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4 avril 2011 1 04 /04 /avril /2011 15:10

A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which is in only one direction, a process known as rectification. Rectifiers have many uses including as components of power supplies and as detectors of radio signals Lenovo E30 Battery 5200mAh .

Rectifiers may be made of solid state diodes, vacuum tube diodes, mercury arc valves, and other components.

A device which performs the opposite function (converting DC to AC) is known as an inverter Lenovo E40 Battery 5200mAh .

When only one diode is used to rectify AC (by blocking the negative or positive portion of the waveform), the difference between the term diode and the term rectifier is merely one of usage, i.e., the term rectifier describes a diode that is being used to convert AC to DC  Lenovo E50 Battery 5200mAh .

Almost all rectifiers comprise a number of diodes in a specific arrangement for more efficiently converting AC to DC than is possible with only one diode. Before the development of silicon semiconductor rectifiers, vacuum tube diodes and copper(I) oxide or selenium rectifier stacks were used Lenovo G460 Battery 5200mAh .

Early radio receivers, called crystal radios, used a "cat's whisker" of fine wire pressing on a crystal of galena (lead sulfide) to serve as a point-contact rectifier or "crystal detector". Rectification may occasionally serve in roles other than to generate direct current per se  Lenovo Y460 Battery 5200mAh .

For example, in gas heating systems flame rectification is used to detect presence of flame. Two metal electrodes in the outer layer of the flame provide a current path, and rectification of an applied alternating voltage will happen in the plasma, but only while the flame is present to generate it Lenovo Y450 Battery 5200mAh .

Half-wave rectification

In half wave rectification, either the positive or negative half of the AC wave is passed, while the other half is blocked. Because only one half of the input waveform reaches the output, it is very inefficient if used for power transfer Lenovo N200 Battery 5200mAh .

Half-wave rectification can be achieved with a single diode in a one-phase supply, or with three diodes in a three-phase supply.

Full-wave rectification

A full-wave rectifier converts the whole of the input waveform to one of constant polarity (positive or negative) at its output Lenovo T410 Battery 5200mAh .

Full-wave rectification converts both polarities of the input waveform to DC (direct current), and is more efficient. However, in a circuit with a non-center tapped transformer, four diodes are required instead of the one needed for half-wave rectification. (See semiconductors,diode) Lenovo T510 Battery 5200mAh .

Four diodes arranged this way are called a diode bridge or bridge rectifier.

A very common vacuum tube rectifier configuration contained one cathode and twin anodes inside a single envelope; in this way, the two diodes required only one vacuum tube. The 5U4 and 5Y3 were popular examples of this configuration Lenovo SL410 Battery 5200mAh .

For three-phase AC, six diodes are used. Typically there are three pairs of diodes, each pair, though, is not the same kind of double diode that would be used for a full wave single-phase rectifier. Instead the pairs are in series (anode to cathode). Typically Lenovo SL510 Battery 5200mAh ,

commercially available double diodes have four terminals so the user can configure them as single-phase split supply use, for half a bridge, or for three-phase use.

Most devices that generate alternating current (such devices are called alternators) generate three-phase AC Lenovo C100 Battery 5200mAh .

For example, an automobile alternator has six diodes inside it to function as a full-wave rectifier for battery charging applications.

Peak loss

An aspect of most rectification is a loss from the peak input voltage to the peak output voltage, caused by the built-in voltage drop across the diodes (around 0.7 V for ordinary silicon p-n-junction diodes and 0.3 V for Schottky diodes) Lenovo T400S Battery 5200mAh .

Half-wave rectification and full-wave rectification using two separate secondaries will have a peak voltage loss of one diode drop. Bridge rectification will have a loss of two diode drops. This may represent significant power loss in very low voltage supplies. In addition  Lenovo Y430 Battery 5200mAh ,

the diodes will not conduct below this voltage, so the circuit is only passing current through for a portion of each half-cycle, causing short segments of zero voltage to appear between each "hump".

Rectifier output smoothing

While half-wave and full-wave rectification suffice to deliver a form of DC output, neither produces constant-voltage DC Ibm ThinkPad G40 Battery 5200mAh .

In order to produce steady DC from a rectified AC supply, a smoothing circuit or filter is required.  In its simplest form this can be just areservoir capacitor or smoothing capacitor, placed at the DC output of the rectifier. There will still remain an amount of AC ripplevoltage where the voltage is not completely smoothed Ibm ThinkPad X200T Battery 5200mAh .

Sizing of the capacitor represents a tradeoff. For a given load, a larger capacitor will reduce ripple but will cost more and will create higher peak currents in the transformer secondary and in the supply feeding it. In extreme cases where many rectifiers are loaded onto a power distribution circuit Apple A1280 battery 5200mAh ,

it may prove difficult for the power distribution authority to maintain a correctly shaped sinusoidal voltage curve.

For a given tolerable ripple the required capacitor size is proportional to the load current and inversely proportional to the supply frequency and the number of output peaks of the rectifier per input cycle  Apple A1281 battery 5200mAh .

The load current and the supply frequency are generally outside the control of the designer of the rectifier system but the number of peaks per input cycle can be affected by the choice of rectifier design.

A half-wave rectifier will only give one peak per cycle and for this and other reasons is only used in very small power suppliesSamsung R522 battery 5200mAh .

A full wave rectifier achieves two peaks per cycle and this is the best that can be done with single-phase input. For three-phase inputs a three-phase bridge will give six peaks per cycle and even higher numbers of peaks can be achieved by using transformer networks placed before the rectifier to convert to a higher phase order Samsung P467 battery 5200mAh .

To further reduce this ripple, a capacitor-input filter can be used. This complements the reservoir capacitor with a choke(inductor) and a second filter capacitor, so that a steadier DC output can be obtained across the terminals of the filter capacitor. The choke presents a high impedance to the ripple current Hp Pavilion dm3 Battery 5400mAh .

A more usual alternative to a filter, and essential if the DC load is very demanding of a smooth supply voltage, is to follow the reservoir capacitor with a voltage regulator. The reservoir capacitor needs to be large enough to prevent the troughs of the ripple getting below the voltage the DC is being regulated to Hp Pavilion dm4 Battery 5200mAh .

The regulator serves both to remove the last of the ripple and to deal with variations in supply and load characteristics. It would be possible to use a smaller reservoir capacitor (these can be large on high-current power supplies) and then apply some filtering as well as the regulator, but this is not a common strategy  Hp Pavilion cq62 Battery 5200mAh .

The extreme of this approach is to dispense with the reservoir capacitor altogether and put the rectified waveform straight into a choke-input filter. The advantage of this circuit is that the current waveform is smoother and consequently the rectifier no longer has to deal with the current as a large current pulse Hp Pavilion cq42 Battery 5200mAh ,

but instead the current delivery is spread over the entire cycle. The downside is that the voltage output is much lower – approximately the average of an AC half-cycle rather than the peak.

Voltage-doubling rectifiers

The simple half wave rectifier can be built in two versions with the diode pointing in opposite direction Hp Pavilion cq72 Battery 5200mAhs,

one version connects the negative terminal of the output direct to the AC supply and the other connects the positive terminal of the output direct to the AC supply. By combining both of these with separate output smoothing it is possible to get an output voltage of nearly double the peak AC input voltage Hp cq20 Battery 2600mAh .

This also provides a tap in the middle, which allows use of such a circuit as a split rail supply.

A variant of this is to use two capacitors in series for the output smoothing on a bridge rectifier then place a switch between the midpoint of those capacitors and one of the AC input terminals  Hp 2230 Battery 2600mAh .

With the switch open this circuit will act like a normal bridge rectifier with it closed it will act like a voltage doubling rectifier. In other words this makes it easy to derive a voltage of roughly 320V (+/- around 15%) DC from any mains supply in the world, this can then be fed into a relatively simple switched mode power supply Dell Inspiron Mini 12 battery 7800mAh .

Cascaded stages of diodes and capacitors can be added to make a voltage multiplier (Cockroft-Walton circuit). These circuits can provide a potential several times that of the peak value of the input AC, although limited in current output and regulation  Dell XPS M1730 battery 7800mAh .

Voltage multipliers are used to provide the high voltage for aCRT in a television receiver, or for powering high-voltage tubes such as image intensifiers or photo multipliers.

Applications

The primary application of rectifiers is to derive DC power from an AC supply. Virtually all electronic devices require DC, so rectifiers find uses inside the power supplies of virtually all electronic equipment  Dell XPS M2010 battery 7800mAh .

Converting DC power from one voltage to another is much more complicated. One method of DC-to-DC conversion first converts power to AC (using a device called an inverter), then use a transformer to change the voltage, and finally rectifies power back to DCDell Studio XPS 1340 battery 5200mAh/7800mAh .

Rectifiers also find a use in detection of amplitude modulated radio signals. The signal may be amplified before detection, but if un-amplified, a very low voltage drop diode must be used. When using a rectifier for demodulation the capacitor and load resistance must be carefully matched Dell Inspiron 9100 battery 7800mAh.

Too low a capacitance will result in the high frequency carrier passing to the output and too high will result in the capacitor just charging and staying charged  Dell Inspiron N4010 battery 5200mAh .

Rectifiers are also used to supply polarised voltage for welding. In such circuits control of the output current is required and this is sometimes achieved by replacing some of the diodes in bridge rectifier with thyristors, whose voltage output can be regulated by means of phase fired controllers Dell Inspiron N3610 battery 5200mAh .

Thyristors are used in various classes of railway rolling stock systems so that fine control of the traction motors can be achieved. Gate turn-off thyristors are used to produce alternating current from a DC supply, for example on the Eurostar Trains to power the three-phase traction motors  Dell Inspiron N5010 battery 5200mAh .

Electromechanical

Early power conversion systems were purely electro-mechanical in design, since electronic devices were not available to handle significant power Dell Inspiron N7010 battery 5200mAh .

Mechanical rectification systems usually rely on some form of rotation or resonant vibration in order to move quickly enough to match the frequency of the input power source, and cannot operate beyond several thousand cycles per second  Dell Inspiron 1464 battery 5200mAh .

Due to the complexity of mechanical systems, they have traditionally needed a high level of maintenance to keep operating correctly. Moving parts will have friction, which requires lubrication and replacement due to wear. Opening mechanical contacts under load results in electrical arcs and sparks that heat and erode the contacts Dell Inspiron 1564 battery 5200mAh .

Synchronous rectifier

To convert AC currents into DC current in electric locomotives, a synchronous rectifier may be used . It consists of a synchronous motor driving a set of heavy-duty electrical contacts  Dell Inspiron 1764 battery 5200mAh .

The motor spins in time with the AC frequency and periodically reverses the connections to the load just when the sinusoidal current goes through a zero-crossing. The contacts do not have to switch a large current, but they need to be able to carry a large current to supply the locomotive's DC traction motors Dell Inspiron 1320 battery 5200mAh .

Vibrator

In the past, the vibrators used in battery-to-high-voltage-DC power supplies often contained a second set of contacts that performed synchronous mechanical rectification of the stepped-up voltage  Dell Inspiron 1320n battery 5200mAh .

Motor-generator set

A motor-generator set, or the similar rotary converter, is not a rectifier in the sense that it doesn't actually rectify current, but rather generates DC from an AC source. In an "M-G set", the shaft of an AC motor is mechanically coupled to that of a DC generatorDell Vostro 1710 battery 5200mAh .

The DC generator produces multiphase alternating currents in its armature windings, and a commutator on the armature shaft converts these alternating currents into a direct current output; or a homopolar generator produces a direct current without the need for a commutator  Dell Vostro 1720 battery 5200mAh .

M-G sets are useful for producing DC for railway traction motors, industrial motors and other high-current applications, and were common in many high power D.C. uses (for example, carbon-arc lamp projectors for outdoor theaters) before high-power semiconductors became widely available  Dell Studio XPS 1640 battery 5200mAh .

Electrolytic

The electrolytic rectifier  was an early device from the 1900s that is no longer used. When two different metals are suspended in an electrolyte solution, it can be found that direct current flowing one way through the metals has less resistance than the other direction  Dell Vostro A840 battery 5200mAh .

These most commonly used an aluminum anode, and a lead or steel cathode, suspended in a solution of tri-ammonium ortho-phosphate.

The rectification action is due to a thin coating of aluminum hydroxide on the aluminum electrode, formed by first applying a strong current to the cell to build up the coating  Dell Vostro A860 battery 5200mAh .

The rectification process is temperature sensitive, and for best efficiency should not operate above 86 °F (30 °C). There is also a breakdown voltage where the coating is penetrated and the cell is short-circuited. Electrochemical methods are often more fragile than mechanical methods Dell Studio 1535 battery 5200mAh/7800mAh ,

and can be sensitive to usage variations which can drastically change or completely disrupt the rectification processes.

Similar electrolytic devices were used as lightning arresters around the same era by suspending many aluminium cones in a tank of tri-ammomium ortho-phosphate solution Dell Studio 1735 battery 5200mAh/7800mAh .

Unlike the rectifier, above, only aluminium electrodes were used, and used on A.C., there was no polarization and thus no rectifier action, but the chemistry was similar.

The modern electrolytic capacitor, an essential component of most rectifier circuit configurations was also developed from the electrolytic rectifier Dell Vostro 1310 battery 5200mAh .

A rectifier used in high-voltage direct current power transmission systems and industrial processing between about 1909 to 1975 is a mercury arc rectifier or mercury arc valve. The device is enclosed in a bulbous glass vessel or large metal tub Dell Latitude E6400 battery,10400mAh/7800mAh .

One electrode, the cathode, is submerged in a pool of liquid mercury at the bottom of the vessel and one or more high purity graphite electrodes, calledanodes, are suspended above the pool. There may be several auxiliary electrodes to aid in starting and maintaining the arc Dell Latitude E6400 Battery,10400mAh/7800mAh .

When an electric arc is established between the cathode pool and suspended anodes, a stream of electrons flows from the cathode to the anodes through the ionized mercury, but not the other way. [In principle, this is a higher-power counterpart to flame rectification Dell Latitude E6400 Battery,10400mAh/7800mAh ,

which uses the same one-way current transmission properties of the plasma naturally present in a flame].

These devices can be used at power levels of hundreds of kilowatts, and may be built to handle one to six phases of AC current Dell Latitude E6500 battery,10400mAh/7800mAh .

Mercury arc rectifiers have been replaced by silicon semiconductor rectifiers and high power thyristor circuits, from the mid 1970s onward. The most powerful mercury arc rectifiers ever built were installed in the Manitoba Hydro Nelson River Bipole HVDC project, with a combined rating of more than one million kilowatts and 450,000 volts Dell Latitude E6500 battery,10400mAh/7800mAh .

Argon gas electron tube

The General Electric Tungar rectifier was an argon gas-filled electron tube device with a tungsten filament cathode and a carbon button anode  Dell Latitude E6500 battery,10400mAh/7800mAh .

It was useful for battery chargers and similar applications from the 1920s until low-cost solid-state rectifiers (the metal rectifiers at first) supplanted it. These were made up to a few hundred volts and a few amperes rating, and in some sizes strongly resembled an incandescent lamp with an additional electrode Dell Latitude E6500 battery,10400mAh/7800mAh .

The 0Z4 was a gas-filled rectifier tube commonly used in vacuum tube car radios in the 1940s and 1950s. It was a conventional full wave rectifier tube with two anodes and one cathode, but was unique in that it had no filament (thus the "0" in its type number) HP Pavilion DV6-1210SA battery - 5200mAh/8800mAh .

The electrodes were shaped such that the reverse breakdown voltage was much higher than the forward breakdown voltage. Once the breakdown voltage was exceeded, the 0Z4 switched to a low-resistance state with a forward voltage drop of about 24 volts  HP Pavilion DV7 battery - 7200mAh .

Vacuum tube (valve)

Since the discovery of the Edison effect or thermionic emission, various vacuum tube devices have been developed to rectify alternating currents. Low-power devices are used as signal detectors, first used in radio by Fleming in 1904 Ibm THINKPAD X200 Battery 8800mAh .

Many vacuum-tube devices also used vacuum rectifiers in their power supplies, for example the All American Five radio receiver. Vacuum rectifiers were made for very high voltages, such as the high voltage power supply for the cathode ray tube of television receivers, and the kenotron used for power supply in X-ray equipment  Ibm THINKPAD X200 Battery 8800mAh .

However, vacuum rectifiers generally had low current capacity owing to the maximum current density that could be obtained by electrodes heated to temperatures compatible with long life. Another limitation of the vacuum tube rectifier was that the heater power supply often required special arrangements to insulate it from the high voltages of the rectifier circuit Ibm THINKPAD X200 Battery 8800mAh .

Once common until replaced by more compact and less costly silicon solid-state rectifiers, these units used stacks of metal plates and took advantage of the semiconductor properties of selenium or copper oxide. While selenium rectifiers were lighter in weight and used less power than comparable vacuum tube rectifiers  SONY VAIO VGN-FW21E Battery ,

they had the disadvantage of finite life expectancy, increasing resistance with age, and were only suitable to use at low frequencies. Both selenium and copper oxide rectifiers have somewhat better tolerance of momentary voltage transients than silicon rectifiersAsus M52N Battery .

Typically these rectifiers were made up of stacks of metal plates or washers, held together by a central bolt, with the number of stacks determined by voltage; each cell was rated for about 20 volts. An automotive battery charger rectifier might have only one cell  Dell Latitude E6400 ATG Battery :

the high-voltage power supply for a vacuum tube might have dozens of stacked plates. Current density in an air-cooled selenium stack was about 600 mA per square inch of active area (about 90 mA per square centimeter).

The AC adapter, AC/DC adapter or AC/DC converter is a type of external power supply SONY VAIO VGN-FW21J Battery ,

often enclosed in what looks like an over-sized AC plug. Other names include plug pack, plug-in adapter, adapter block, domestic mains adapter, line power adapter, or power adapter. Informal terms include wall wart, wall cubeand power brick  SONY VAIO VGN-FW21L Battery .

AC adapters are typically used with electrical devices that do not contain their own internal power supply. The internal circuitry of an external power supply is very similar to the design that would be used for a built-in or internal supply, but there are several advantages of separating the power supply from the main body of the electronic device, as explained below SONY VAIO VGN-FW21M Battery .

Modes of operation

Originally, AC adapters were linear power supplies, containing a transformer to convert the mains electricity voltage to a lower voltage, a rectifier to convert it to pulsating DC, and a filter to smooth the pulsating waveform to DC SONY VAIO VGN-FW21Z Battery,

with residual ripple variations small enough not to affect the device being powered. Size and weight of the device was largely determined by the transformer, which in turn was determined by the power output and mains frequency. Ratings over a few watts made the devices too large and heavy to be physically supported by a wall outlet   SONY VAIO VGN-FW25T/B Battery .

The output voltage of these adapters varied with load; for equipment requiring a more stable voltage, linear voltage regulator circuitry was added. Losses in the transformer and the linear regulator were considerable; efficiency was relatively low, and significant power dissipated as heat even when not driving a load    SONY VAIO VGN-FW26T/B Battery .

More recently, switched-mode power supplies (SMPSs) have become almost ubiquitous for this purpose. Mains voltage is rectified to a high direct voltage driving a switching circuit, which has a transformer operating at a high frequency and outputs direct current at the desired voltage  SONY VAIO VGN-FW27/B Battery .

The high-frequency ripple is more easily filtered out than mains-frequency. The high frequency allows the transformer to be small, which reduces its losses; and the switching regulator is more efficient than a linear regulator. The result is a much more efficient, smaller, and lighter device SONY VAIO VGN-FW27/W Battery .

Safety is ensured, same as in the older circuit, because the transformer electrically isolates the output from the mains.

Unless very carefully designed and using suitable components, switching adapters can be more likely to fail than the older type, due in part to complex circuitry and the use of semiconductors  SONY VAIO VGN-FW31ZJ Battery .

Unless designed well, these adapters may be easily damaged by overloads, even transient ones, which can come from lightning, brief mains overvoltage (sometimes caused by an incandescent light failing), component degradation, etc. A very common mode of failure is due to the use of electrolytic capacitors whose equivalent series resistance (ESR) increases with age SONY VAIO VGN-FW27T/H Battery ;

switching regulators are very sensitive to high ESR (the older circuit also used electrolytic capacitors, but the effect of degradation is much less dramatic). Well-designed circuits pay attention to the ESR, ripple current rating, pulse operation, and temperature rating of capacitors Sometimes circuit designers design SMPSs SONY VAIO VGN-FW29/B Battery ,

and accountants then cut the cost by replacing components such as capacitors with cheaper ones with the same headline characteristics but worse ESR, etc.  The cost-reduced power supply may appear to be just as functional as before, but then suffers a high failure rate after a few years  SONY VAIO VGN-FW290JRB Battery .

Advantages

External AC adapters are widely used to power small or portable electronic devices, because of a number of advantages:

  • Safety — External power adapters can free product designers from worrying about some safety issues SONY VAIO VGN-FW290JTB Battery .
  • The designer builds a product to run off a low voltage (usually supplied through a coaxial power connector) and does not have to worry about hazardous voltages inside the product enclosure. External power supplies tested and certified by safety agencies such as SONY VAIO VGN-FW290JTH Battery
  • Underwriters Laboratories (UL) and DIN allow a low-power electronic product to be produced and later modified as needed, without requiring expensive and time-consuming recertification each time a design is changed. Nevertheless, care should be taken if more than a few watts are supplied to a product SONY VAIO VGN-FW33G/E1 Battery ,
  • even at low voltages, since a malfunction could still start a fire. Thus, an internal fuse, circuit breaker, or thermal limiter may be useful in a powered product, even when a safety-certified external power supply is used. An external, removable AC adapter can be used to help achieve a design withintrinsic safety SONY VAIO VGN-FW33G/W Battery .
  • Heat reduction — Heat reduces reliability and longevity of electronic components, and can cause sensitive circuits to drift or malfunction. A separately-enclosed power supply moves what is often the primary source of heat to a cables-length distanceSONY VAIO VGN-FW33GW Battery .
  • Electrical noise reduction — Because radiated electrical noise falls off with the inverse square of the distance, it is advantageous to convert potentially-noisy AC line power or automotive power to "clean", filtered DC in an external adapter, at a safe distance from noise-sensitive circuitry Dell XPS M1210 Batteryhttp://www.hdd-shop.co.uk .
  • Weight reduction — When most power adapters were simple transformer-based designs, they were quite heavy, and would have added considerably to the weight of portable or hand-held devices. Even modern light-weight switching circuits for AC line power still would add appreciable weight to hand-held devices Dell KM958 battery ,
  • whose design weight budget is often denominated in grams. Moving these heavy power-handling components to a separate power supply reduces the dead weight of parts that are only in use during a battery charging cycle Dell Studio 1555 battery .
  • Size reduction — The physical bulk of power supply components can be shed from portable or handheld devices, slimming them considerably Sony VGP-BPS13 battery .
  • Ease of replacement — Power supplies are prone to failure, due to their exposure to power spikes and their internal generation of waste heat. External power supplies exemplify the design principle of modularity, in that they can be easily unplugged and replaced if they should fail  Sony VGP-BPS13/B battery .
  • Configuration versatility — Externally-powered electronic products can be used with different power sources as needed (e.g. 120VAC, 240VAC, 12VDC, or external battery pack), for convenient use in the field, or when traveling Sony VGP-BPS13/S battery .
  • Simplified product inventory and distribution — An electronic product that is sold and used internationally must be powered from a wide range of power sources, and must meet product safety regulations in many jurisdictions  Sony VGP-BPS13A/B battery .
  • The configuration modularity enabled by a separate, external power supply and power cord allow satisfying these diverse power requirements with a small number of product SKUs, reducing product inventory costs and overhead costs of warehousing and distribution  Sony VGP-BPS13B/B battery .

Problems

Whilst useful for many purposes, some external AC adapters have attracted criticism. Problems with this type of power supply may include  Sony VGP-BPL9 battery :

  • Size — The power supply may obscure other power sockets; some power strips attempt to relieve this problem by varying the spacing between their sockets.
  • Weight — Some AC adapters can be heavy, exerting excess weight on the plug socket (this depends on the socket design of the country in question)  Sony VGP-BPS13B/B battery .
  • Some external power supplies are "power bricks" having a short AC cord so they can lie on the floor, thus relieving strain, at the expense of clutter. Other wall-hanging types are made long and thin, minimizing the leverage of their weight vector that pulls the plug out, at the expense of exacerbating the size problem  Sony VGP-BPL15 battery .
  • Heavy adapters are a burden on travelers, especially if several different ones must be carried for various equipment.
  • Inefficiency — Some AC adapters, especially older linear power supplies, waste significant electrical energy while in use, or even when the powered device is not in use or disconnected  Dell Inspiron E1505 battery .
  • Confusion — Some consumers are unable to choose or obtain a suitable replacement AC adapter for a given small appliance. This can result in the large scale discarding of "orphaned" adapters and equipment which have been separated from each otherHP Pavilion dv6000 Battery .
  • The sheer number of partially incompatible adapters in use can be overwhelming, especially in the absence of clear labeling. Still worse, mismatching of adapters can cause severe equipment damage, or even fire HP Pavilion dv8000 battery .

Efficiency

The issue of inefficiency of some power supplies has become more well known, with US president George W. Bush referring in 2001 to such devices as "Energy Vampires".  Legislation is being enacted in the EU and a number of states, including California, to reduce the level of energy wasted by some of these devices HP PAVILION DV6000 Battery,  8800mAh .

See standby power and the One Watt Initiative for more details.

But others have argued that these inefficient devices are low powered, e.g., devices that are used for small battery chargers, so even if they have a low efficiency, the amount of energy they waste is less than 1% of household consumption of electric energy HP PAVILION DV8000 Battery,  8800mAh .

Considering the total efficiency of power supplies for small electronic equipment, the older mains-frequency linear transformer-based power supply was found in a 2002 report to have efficiencies from 20–75%, and have considerable energy loss even when powered up but not supplying power  SONY VAIO VGN-FZ Battery .

Switched-mode power supplies (SMPSs) are much more efficient; a good design can be 80–90% efficient, and is also much smaller and lighter. In 2002 most external plug-in "wall wart" power adapters commonly used for low-power consumer electronics devices were of linear design, as well as supplies built into some equipment  HP Pavilion DV6000 battery - 8800mAh .

External supplies are usually left plugged in even when not in use, and consume from a few watts to 35 watts of power in that state. The report concluded that about 32 billion kilowatt-hours (kWh) per year, about 1% of total electrical energy consumption, could be saved in the United States by replacing all linear power supplies HP Pavilion DV8000 battery - 7800mAh

(average efficiency 40–50%) with advanced switching designs (efficiency 80–90%), by replacing older switching supplies (efficiencies of less than 70%) with advanced designs (efficiency of at least 80%), and by reducing standby consumption of supplies to not more than 1 watt  HP Pavilion DV6000 battery - 8800mAh .

Since the report was published, SMPSs have indeed replaced linear supplies to a great extent, even in wall warts. The 2002 report estimated that 6% of electrical energy used in the US "flows through" power supplies (not counting only the wall warts)  HP Pavilion DV8000 battery - 6600mAh .

The website where the report was published said in 2010 that despite the spread of SMPSs, "today's power supplies consume at least 2% of all U.S. electricity production. More efficient power supply designs could cut that usage in half"

Since wasted electrical energy is released as heat, an inefficient power supply is hot to the touch SONY VAIO VGN-FZ18 Battery ,

as is one that wastes power without an electrical load. This waste heat is itself a problem in warm weather, since it often requires additional air conditioning to remove the unwanted heat.

Reuse

AC adapters are often reused on other appliances, but there are 5 parameters which all must suit the appliance SONY VAIO VGN-FZ21E Battery :

  • Voltage
  • Current capacity
  • Polarity (or AC)
  • Voltage regulation (or stabilization)
  • Connector type

"Universal" adapters are available  on which the user can adjust these parameters  SONY VAIO VGN-FW31J Battery 10400mAh .

Universal power adapters

One inherent disadvantage of external power adapters is that they can get separated from the product they are intended to power. Consequently, there is a market for replacement adapters. In addition, failed power supplies must be replaced  SONY VAIO VGN-FW31J Battery10400mAh .

Not only must the replacement match voltage, current, and polarity requirements, but it must also match the connector. Many electrical products are poorly labeled with information about the power supply they require, so it is prudent to record the specifications of the original power supply in advance, to ease replacement if the original is later lost SONY VAIO VGN-FW21E Battery .

Careful labeling of power adapters can also reduce the likelihood of a disastrous mixup which could cause equipment damage.

Some so-called "universal" replacement power supplies allow the voltage and polarity to be switched, which can ease the matching problem. In addition, the power connector must be matched  SONY VAIO VGN-NR11S/S Battery .

Four-way X connectors or six-way star connectors, also known as spider connectors, with multiple plug sizes and types are common on generic power supplies. Other replacement power supplies have arrangements for changing the power connector, with from four to nine different alternatives available when purchased in a set SONY VAIO VGN-NR11M/S Battery .

RadioShack sells universal AC adapters of various capacities, branded as "Enercell Adaptaplug", and fitted with 2-pin female sockets compatible with their Adaptaplug connector lineup. This allows many different configurations of AC adapters to be put together, without requiring soldering  SONY VAIO VGN-NR11Z/S Battery .

Philmore and other competing brands offer similar AC adapters with interchangeable connectors.

A suitable power supply for a particular use must have the matching plug dimensions, the matching DC (or AC) voltage and polarity, and the ability to supply at least the required current  SONY VAIO VGN-NR11Z/T Battery .

The input voltage must match the wall socket (115/230 VAC at 60/50 Hz) or other power source, such as 12VDC automotive battery power.

But the label on a power supply may not be a reliable guide to the actual voltage it supplies under varying conditions  Sony VAIO VGN-FZ21E Battery .

Most low-cost power supplies are "unregulated", in that their voltage can change appreciably with load. If they are lightly loaded, they may put out much more than the nominal "name plate" voltage, which could damage the load. If they are heavily loaded, the output voltage may droop appreciably  Sony VAIO VGN-FW21E Battery ,

in some cases well below the nominal label voltage even within the nominal rated current, causing the equipment being supplied to malfunction or be damaged. Cheap external power supplies of traditional design with undersized transformers tend to have poor regulation, whether originally-supplied or replacement units  Sony VAIO VGN-NR11S/S Battery .

In general, more modern high-quality switched-mode power supplies (SMPSs) are smaller, more efficient, and put out a much more constant voltage even as the input voltage and the load current may vary. Configurable switched-mode power supplies have come down considerably in price  Sony VAIO VGN-NR11Z/S Battery ,

and they are especially convenient for use when traveling because of their decreased weight and size.

Standards

A de facto standard has emerged in low-power AC adapters, namely the USB connector  Sony VAIO VGN-NR11M/S Battery .

Although the original purpose of this design was to enable serial digital data exchange, the USB standard also provided the option of providing 5VDC power, up to 500mA per device. This standard has been become sufficiently widespread that numerous accessory gadgets ("USB decorations") are available which connect to USB only for DC power, and not for data interchange Sony VAIO VGN-NR11Z/T Battery .

Electric fans, lamps, alarms, coffee warmers, battery chargers, and even toys are designed to tap power from a USB connector. Adapters are widely available to convert 115VAC or 230VAC power or 12VDC automotive power to USB power (see photo at right)SONY VAIO VGN-FZ180E Battery .

Portable "USB chargers" are available which convert energy from an internal battery to deliver DC power via a standard USB connnector, which can used to power a variety of portable electronics (e.g. MP3 player, cellphone). There is even a popular DIY Open Hardware USB charger which can be built from a kit  SONY VAIO VGN-FZ220E Battery .

Also, pocket-sized portable "USB battery packs" are available which can charge from a powered USB port, and can later in turn provide power from their own USB port.

The trend towards more-compact electronic devices has driven a shift towards the micro-USB connector, which is identical in function to the original USB connector but physically smaller SONY VAIO VGN-FZ340E Battery .

In 2009, the International Telecommunication Union (ITU) announced adoption of a world standard based on the micro-USB connector and interchangeable chargers for standards-compliant cellphones. This should markedly reduce the profusion of non-interchangeable power adapters previously needed for each year's new crop of cellphone models SONY VAIO VGN-FZ430E Battery .

Note that the previous assertions regarding USB power and the micro-USB charger standards are fairly-well documented in the USB article, and interested readers are referred there for much more detailed information  SONY VAIO VGN-FZ460E Battery .

Substitutes

A "Green Plug" system has been proposed, based on USB technology, by which the consuming device would tell the external power supply what kind of power is needed. Some commercial adapters, for example those by iGo , already automatically set their output according to which of a range of interchangeable tips is fitted SONY VAIO VGN-FZ480E Battery ;

tips are available to fit and supply appropriate power to many notebook computers and mobile devices. Adapters can be manufactured to operate from any AC mains supply anywhere in the world (however iGo products are not available in many countries, including Australia)  SONY VAIO VGN-FZ4000 Battery

with an appropriate interchangeable plug and voltage-tolerant circuitry, and also operate from 12V DC vehicle supplies—a truly universal device.

Larry Page, a founder of Google, has proposed a 12V 15A standard for almost all equipment requiring an external converter SONY VAIO VGN-FZ31E Battery .

New buildings would also have 12V DC wiring, so that in effect the AC adapter would be built into the wall.

In electronics, a diode is a two-terminal electronic component that conducts electric current in only one direction. The term usually refers to a semiconductor diode, the most common type today   SONY VAIO VGN-FZ31B Battery .

This is a crystalline piece of semiconductor material connected to two electrical terminals. A vacuum tube diode (now little used except in some high-power technologies) is a vacuum tube with two electrodes: a plate and a cathode.

The most common function of a diode is to allow an electric current to pass in one direction (called the diode's forward direction) while blocking current in the opposite direction (the reverse direction)  SONY VAIO VGN-FZ31J Battery .

Thus, the diode can be thought of as an electronic version of a check valve. This unidirectional behavior is calledrectification, and is used to convert alternating current to direct current, and to extract modulation from radio signals in radio receivers SONY VAIO VGN-FZ31M Battery .

However, diodes can have more complicated behavior than this simple on-off action. This is due to their complex non-linear electrical characteristics, which can be tailored by varying the construction of their P-N junction. These are exploited in special purpose diodes that perform many different functions  SONY VAIO VGN-FZ31Z Battery .

For example, specialized diodes are used to regulate voltage (Zener diodes), to electronically tune radio and TV receivers (varactor diodes), to generate radio frequencyoscillations (tunnel diodes), and to produce light (light emitting diodes). Tunnel diodes exhibit negative resistance, which makes them useful in some types of circuits  SONY VAIO VGN-FZ38M Battery .

Diodes were the first semiconductor electronic devices. The discovery of crystals' rectifying abilities was made by German physicist Ferdinand Braun in 1874. The first semiconductor diodes, called cat's whisker diodes, developed around 1906, were made of mineral crystals such as galena  SONY VGP-BPS8 Battery .

Today most diodes are made of silicon, but other semiconductors such as germanium are sometimes used.

History

Although the crystal semiconductor diode was popular before the thermionic diode, thermionic and solid state diodes were developed in parallel  SONY VGP-BPS13/S Battery .

In 1873 Frederick Guthrie discovered the basic principle of operation of thermionic diodes. Guthrie discovered that a positively charged electroscope could be discharged by bringing a grounded piece of white-hot metal close to it (but not actually touching it)SONY VGP-BPS13A/B Battery .

The same did not apply to a negatively charged electroscope, indicating that the current flow was only possible in one direction.

Thomas Edison independently rediscovered the principle on February 13, 1880. At the time, Edison was investigating why the filaments of his carbon-filament light bulbs nearly always burned out at the positive-connected end SONY VGP-BPS13B/B Battery .

He had a special bulb made with a metal plate sealed into the glass envelope. Using this device, he confirmed that an invisible current flowed from the glowing filament through the vacuum to the metal plate, but only when the plate was connected to the positive supply  SONY VGP-BPS13A/S Battery .

Edison devised a circuit where his modified light bulb effectively replaced the resistor in a DC voltmeter. Edison was awarded a patent for this invention in 1884.There was no apparent practical use for such a device at the time. So, the patent application was most likely simply a precaution in case someone else did find a use for the so-called Edison effect SONY VGP-BPS13AS Battery .

About 20 years later, John Ambrose Fleming (scientific adviser to the Marconi Company and former Edison employee) realized that the Edison effect could be used as a precision radio detector. Fleming patented the first true thermionic diode in Britain on November 16, 1904  (followed by U.S. Patent 803,684 in November 1905)  Dell Inspiron 1320n Battery .

In 1874 German scientist Karl Ferdinand Braun discovered the "unilateral conduction" of crystals. Braun patented the crystal rectifier in 1899. Copper oxideand selenium rectifiers were developed for power applications in the 1930s   Dell Studio 1450 Battery .

Indian scientist Jagadish Chandra Bose was the first to use a crystal for detecting radio waves in 1894. The crystal detector was developed into a practical device for wireless radio reception by Greenleaf Whittier Pickard, who invented a silicon crystal detector in 1903 and received a patent for it on November 20, 1906  Dell Studio 1457 Battery .

Other experimenters tried a variety of other substances, of which the most widely used was the mineral galena (lead sulfide). Other substances offered slightly better performance, but galena was most widely used because it had the advantage of being cheap and easy to obtain   Dell Latitude D610 Battery .

The crystal detector in these early radio sets consisted of an adjustable wire point-contact (the so-called "cat's whisker") which could be manually moved over the face of the crystal in order to obtain optimum signal. This troublesome device was quickly superseded by thermionic diodes  Toshiba NB100 Battery ,

but the crystal detector later returned to dominant use with the advent of inexpensive fixed-germanium diodes in the 1950s.

At the time of their invention, such devices were known as rectifiers. In 1919, William Henry Eccles coined the term diode from the Greek roots dia, meaning “through”, and ode (from ????), meaning “path”  Toshiba Satellite M65 battery .

Thermionic and gaseous state diodes

Thermionic diodes are thermionic-valve devices (also known as vacuum tubes, tubes, or valves), which are arrangements of electrodes surrounded by a vacuum within a glass envelope. Early examples were fairly similar in appearance to incandescent light bulbs  Toshiba Satellite M60 battery .

In thermionic valve diodes, a current through the heater filament indirectly heats the cathode, another internal electrode treated with a mixture of barium and strontiumoxides, which are oxides of alkaline earth metals; these substances are chosen because they have a small work function  Dell Latitude D830 Battery .

(Some valves use direct heating, in which a tungsten filament acts as bo

th heater and cathode.) The heat causes thermionic emission of electrons into the vacuum. In forward operation, a surrounding metal electrode called the anode is positively charged so that it electrostatically attracts the emitted electrons   Dell Latitude D620 Battery .

However, electrons are not easily released from the unheated anode surface when the voltage polarity is reversed. Hence, any reverse flow is negligible.

For much of the 20th century, thermionic valve diodes were used in analog signal applications, and as rectifiers in many power supplies  Dell Inspiron Mini 10 Battery .

Today, valve diodes are only used in niche applications such as rectifiers in electric guitar and high-end audio amplifiers as well as specialized high-voltage equipment   Sony VGN-FW11S Battery .

Semiconductor diodes

A modern semiconductor diode is made of a crystal of semiconductor like silicon that has impurities added to it to create a region on one side that contains negativecharge carriers (electrons), called n-type semiconductor, and a region on the other side that contains positive charge carriers (holes), called p-type semiconductor  Sony VGN-FW11M Battery .

The diode's terminals are attached to each of these regions. The boundary within the crystal between these two regions, called a PN junction, is where the action of the diode takes place. The crystal conducts a current of electrons in a direction from the N-type side (called the cathode) to the P-type side (called the anode)  Sony VGN-FW139E/H battery ,

but not in the opposite direction; that is, a conventional current flows from anode to cathode (opposite to the electron flow, since electrons have negative charge).

Another type of semiconductor diode, the Schottky diode, is formed from the contact between a metal and a semiconductor rather than by a p-n junction  Dell Latitude E5400 Battery .

Current–voltage characteristic

A semiconductor diode’s behavior in a circuit is given by its current–voltage characteristic, or I–V graph (see graph below). The shape of the curve is determined by the transport of charge carriers through the so-called depletion layer or depletion region that exists at the p-n junction between differing semiconductors  Dell Latitude E4200 Battery .

When a p-n junction is first created, conduction band (mobile) electrons from the N-doped region diffuse into the P-doped region where there is a large population of holes (vacant places for electrons) with which the electrons “recombine”. When a mobile electron recombines with a hole, both hole and electron vanish  Dell Inspiron 300M Battery ,

leaving behind an immobile positively charged donor (dopant) on the N-side and negatively charged acceptor (dopant) on the P-side. The region around the p-n junction becomes depleted of charge carriers and thus behaves as an insulator  Dell Studio 1737 battery .

However, the width of the depletion region (called the depletion width) cannot grow without limit. For each electron-hole pair that recombines, a positively charged dopant ion is left behind in the N-doped region, and a negatively charged dopant ion is left behind in the P-doped region  Dell Inspiron E1505 battery .

As recombination proceeds more ions are created, an increasing electric field develops through the depletion zone which acts to slow and then finally stop recombination. At this point, there is a “built-in” potential across the depletion zone  Dell RM791 battery .

If an external voltage is placed across the diode with the same polarity as the built-in potential, the depletion zone continues to act as an insulator, preventing any significant electric current flow (unless electron/hole pairs are actively being created in the junction by, for instance, light. see photodiode)  Toshiba Satellite P10 Battery .

This is the reverse bias phenomenon. However, if the polarity of the external voltage opposes the built-in potential, recombination can once again proceed, resulting in substantial electric current through the p-n junction (i.e. substantial numbers of electrons and holes recombine at the junction)  Acer Aspire One battery 10400mAh .

For silicon diodes, the built-in potential is approximately 0.7 V (0.3 V for Germanium and 0.2 V for Schottky). Thus, if an external current is passed through the diode, about 0.7 V will be developed across the diode such that the P-doped region is positive with respect to the N-doped region and the diode is said to be “turned on” as it has a forward bias SONY VGN-FZ210CE Battery .

A diode’s 'I–V characteristic' can be approximated by four regions of operation.

At very large reverse bias , beyond the peak inverse voltage or PIV, a process called reverse breakdown occurs which causes a large increase in current (i.e. a large number of electrons and holes are created at, and move away from the pn junction) that usually damages the device permanently Dell Precision M70 Battery .

The avalanche diode is deliberately designed for use in the avalanche region. In the zener diode, the concept of PIV is not applicable. A zener diode contains a heavily doped p-n junction allowing electrons to tunnel from the valence band of the p-type material to the conduction band of the n-type material Toshiba Satellite L305 Battery ,

such that the reverse voltage is “clamped” to a known value (called the zener voltage), and avalanche does not occur. Both devices, however, do have a limit to the maximum current and power in the clamped reverse voltage region. Also, following the end of forward conduction in any diode, there is reverse current for a short time Toshiba Satellite T4900 Battery .

The device does not attain its full blocking capability until the reverse current ceases .

The second region, at reverse biases more positive than the PIV, has only a very small reverse saturation current. In the reverse bias region for a normal P-N rectifier diode, the current through the device is very low (in the µA range) Toshiba PA3399U-2BRS battery .

However, this is temperature dependent, and at sufficiently high temperatures, a substantial amount of reverse current can be observed (mA or more).

The third region is forward but small bias, where only a small forward current is conducted Sony VGP-BPS10 battery 10400mAh .

As the potential difference is increased above an arbitrarily defined “cut-in voltage” or “on-voltage” or “diode forward voltage drop (Vd)”, the diode current becomes appreciable (the level of current considered “appreciable” and the value of cut-in voltage depends on the application), and the diode presents a very low resistance Sony VGP-BPS9 battery 10400mAh .

The current–voltage curve is exponential. In a normal silicon diode at rated currents, the arbitrary “cut-in” voltage is defined as 0.6 to 0.7 volts. The value is different for other diode types — Schottky diodes can be rated as low as 0.2 V, Germanium diodes 0.25 to 0.3 V, and red or blue light-emitting diodes (LEDs) can have values of 1.4 V and 4.0 V respectively Sony VGP-BPS11 battery 10400mAh .

At higher currents the forward voltage drop of the diode increases. A drop of 1 V to 1.5 V is typical at full rated current for power diodes.

Shockley diode equation

The Shockley ideal diode equation or the diode law (named after transistor co-inventor William Bradford Shockley Sony VGP-BPS22 battery 10400mAh ,

not to be confused with tetrode inventor Walter H. Schottky) gives the I–V characteristic of an ideal diode in either forward or reverse bias (or no bias). The equation is:

I=I_\mathrm{S} \left( e^{V_\mathrm{D}/(n V_\mathrm{T})}-1 \right),\,

where

I is the diode current, Sony VGP-BPL8 battery  10400mAh
IS is the reverse bias saturation current (or scale current),
VD is the voltage across the diode,
VT is the thermal voltage, and  Sony VGP-BPS14 battery 10400mAh
n is the ideality factor, also known as the quality factor or sometimes emission coefficient. The ideality factor n varies from 1 to 2 depending on the fabrication process and semiconductor material and in many cases is assumed to be approximately equal to 1 (thus the notation n is omitted) Sony VGP-BPS21 battery   10400mAh .

The thermal voltage VT is approximately 25.85 mV at 300 K, a temperature close to “room temperature” commonly used in device simulation software. At any temperature it is a known constant defined by:

V_\mathrm{T} = \frac{k T}{q} \, , Toshiba Satellite A200 Battery

where k is the Boltzmann constant, T is the absolute temperature of the p-n junction, and q is the magnitude of charge on an electron (the elementary charge).

The Shockley ideal diode equation or the diode law is derived with the assumption that the only processes giving rise to the current in the diode are drift (due to electrical field)  Toshiba Satellite 1200 Battery ,

diffusion, and thermal recombination-generation. It also assumes that the recombination-generation (R-G) current in the depletion region is insignificant. This means that the Shockley equation doesn’t account for the processes involved in reverse breakdown and photon-assisted R-G  Toshiba Satellite M300 Battery .

Additionally, it doesn’t describe the “leveling off” of the I–V curve at high forward bias due to internal resistance.

Under reverse bias voltages (see Figure 5) the exponential in the diode equation is negligible, and the current is a constant (negative) reverse current value of ?IS. The reverse breakdown region is not modeled by the Shockley diode equation SONY VGP-BPS13A/Q Battery .

For even rather small forward bias voltages (see Figure 5) the exponential is very large because the thermal voltage is very small, so the subtracted ‘1’ in the diode equation is negligible and the forward diode current is often approximated as

I=I_\mathrm{S} e^{V_\mathrm{D}/(n V_\mathrm{T})}

The use of the diode equation in circuit problems is illustrated in the article on diode modeling SONY VGP-BPS13A/Q Battery .

Small-signal behavior

For circuit design, a small-signal model of the diode behavior often proves useful. A specific example of diode modeling is discussed in the article on small-signal circuits SONY VGP-BPS13B/Q Battery .

Reverse-recovery effect

Following the end of forward conduction in a PN type diode, a reverse current flows for a short time. The device does not attain its full blocking capability until the reverse current ceases   SONY VGP-BPS13B/Q Battery .

The effect can be significant when switching large currents very quickly (di/dt on the order of 100 A/µs or more). A certain amount of "reverse recovery time" tr (on the order of tens of nanoseconds) may be required to remove the "reverse recovery charge" Qr (on the order of tens of nanocoulombs) from the diode  SONY VGP-BPS13/Q Battery .

During this recovery time, the diode can actually conduct in the reverse direction. In certain real-world cases it can be important to consider the losses incurred by this non-ideal diode effect.  However, when the slew rate of the current is not so severe (di/dt on the order of 10 A/µs or less), the effect can be safely ignored  SONY VGP-BPS13/Q Battery .

For most applications, the effect is also negligible for Schottky diodes.

The reverse current ceases abruptly when the stored charge is depleted, which is exploited in step recovery diodes for generation of extremely short pulses  SONY VGP-BPS21A/B Battery .

Types of semiconductor diode

There are several types of junction diodes, which either emphasize a different physical aspect of a diode often by geometric scaling, doping level, choosing the right electrodes, are just an application of a diode in a special circuit, or are really different devices like the Gunn and laser diode and the MOSFET  SONY VGP-BPS21/S Battery :

Normal (p-n) diodes, which operate as described above, are usually made of doped silicon or, more rarely, germanium. Before the development of modern silicon power rectifier diodes, cuprous oxide and later selenium was used; its low efficiency gave it a much higher forward voltage drop SONY VGP-BPS21B Battery

(typically 1.4 to 1.7 V per “cell”, with multiple cells stacked to increase the peak inverse voltage rating in high voltage rectifiers), and required a large heat sink (often an extension of the diode’s metal substrate), much larger than a silicon diode of the same current ratings would require  SONY VGP-BPS21A Battery .

The vast majority of all diodes are the p-n diodes found in CMOS integrated circuits, which include two diodes per pin and many other internal diodes.

Avalanche diodes   SONY VGP-BPS21 Battery

Diodes that conduct in the reverse direction when the reverse bias voltage exceeds the breakdown voltage. These are electrically very similar to Zener diodes, and are often mistakenly called Zener diodes, but break down by a different mechanism, the avalanche effect  Sony VGP-BPS21A/B Battery .
This occurs when the reverse electric field across the p-n junction causes a wave of ionization, reminiscent of an avalanche, leading to a large current. Avalanche diodes are designed to break down at a well-defined reverse voltage without being destroyed   Sony VGP-BPS21/S Battery .
The difference between the avalanche diode (which has a reverse breakdown above about 6.2 V) and the Zener is that the channel length of the former exceeds the “mean free path” of the electrons, so there are collisions between them on the way out  Sony VGP-BPS21A Battery .
The only practical difference is that the two types have temperature coefficients of opposite polarities.

Cat’s whisker or crystal diodes

These are a type of point-contact diode     Sony VGP-BPS21 Battery .
The cat’s whisker diode consists of a thin or sharpened metal wire pressed against a semiconducting crystal, typically galena or a piece of coal.  The wire forms the anode and the crystal forms the cathode. Cat’s whisker diodes were also called crystal diodes and found application in crystal radio receivers  Sony Vaio PCG-5G2L Battery .
Cat’s whisker diodes are generally obsolete, but may be available from a few manufacturers.

Constant current diodes

These are actually a JFET  with the gate shorted to the source, and function like a two-terminal current-limiter analog to the Zener diode, which is limiting voltage    Sony Vaio PCG-5G3L Battery .
They allow a current through them to rise to a certain value, and then level off at a specific value. Also called CLDs, constant-current diodes, diode-connected transistors, or current-regulating diodes.

Esaki or tunnel diodes

These have a region of operation showing negative resistance caused by quantum tunneling, Sony Vaio PCG-5J1L Battery
allowing amplification of signals and very simple bistable circuits. Due to the high carrier concentration, tunnel diodes are very fast, may be used at low (mK) temperatures, high magnetic fields, and in high radiation environments. Because of these properties, they are often used in spacecraft   Sony Vaio PCG-5K2L Battery .

Gunn diodes

These are similar to tunnel diodes in that they are made of materials such as GaAs or InP that exhibit a region of negative differential resistance. With appropriate biasing, dipole domains form and travel across the diode, allowing high frequency microwave oscillators to be built  Sony Vaio PCG-5J2L Battery .

Light-emitting diodes (LEDs)

In a diode formed from a direct band-gap semiconductor, such as gallium arsenide, carriers that cross the junction emit photons when they recombine with the majority carrier on the other side  Sony Vaio PCG-5K1L Battery .
Depending on the material, wavelengths (or colors)from the infrared to the near ultraviolet may be produced. The forward potential of these diodes depends on the wavelength of the emitted photons: 1.2 V corresponds to red, 2.4 V to violet  Sony Vaio PCG-6S2L Battery .
The first LEDs were red and yellow, and higher-frequency diodes have been developed over time. All LEDs produce incoherent, narrow-spectrum light; “white” LEDs are actually combinations of three LEDs of a different color, or a blue LED with a yellow scintillator coating  Sony Vaio PCG-6S3L Battery .
LEDs can also be used as low-efficiency photodiodes in signal applications. An LED may be paired with a photodiode or phototransistor in the same package, to form an opto-isolator Sony Vaio PCG-6V1L Battery .

Laser diodes

When an LED-like structure is contained in a resonant cavity formed by polishing the parallel end faces, a laser can be formed. Laser diodes are commonly used in optical storage devices and for high speed optical communication  Sony Vaio PCG-6W1L Battery .

Thermal diodes

This term is used both for conventional PN diodes used to monitor temperature due to their varying forward voltage with temperature, and for Peltier heat pumps for thermoelectric heating and cooling.. Peltier heat pumps may be made from semiconductor  Sony Vaio PCG-6W2L Battery ,
though they do not have any rectifying junctions, they use the differing behaviour of charge carriers in N and P type semiconductor to move heat.

Photodiodes

All semiconductors are subject to optical charge carrier generation  Sony Vaio PCG-6W3L Battery .
This is typically an undesired effect, so most semiconductors are packaged in light blocking material. Photodiodes are intended to sense light(photodetector), so they are packaged in materials that allow light to pass, and are usually PIN (the kind of diode most sensitive to light) Sony Vaio PCG-7111L Battery .
photodiode can be used in solar cells, in photometry, or in optical communications. Multiple photodiodes may be packaged in a single device, either as a linear array or as a two-dimensional array. These arrays should not be confused with charge-coupled devices  Sony Vaio PCG-7112L Battery .

Point-contact diodes

These work the same as the junction semiconductor diodes described above, but their construction is simpler. A block of n-type semiconductor is built, and a conducting sharp-point contact made with some group-3 metal is placed in contact with the semiconductor  Sony Vaio PCG-7113L Battery .
Some metal migrates into the semiconductor to make a small region of p-type semiconductor near the contact. The long-popular 1N34 germanium version is still used in radio receivers as a detector and occasionally in specialized analog electronics   Sony Vaio PCG-7133L Battery .

PIN diodes

A PIN diode has a central un-doped, or intrinsic, layer, forming a p-type/intrinsic/n-type structure.  They are used as radio frequency switches and attenuators. They are also used as large volume ionizing radiation detectors and as photodetectorsSony Vaio PCG-7Z2L Battery .
PIN diodes are also used in power electronics, as their central layer can withstand high voltages. Furthermore, the PIN structure can be found in many power semiconductor devices, such as IGBTs, power MOSFETs, and thyristors  Sony Vaio PCG-8Y1L Battery .

Schottky diodes

Schottky diodes are constructed from a metal to semiconductor contact. They have a lower forward voltage drop than p-n junction diodes. Their forward voltage drop at forward currents of about 1 mA is in the range 0.15 V to 0.45 V, which makes them useful in voltage clamping applications and prevention of transistor saturation Sony Vaio PCG-8Y2L Battery .
They can also be used as low loss rectifiers although their reverse leakage current is generally higher than that of other diodes. Schottky diodes are majority carrier devices and so do not suffer from minority carrier storage problems that slow down many other diodes — so they have a faster “reverse recovery” than p-n junction diodes  Sony Vaio PCG-8Z1L Battery .
They also tend to have much lower junction capacitance than p-n diodes which provides for high switching speeds and their use in high-speed circuitry and RF devices such as switched-mode power supply, mixers and detectors Sony Vaio PCG-8Z2L Battery .

Super barrier diodes

Super barrier diodes are rectifier diodes that incorporate the low forward voltage drop of the Schottky diode with the surge-handling capability and low reverse leakage current of a normal p-n junction diode  SONY VAIO VGN-FW31J Battery .

Gold-doped diodes

As a dopant, gold (or platinum) acts as recombination centers, which help a fast recombination of minority carriers. This allows the diode to operate at signal frequencies, at the expense of a higher forward voltage drop. Gold doped diodes are faster than other p-n diodes (but not as fast as Schottky diodes) SONY VAIO VGN-FW31E Battery .
They also have less reverse-current leakage than Schottky diodes (but not as good as other p-n diodes). A typical example is the 1N914.

Snap-off or Step recovery diodes

The term step recovery relates to the form of the reverse recovery characteristic of these devices SONY VAIO VGN-FW31B Battery .
After a forward current has been passing in an SRD and the current is interrupted or reversed, the reverse conduction will cease very abruptly (as in a step waveform). SRDs can therefore provide very fast voltage transitions by the very sudden disappearance of the charge carriers SONY VAIO VGN-FW31M Battery .

Transient voltage suppression diode (TVS)

These are avalanche diodes designed specifically to protect other semiconductor devices from high-voltage transients SONY VAIO VGN-FW31Z Battery .
Their p-n junctions have a much larger cross-sectional area than those of a normal diode, allowing them to conduct large currents to ground without sustaining damage.

Varicap or varactor diodes

These are used as voltage-controlled capacitors  SONY VAIO VGN-FW38M Battery .
These are important in PLL (phase-locked loop) and FLL (frequency-locked loop) circuits, allowing tuning circuits, such as those in television receivers, to lock quickly, replacing older designs that took a long time to warm up and lock. A PLL is faster than an FLL, but prone to integer harmonic locking (if one attempts to lock to a broadband signal) SONY VAIO VGN-AW41JF Battery .
They also enabled tunable oscillators in early discrete tuning of radios, where a cheap and stable, but fixed-frequency, crystal oscillator provided the reference frequency for avoltage-controlled oscillator.

Zener diodes

Diodes that can be made to conduct backwards  SONY VAIO VGN-AW41JF/H Battery .
This effect, called Zener breakdown, occurs at a precisely defined voltage, allowing the diode to be used as a precision voltage reference. In practical voltage reference circuits Zener and switching diodes are connected in series and opposite directions to balance the temperature coefficient to near zero SONY VAIO VGN-AW41MF Battery .
Some devices labeled as high-voltage Zener diodes are actually avalanche diodes (see above). Two (equivalent) Zeners in series and in reverse order, in the same package, constitute a transient absorber (or Transorb, a registered trademark) SONY VAIO VGN-AW41MF/H Battery .
The Zener diode is named for Dr. Clarence Melvin Zener of Carnegie Mellon University, inventor of the device.

Other uses for semiconductor diodes include sensing temperature, and computing analog logarithms (see Operational amplifier applications#Logarithmic)  SONY VAIO VGN-FW30B Battery .

Numbering and coding schemes

There are a number of common, standard and manufacturer-driven numbering and coding schemes for diodes; the two most common being the EIA/JEDEC standard and the European Pro Electron standard  SONY VAIO VGN-FW50B Battery . :

EIA/JEDEC

A standardized 1N-series numbering system was introduced in the US by EIA/JEDEC (Joint Electron Device Engineering Council) about 1960. Among the most popular in this series were: 1N34A/1N270 (Germanium signal), 1N914/1N4148 (Silicon signal), 1N4001-1N4007 (Silicon 1A power rectifier) and 1N54xx (Silicon 3A power rectifier)SONY VAIO VGN-FW51B/W Battery

Pro Electron

The European Pro Electron coding system for active components was introduced in 1966 and comprises two letters followed by the part code  SONY VAIO VGN-FW51MF/H Battery .

The first letter represents the semiconductor material used for the component (A = Germanium and B = Silicon) and the second letter represents the general function of the part (for diodes: A = low-power/signal, B = Variable capacitance, X = Multiplier, Y = Rectifier and Z = Voltage reference), for example  SONY VAIO VGN-FW51ZF Battery :

  • AA-series germanium low-power/signal diodes (e.g.: AA119)
  • BA-series silicon low-power/signal diodes (e.g.: BAT18 Silicon RF Switching Diode)
  • BY-series silicon rectifier diodes (e.g.: BY127 1250V, 1A rectifier diode) SONY VAIO VGN-FW51ZF/H Battery
  • BZ-series silicon zener diodes (e.g.: BZY88C4V7 4.7V zener diode)

Other common numbering / coding systems (generally manufacturer-driven) include:

  • GD-series germanium diodes (ed: GD9) — this is a very old coding system  SONY VAIO VGN-FW52JB Battery
  • OA-series germanium diodes (e.g.: OA47) — a coding sequence developed by Mullard, a UK company

As well as these common codes, many manufacturers or organisations have their own systems too — for example:

Related devices

In optics, an equivalent device for the diode but with laser light would be the Optical isolator, also known as an Optical Diode, that allows light to only pass in one direction. It uses a Faraday rotator as the main component  SONY VAIO VGN-FW71DB/W Battery .

Radio demodulation

The first use for the diode was the demodulation of amplitude modulated (AM) radio broadcasts. The history of this discovery is treated in depth in the radio article  SONY VAIO VGN-FW72JGB Battery .

In summary, an AM signal consists of alternating positive and negative peaks of voltage, whose amplitude or “envelope” is proportional to the original audio signal. The diode (originally a crystal diode) rectifies the AM radio frequency signal, leaving an audio signal which is the original audio signal, minus atmospheric noise  SONY VAIO VGN-FW73JGB Battery .

The audio is extracted using a simple filter and fed into an audio amplifier or transducer, which generates sound waves.

Power conversion

Rectifiers are constructed from diodes, where they are used to convert alternating current (AC) electricity into direct current (DC)SONY VAIO VGN-FW74FB Battery .

Automotive alternators are a common example, where the diode, which rectifies the AC into DC, provides better performance than the commutator of earlier dynamo. Similarly, diodes are also used in Cockcroft–Walton voltage multipliers to convert AC into higher DC voltages  SONY VAIO VGN-FW81HS Battery .

Over-voltage protection

Diodes are frequently used to conduct damaging high voltages away from sensitive electronic devices. They are usually reverse-biased (non-conducting) under normal circumstances. When the voltage rises above the normal range, the diodes become forward-biased (conducting)  SONY VAIO VGN-FW81NS Battery .

For example, diodes are used in (stepper motor and H-bridge) motor controller and relay circuits to de-energize coils rapidly without the damaging voltage spikes that would otherwise occur. (Any diode used in such an application is called a flyback diode)SONY VAIO VGN-FW81S Battery .

Many integrated circuits also incorporate diodes on the connection pins to prevent external voltages from damaging their sensitive transistors. Specialized diodes are used to protect from over-voltages at higher power (see Diode types above)  SONY VAIO VGN-FW82DS Battery .

Logic gates

Diodes can be combined with other components to construct AND and OR logic gates. This is referred to as diode logic.

Ionizing radiation detectors

In addition to light, mentioned above, semiconductor diodes are sensitive to more energetic radiation  SONY VAIO VGN-FW82JS Battery .

In electronics, cosmic rays and other sources of ionizing radiation cause noise pulses and single and multiple bit errors. This effect is sometimes exploited by particle detectors to detect radiation. A single particle of radiation, with thousands or millions of electron volts of energy, generates many charge carrier pairs, as its energy is deposited in the semiconductor material  SONY VAIO VGN-FW82XS Battery .

If the depletion layer is large enough to catch the whole shower or to stop a heavy particle, a fairly accurate measurement of the particle’s energy can be made, simply by measuring the charge conducted and without the complexity of a magnetic spectrometer or etc   SONY VAIO VGN-FW83DS Battery .

These semiconductor radiation detectors need efficient and uniform charge collection and low leakage current. They are often cooled by liquid nitrogen. For longer range (about a centimetre) particles they need a very large depletion depth and large area  SONY VAIO VGN-FW83JS Battery .

For short range particles, they need any contact or un-depleted semiconductor on at least one surface to be very thin. The back-bias voltages are near breakdown (around a thousand volts per centimetre). Germanium and silicon are common materials. Some of these detectors sense position as well as energy  SONY VAIO VGN-FW83XS Battery .

They have a finite life, especially when detecting heavy particles, because of radiation damage. Silicon and germanium are quite different in their ability to convert gamma rays to electron showers.

Semiconductor detectors for high energy particles are used in large numbers   SONY VAIO VGN-FW90NS Battery .

Because of energy loss fluctuations, accurate measurement of the energy deposited is of less use.

Temperature measurements

A diode can be used as a temperature measuring device, since the forward voltage drop across the diode depends on temperature, as in a Silicon bandgap temperature sensor. From the Shockley ideal diode equation given above, it appears the voltage has a positive temperature coefficient Lenovo S10-2 Battery 5200mAh

(at a constant current) but depends on doping concentration and operating temperature (Sze 2007). The temperature coefficient can be negative as in typical thermistors or positive for temperature sense diodes down to about 20 kelvins. Typically, silicon diodes have approximately ?2 mV/?C temperature coefficient at room temperature Lenovo Y730 Battery 5200mAh .

Current steering

Diodes will prevent currents in unintended directions. To supply power to an electrical circuit during a power failure, the circuit can draw current from a battery. An Uninterruptible power supply may use diodes in this way to ensure that current is only drawn from the battery when necessary  Lenovo Y710 Battery 5200mAh .

Similarly, small boats typically have two circuits each with their own battery/batteries: one used for engine starting; one used for domestics. Normally both are charged from a single alternator, and a heavy duty split charge diode is used to prevent the higherLenovo Y530 Battery 5200mAh

charge battery (typically the engine battery) from discharging through the lower charged battery when the alternator is not running.

Diodes are also used in electronic musical keyboards. To reduce the amount of wiring needed in electronic musical keyboards, these instruments often use keyboard matrix circuits Lenovo Y510 Battery 5200mAh .

The keyboard controller scans the rows and columns to determine which note the player has pressed. The problem with matrix circuits is that when several notes are pressed at once, the current can flow backwards through the circuit and trigger "phantom keys" that cause “ghost” notes to play Lenovo S10-3 Battery 5200mAh .

To avoid triggering unwanted notes, most keyboard matrix circuits have diodes soldered with the switch under each key of the musical keyboard. The same principle is also used for the switch matrix in solid state pinball machines Lenovo G450 Battery 5200mAh .

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