Telecommunications Equipment. An antenna is used to subject the device to electromagnetic waves, covering a large frequency range (usually from 30 MHz to 2.9 GHz). |
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| 50. Residual-current Device |
A two-pole residual current device. A residual-current device (RCD), similar to a Residual Current Circuit Breaker (RCCB), is an electrical wiring device that disconnects a circuit whenever it detects that the electric current is not balanced between the energized conductor and the return neutral conductor. Such an imbalance is sometimes caused by current leakage through the body of a person who is grounded and accidentally touching the energized part of the circuit. A lethal shock can result from these conditions. RCDs are designed to disconnect quickly enough to mitigate the harm caused by such shocks although they are not intended to provide protection against overload or short-circuit conditions.
In the United States and Canada, a residual current device is also known as a ground fault circuit interrupter (GFCI), ground fault interrupter (GFI) or an appliance leakage current interrupter (ALCI). In Australia they are sometimes known as "safety switches" or simply "RCD" and in the United Kingdom they can be referred to as "trips" or "trip switches". They can be found in kitchens, bathrooms, and other places that can be wet. |
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| 51. Ripple |
The ripples in electricity. For ripples on fluid interfaces, see capillary wave. For other uses, see ripple. The most common meaning of ripple in electrical science, is the small unwanted residual periodic variation of the direct current (dc) output of a power supply which has been derived from an alternating current (ac) source. This ripple is due to incomplete suppression of the alternating waveform within the power supply.
As well as this time-varying phenomenon, there is a frequency domain ripple that arises in some classes of filter and other signal processing networks. In this case the periodic variation is a variation in the insertion loss of the network against increasing frequency. The variation may not be strictly linearly periodic. In this meaning also, ripple is usually to be considered an unwanted effect, its existence being a compromise between the amount of ripple and other design parameters. |
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| 52. Safety Tests |
| There are three main types of compliance test for electronic devices, emissions tests, immunity tests, and safety tests. All of these tests are specified in Telcordia GR-1089, Electromagnetic Compatibility and Electrical Safety - Generic Criteria for Network Telecommunications Equipment. Emissions tests ensure that a product will not emit harmful interference by electromagnetic radiation and/or electrical signals in communication and power lines. Immunity tests ensure that a product is immune to common electrical signals and Electromagnetic interference (EMI) that will be found in its operating environment, such as electromagnetic radiation from a local radio station or interference from nearby products. Safety tests ensure that a product will not create a safety risk from situations such as a failed or shorted power supply, blocked cooling vent, and power line voltage spikes and dips. |
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| 53. SCR Regulators |
| Regulators powered from AC power circuits can use silicon controlled rectifiers (SCRs) as the series device. Whenever the output voltage is below the desired value, the SCR is triggered, allowing electricity to flow into the load until the AC mains voltage passes through zero (ending the half cycle). SCR regulators have the advantages of being both very efficient and very simple, but because they can not terminate an on-going half cycle of conduction, they are not capable of very accurate voltage regulation in response to rapidly-changing loads. |
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| 54. Service Life Examples |
For maintainable items, those wear-out items that are determined by logistical analysis to be provisioned for sparing and replacement will assure a longer service life than manufactured items without such planning. A simple example is automotive tires - failure to plan for this wear out item would limit automotive service life to the extent of a single set of tires.
An individual tire's life follows the bathtub curve, to boot. After installation, there is a not-small probability of failure which may be related to material or workmanship or even to the process for mounting the tire which may introduce some small damage. After the initial period, the tire will perform, given no defect introducing event such as encountering a road hazard (a nail or a pothole), for a long duration relative to its expected service life which is a function of several variables (design, material, process). After a period, the failure probability will rise; for some tires, this will occur after the tread is worn out. Then, a secondary market for tires puts a retread on the tire thereby extending the service life. It is not uncommon for an 80,000-mile tire to perform well beyond that limit.
It may be difficult to obtain reliable longevity data about many consumer products as, in general, efforts at actuarial analysis are not taken to the same extent as found with that needed to support insurance decisions. However, some attempts to provide this type of information have been made. An example is the collection of estimates for household components provided by the Old House Web] which gathers data from the Appliance Statistical Review and various institutes involved with the homebuilding trade. |
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| 55. Shielding |
Electromagnetic shielding is the process of limiting the penetration of electromagnetic fields into a space, by blocking them with a barrier made of conductive material. Typically it is applied to enclosures, separating electrical devices from the 'outside world', and to cables, separating wires from the environment the cable runs through. Electromagnetic shielding used to block radio frequency electromagnetic radiation is also known as RF shielding.
The shielding can reduce the coupling of radio waves, electromagnetic fields and electrostatic fields, though not static or low-frequency magnetic fields (a conductive enclosure used to block electrostatic fields is also known as a Faraday cage). The amount of reduction depends very much upon the material used, its thickness, the size of the shielded volume and the frequency of the fields of interest and the size, shape and orientation of apertures in a shield to an incident electromagnetic field. |
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| 56. Shunt |
| A shunt is a device which allows electric current to pass around another point in the circuit. The term is also widely used in photovoltaics to describe an unwanted short circuit between the front and back surface contacts of a solar cell, usually caused by wafer damage. |
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| 57. Simple Voltage Stabilizer |
In the simplest case emitter follower is used, the base of the regulating transistor is directly connected to the voltage reference:
The stabilizer uses the power source, having voltage Uin that may vary over time. It delivers the relatively constant voltage Uout. The output load RL can also vary over time. For such a device to work properly, the input voltage must be larger than the output voltage and Voltage drop must not exceed the limits of the transistor used.
The output voltage of the stabilizer is equal to UZ - UBE where UBE is about 0.7v and depends on the load current. If the output voltage drops below that limit, this increases the voltage difference between the base and emitter (Ube), opening the transistor and delivering more current. Delivering more current through the same output resistor RL increases the voltage again. |
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| 58. Switched-mode Power Supply |
computer's switched mode power supply unit
A switched-mode power supply (SMPS) works on a different principle. AC mains input is directly rectified without the use of a transformer, to obtain a DC voltage. This voltage is then sliced into small pieces by a high-speed electronic switch. The size of these slices grows larger as power output requirements increase.
The input power slicing occurs at a very high speed (typically 10 kHz — 1 MHz). High frequency and high voltages in this first stage permit much smaller step down transformers and smoothing capacitors than are in a linear power supply. After the transformer secondary, the AC is again rectified to DC. To keep output voltage constant, the power supply needs a sophisticated feedback controller to monitor current drawn by the load.
Modern switched-mode power supplies often include additional safety features such as the crowbar circuit to help protect the device and the user from harm.[4] In the event that an abnormal high current power draw is detected, the switched-mode supply can assume this is a direct short and will shut itself down before damage is done. For decades PC power supplies have also provided a power good signal to the motherboard which prevents operation when abnormal supply voltages are present.
Switched mode power supplies have an absolute limit on their minimum current output.[5] They are only able to output above a certain power level and cannot function below that point. In a no-load condition the frequency of the power slicing circuit increases to great speed, causing the isolated transformer to act as a Tesla coil, causing damage due to the resulting very high voltage power spikes. Switched-mode supplies with protection circuits may briefly turn on but then shut down when no load has been detected. A very small low-power dummy load such as a ceramic power resistor or 10-watt light bulb can be attached to the supply to allow it to run with no primary load attached.
Power factor has become a recent issue of concern for computer manufacturers. Switched mode power supplies have traditionally been a source of power line harmonics and have a very poor power factor. Many computer power supplies built in the last few years now include power factor correction built right into the switched-mode supply, and may advertise the fact that they offer 1.0 power factor.
By slicing up the sinusoidal AC wave into very small discrete pieces, a portion of unused alternating current stays in the power line as very small spikes of power that cannot be utilized by AC motors and results in waste heating of power line transformers. Hundreds of switched mode power supplies in a building can result in poor power quality for other customers surrounding that building, and high electric bills for the company if they are billed according to their power factor in addition to the actual power used. Filtering capacitor banks may be needed on the building power mains to suppress and absorb these negative power factor effects. |
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| 59. Switcher |
A switched-mode power supply (switching-mode power supply/SMPS, or simply switcher) is an electronic power supply unit (PSU) that converts the volt and current characteristics from one form to another and incorporates a switching regulator in order to be highly efficient. A SMPS power converter that transfers power from a source like a battery or the electrical power grid to a load (e.g., a personal computer). The function of the converter is usually to provide a regulated output voltage usually at a different level from the input voltage.
Unlike a linear power supply, the pass transistor of a switching mode supply switches very quickly (typically between 50 kHz and 1 MHz) between full-on and full-off states, which minimizes wasted energy. Voltage regulation is provided by varying the ratio of on to off time. In contrast, a linear power supply must dissipate the excess voltage to regulate the output. This higher efficiency is the chief advantage of a switch-mode power supply.
Switching regulators are used as replacements for the linear regulators when higher efficiency, smaller size or lighter weight are required. They are, however, more complicated, their switching currents can cause electrical noise problems if not carefully suppressed, and simple designs may have a poor power factor. |
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| 60. Switching Regulators |
Switching regulators rapidly switch a series device on and off. The duty cycle of the switch sets how much charge is transferred to the load. This is controlled by a similar feedback mechanism as in a linear regulator. Because the series element is either fully conducting, or switched off, it dissipates almost no power; this is what gives the switching design its efficiency. Switching regulators are also able to generate output voltages which are higher than the input, or of opposite polarity — something not possible with a linear design.
Like linear regulators, nearly-complete switching regulators are also available as integrated circuits. Unlike linear regulators, these usually require one external component: an inductor that acts as the energy storage element. (Large-valued inductors tend to be physically large relative to almost all other kinds of componentry, so they are rarely fabricated within integrated circuits and IC regulators — with some exceptions.) |
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| 61. Telcordia Technologies |
| It provides conformance testing for all telecommunications electronic products. The conformance testing is performed based on the specific Generic Requirements for the specific product under test. Conformance testing follows a rigid process of requirements review, test plan development, testing, and documentation of test results. Conformance testing for telecommunication products is required by major telecommunication carriers to help ensure performance and quality when products are deployed in networks. |
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| 62. Temperature Coefficient |
The temperature coefficient is the relative change of a physical property when the temperature is changed by 1 K. In the following formula, let R be the physical property to be measured and T be the temperature at which the property is measured. T0 is the reference temperature, and ΔT is the difference between T and T0. Finally, α is the (linear) temperature coefficient. Given these definitions, the physical property is:
Here α has the dimensions of an inverse temperature (1/K or K-1).
This equation is linear with respect to temperature. For quantities that vary polynomially or logarithmically with temperature, it may be possible to calculate a temperature coefficient that is a useful approximation for a certain range of temperatures. For quantities that vary exponentially with temperature, such as the rate of a chemical reaction, any temperature coefficient would be valid only over a very small temperature range. Different temperature coefficients are specified for various applications, including nuclear, electrical and magnetic. |
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| 63. Thermal Cutouts |
| Some PSUs use a thermal cutout buried in the transformer rather than a fuse. The advantage is it allows greater current to be drawn for limited time than the unit can supply continuously. Some such cutouts are self resetting, some are single use only. |
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| 64. Thermal Shock |
Thermal shock is the name given to cracking as a result of rapid temperature change. Glass and ceramic objects are particularly vulnerable to this form of failure, due to their low toughness, low thermal conductivity, and high thermal expansion coefficients. However, they are used in many high temperature applications due to their high melting point.
Thermal shock occurs when a thermal gradient causes different parts of an object to expand by different amounts. This differential expansion can be understood in terms of stress or of strain, equivalently. At some point, this stress overcomes the strength of the material, causing a crack to form. If nothing stops this crack from propagating through the material, it will cause the object's structure to fail.
Thermal shock can be prevented by:
- Reducing the thermal gradient seen by the object, by
- changing its temperature more slowly
- increasing the material's thermal conductivity
- Reducing the material's coefficient of thermal expansion
- Increasing its strength
- Decreasing its Young's modulus
- Increasing its toughness, by
- crack tip blunting, i.e., plasticity or phase transformation
- crack deflection
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| 65. Uninterruptible Power Supply |
| An uninterruptible power supply (UPS) takes its power from two or more sources simultaneously. It is usually powered directly from the AC mains, while simultaneously charging a storage battery. Should there be a dropout or failure of the mains, the battery instantly takes over so that the load never experiences an interruption. Such a scheme can supply power as long as the battery charge suffices, e.g., in a computer installation, giving the operator sufficient time to effect an orderly system shutdown without loss of data. Other UPS schemes may use an internal combustion engine or turbine to continuously supply power to a system in parallel with power coming from the AC mains. The engine-driven generators would normally be idling, but could come to full power in a matter of a few seconds in order to keep vital equipment running without interruption. Such a scheme might be found in hospitals or telephone central offices. |
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| 66. Unregulated Power Supply |
A home-made linear power supply (used here to power amateur radio equipment
An AC powered unregulated power supply usually uses a transformer to convert the voltage from the wall outlet (mains) to a different, usually a lower voltage. If it is used to produce DC, a rectifier is used. A capacitor is used to smooth the pulsating current from the rectifier. Some small periodic deviations from smooth direct current will remain, which is known as ripple. These pulsations occur at a frequency related to the AC power frequency (for example, a multiple of 50 or 60 Hz). The simplest unregulated DC power supply circuit consists of a single diode and resistor in series with the AC supply. This circuit is common in rechargeable flashlights. |
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| 67. Voltage Multipliers |
Voltage multipliers, as the name implies, are circuits designed to multiply the input voltage. The input voltage may be doubled (voltage doubler), tripled (voltage tripler), quadrupled (voltage quadrupler), etc. Voltage multipliers are also power converters. An AC input is converted to a higher DC output. These circuits allow high voltages to be obtained using a much lower voltage AC source.
Typically, voltage multipliers are composed of half-wave rectifiers, capacitors, and diodes. For example, a voltage tripler consists of three half-wave rectifiers, three capacitors, and three diodes (see Cockroft Walton Multiplier). Full-wave rectifiers may be used in a different configuration to achieve even higher voltages. Also, both parallel and series configurations are available. For parallel multipliers, a higher voltage rating is required at each consecutive multiplication stage, but less capacitance is required. The voltage capability of the capacitor limits the maximum output voltage.
Voltage multipliers have many applications. For example, voltage multipliers can be found in everyday items like televisions and photocopiers. Even more applications can be found in the laboratory, such as cathode ray tubes, oscilloscopes, and photomultiplier tubes. |
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| 68. Voltage Regulator |
Electronic symbol for Voltage regulator
A voltage regulator is an electrical regulator designed to automatically maintain a constant voltage level. It may use an electromechanical mechanism, or passive or active electronic components. Depending on the design, it may be used to regulate one or more AC or DC voltages.
With the exception of passive shunt regulators, all modern electronic voltage regulators operate by comparing the actual output voltage to some internal fixed reference voltage. Any difference is amplified and used to control the regulation element in such a way as to reduce the voltage error. This forms a negative feedback control loop; increasing the open-loop gain tends to increase regulation accuracy but reduce stability (avoidance of oscillation, or ringing during step changes). There will also be a trade-off between stability and the speed of the response to changes. If the output voltage is too low (perhaps due to input voltage reducing or load current increasing), the regulation element is commanded, up to a point, to produce a higher output voltage - by dropping less of the input voltage (for linear series regulators and buck switching regulators), or to draw input current for longer periods (boost-type switching regulators); if the output voltage is too high, the regulation element will normally be commanded to produce a lower voltage. However, many regulators have over-current protection, so that they will entirely stop sourcing current (or limit the current in some way) if the output current is too high, and some regulators may also shut down if the input voltage is outside a given range (see also: crowbar circuits). |
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| 69. Voltage Stabilizer |
A voltage stabilizer is an electronic device able to deliver relatively constant output voltage while input voltage and load current changes over time.
The voltage stabilizer is the shunt regulator such as a Zener diode or avalanche diode. Each of these devices begins conducting at a specified voltage and will conduct as much current as required to hold its terminal voltage to that specified voltage. Hence the shunt regulator can be viewed as the limited power parallel stabilizer. The shunt regulator output is used as a voltage reference.
The Zener diode and avalanche diode have opposite threshold voltage dependence on temperature. By connecting these two devices sequentially, it is possible to construct a voltage reference with improved thermal stability. Sometimes (mostly for the voltages around 5.6 V) both effects are combined in the same diode. |
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| 70. Welding Power Supply |
| Arc welding uses electricity to melt the surfaces of the metals in order to join them together through coalescence. The electricity is provided by a welding power supply, and can either be AC or DC. Arc welding typically requires high currents typically between 100 and 350 amps. Some types of welding can use as few as 10 amps, while some applications of spot welding employ currents as high as 60,000 amps for an extremely short time. Older welding power supplies consisted of transformers or engines driving generators. More recent supplies use semiconductors and microprocessors reducing their size and weight. |
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