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76-422: TRIACs are a subset of thyristors (analogous to a relay in that a small voltage and current can control a much larger voltage and current) and are related to silicon controlled rectifiers (SCRs). TRIACs differ from SCRs in that they allow current flow in both directions, whereas an SCR can only conduct current in a single direction. Most TRIACs can be triggered by applying either a positive or negative voltage to

152-502: A TRIAC , is able to work in both directions. This added capability, though, also can become a shortfall. Because the TRIAC can conduct in both directions, reactive loads can cause it to fail to turn off during the zero-voltage instants of the AC power cycle. Because of this, use of TRIACs with (for example) heavily inductive motor loads usually requires the use of a " snubber " circuit around

228-423: A bistable switch (or a latch). There are two designs, differing in what triggers the conducting state. In a three-lead thyristor, a small current on its gate lead controls the larger current of the anode-to-cathode path. In a two-lead thyristor, conduction begins when the potential difference between the anode and cathode themselves is sufficiently large (breakdown voltage). The thyristor continues conducting until

304-454: A TRIAC in a circuit. In this section, a few are summarized. A TRIAC starts conducting when a current flowing into or out of its gate is sufficient to turn on the relevant junctions in the quadrant of operation. The minimum current able to do this is called gate threshold current and is generally indicated by I GT . In a typical TRIAC, the gate threshold current is generally a few milliamperes, but one has to take into account also that: When

380-484: A combination of Greek language θύρα , meaning "door" or "valve", and transistor ) is a solid-state semiconductor device which can be thought of as being a highly robust and switchable diode , allowing the passage of current in one direction but not the other, often under control of a gate electrode, that is used in high power applications like inverters and radar generators. It usually consists of four layers of alternating P- and N-type materials. It acts as

456-504: A critical part of flashes (strobes). Thyristors can be triggered by a high rise-rate of off-state voltage. Upon increasing the off-state voltage across the anode and cathode of the thyristor, there will be a flow of charges similar to the charging current of a capacitor. The maximum rate of rise of off-state voltage or dV/dt rating of a thyristor is an important parameter since it indicates the maximum rate of rise of anode voltage that does not bring thyristor into conduction when no gate signal

532-469: A failure in the power supply from damaging downstream components. A thyristor is used in conjunction with a Zener diode attached to its gate, and if the output voltage of the supply rises above the Zener voltage, the thyristor will conduct and short-circuit the power supply output to ground (in general also tripping an upstream breaker or fuse ). This kind of protection circuit is known as a crowbar , and has

608-433: A lower value of V AK . By selecting an appropriate value of V G , the thyristor can be switched into the on state quickly. Once avalanche breakdown has occurred, the thyristor continues to conduct, irrespective of the gate voltage, until: (a) the potential V AK is removed or (b) the current through the device (anode−cathode) becomes less than the holding current specified by the manufacturer. Hence V G can be

684-696: A relatively large amount of power and voltage with a small device, they find wide application in control of electric power, ranging from light dimmers and electric motor speed control to high-voltage direct-current power transmission. Thyristors may be used in power-switching circuits, relay-replacement circuits, inverter circuits, oscillator circuits, level-detector circuits, chopper circuits, light-dimming circuits, low-cost timer circuits, logic circuits, speed-control circuits, phase-control circuits, etc. Originally, thyristors relied only on current reversal to turn them off, making them difficult to apply for direct current; newer device types can be turned on and off through

760-449: A resistor built in to safeguard against spurious dv/dt triggering. This will mask the gate's supposed diode-type behaviour when testing a TRIAC with a multimeter . In datasheets, the static d v /d t is usually indicated as ( d ⁡ v d ⁡ t ) s {\displaystyle \left({\frac {\operatorname {d} v}{\operatorname {d} t}}\right)_{s}} and, as mentioned before,

836-465: A similar electronic switching capability, where a small control voltage could switch a large current. It is from a combination of "thyratron" and " transistor " that the term "thyristor" is derived. In recent years, some manufacturers have developed thyristors using silicon carbide (SiC) as the semiconductor material. These have applications in high temperature environments, being capable of operating at temperatures up to 350 °C. The thyristor

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912-459: A snubber circuit (usually of the resistor/capacitor or resistor/capacitor/inductor type) between MT1 and MT2. Snubber circuits are also used to prevent premature triggering, caused for example by voltage spikes in the mains supply. Because turn-ons are caused by internal capacitive currents flowing into the gate as a consequence of a high d v /d t , (i.e., rapid voltage change) a gate resistor or capacitor (or both in parallel) may be connected between

988-457: A thyristor can be understood in terms of a pair of tightly coupled bipolar junction transistors , arranged to cause a self-latching action. Thyristors have three states: The thyristor has three p-n junctions (serially named J 1 , J 2 , J 3 from the anode). When the anode is at a positive potential V AK with respect to the cathode with no voltage applied at the gate, junctions J 1 and J 3 are forward biased, while junction J 2

1064-814: A trigger at a controlled phase angle of the AC in the main circuit allows control of the average current flowing into a load ( phase control ). This is commonly used for controlling the speed of a universal motor , dimming lamps, and controlling electric heaters. TRIACs are Bipolar devices. To understand how TRIACs work, consider the triggering in each of the four possible combinations of gate and MT2 voltages with respect to MT1. The four separate cases (quadrants) are illustrated in Figure 1. Main Terminal 1 (MT1) and Main Terminal 2 (MT2) are also referred to as Anode 1 (A1) and Anode 2 (A2) respectively. The relative sensitivity depends on

1140-511: A triggering in quadrant III, this lowers the potential of the n-layer and turns on the PNP transistor formed by the n-layer and the two p-layers next to it. The lower p-layer works as the collector of this PNP transistor and has its voltage heightened: this p-layer also acts as the base of an NPN transistor made up by the last three layers just over the MT2 terminal, which, in turn, gets activated. Therefore,

1216-501: A voltage drop between the gate region and the MT1 region which may make the TRIAC stay turned on. In a datasheet, the commutating d i /d t is usually indicated as ( d ⁡ i d ⁡ t ) c {\displaystyle \left({\frac {\operatorname {d} i}{\operatorname {d} t}}\right)_{c}} and is generally in the order of some amperes per microsecond. The commutating d v /d t

1292-429: A voltage pulse, such as the voltage output from a UJT relaxation oscillator . The gate pulses are characterized in terms of gate trigger voltage ( V GT ) and gate trigger current ( I GT ). Gate trigger current varies inversely with gate pulse width in such a way that it is evident that there is a minimum gate charge required to trigger the thyristor. In a conventional thyristor, once it has been switched on by

1368-423: Is a four-layered, three-terminal semiconductor device, with each layer consisting of alternating N-type or P-type material, for example P-N-P-N. The main terminals, labelled anode and cathode, are across all four layers. The control terminal, called the gate, is attached to p-type material near the cathode. (A variant called an SCS—silicon controlled switch—brings all four layers out to terminals.) The operation of

1444-474: Is absent, if the power is not removed and the polarities of the cathode and anode have not yet reversed, the LASCR is still in the "on" state. A light-activated TRIAC resembles a LASCR, except that it is designed for alternating currents. Thyristor manufacturers generally specify a region of safe firing defining acceptable levels of voltage and current for a given operating temperature . The boundary of this region

1520-403: Is active, but the N region attached to MT1 only participates in the initial triggering, not the bulk current flow. In most applications, the gate current comes from MT2, so quadrants 1 and 3 are the only operating modes (both gate and MT2 positive or negative against MT1). Other applications with single polarity triggering from an IC or digital drive circuit operate in quadrants 2 and 3, where MT1

1596-401: Is always greater than holding current. In the above figure I L has to come above the I H on y-axis since I L > I H . A thyristor can be switched off if the external circuit causes the anode to become negatively biased (a method known as natural, or line, commutation). In some applications this is done by switching a second thyristor to discharge a capacitor into the anode of

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1672-408: Is applied. When the flow of charges due to rate of rise of off-state voltage across the anode and cathode of the thyristor becomes equal to the flow of charges as injected when the gate is energized then it leads to random and false triggering of thyristor which is undesired. This is prevented by connecting a resistor - capacitor (RC) snubber circuit between the anode and cathode in order to limit

1748-508: Is cooled with deionized water , and the entire arrangement becomes one of multiple identical modules forming a layer in a multilayer valve stack called a quadruple valve . Three such stacks are typically mounted on the floor or hung from the ceiling of the valve hall of a long-distance transmission facility. The functional drawback of a thyristor is that, like a diode, it only conducts in one direction so it cannot be safely used with AC current . A similar self-latching 5-layer device, called

1824-501: Is coupled by an optical fiber . Since no electronic boards need to be provided at the potential of the thyristor in order to trigger it, light-triggered thyristors can be an advantage in high-voltage applications such as HVDC . Light-triggered thyristors are available with in-built over-voltage (VBO) protection, which triggers the thyristor when the forward voltage across it becomes too high; they have also been made with in-built forward recovery protection , but not commercially. Despite

1900-438: Is due to a parasitic capacitive coupling of the gate terminal with the MT2 terminal, which lets currents into the gate in response to a large rate of voltage change at MT2. One way to cope with this limitation is to design a suitable RC or RCL snubber network. In many cases this is sufficient to lower the impedance of the gate towards MT1. By putting a resistor or a small capacitor (or both in parallel) between these two terminals,

1976-437: Is easier for the capacitive current due to d v /d t to turn on the device again. Another important factor during a commutation from on-state to off-state is the d i /d t of the current from MT1 to MT2. This is similar to the recovery in standard diodes: the higher the d i /d t , the greater the reverse current. Because in the TRIAC there are parasitic resistances, a high reverse current in the p-n junctions inside it can provoke

2052-406: Is generally in the order of up to some volts per microsecond. The reason why commutating dv/dt is less than static dv/dt is that, shortly before the device tries to turn off, there is still some excess minority charge in its internal layers as a result of the previous conduction. When the TRIAC starts to turn off, these charges alter the internal potential of the region near the gate and MT1, so it

2128-405: Is in relation to the tendency of a TRIAC to turn on from the off state after a large voltage rate of rise even without applying any current in the gate. A high rate of rise of the current between MT1 and MT2 (in either direction) when the device is turning on can damage or destroy the TRIAC even if the pulse duration is very short. The reason is that during the commutation, the power dissipation

2204-559: Is lost through the ohmic path across the p-silicon, flowing directly into MT1 without passing through the NPN transistor base. In this case, the injection of holes in the p-silicon makes the stacked n, p and n layers beneath MT1 behave like a NPN transistor, which turns on due to the presence of a current in its base. This, in turn, causes the p, n and p layers over MT2 to behave like a PNP transistor, which turns on because its n-type base becomes forward-biased with respect to its emitter (MT2). Thus,

2280-471: Is more complex than triggering a TRIAC. TRIACs may also fail to turn on reliably with reactive loads if the current phase shift causes the main circuit current to be below the holding current at trigger time. To overcome the problem DC or a pulse train may be used to repeatedly trigger the TRIAC until it turns on. Low-power TRIACs are used in many applications such as light dimmers , speed controls for electric fans and other electric motors , and in

2356-422: Is not to be confused with asymmetrical operation, as the output is unidirectional, flowing only from cathode to anode, and so is asymmetrical in nature. Thyristors can be used as the control elements for phase angle triggered controllers, also known as phase fired controllers . They can also be found in power supplies for digital circuits , where they are used as a sort of "enhanced circuit breaker " to prevent

TRIAC - Misplaced Pages Continue

2432-453: Is not uniformly distributed across the device. When switching on, the device starts to conduct current before the conduction finishes to spread across the entire junction. The device typically starts to conduct the current imposed by the external circuitry after some nanoseconds or microseconds but the complete switch on of the whole junction takes a much longer time, so too swift a current rise may cause local hot spots that can permanently damage

2508-444: Is partly determined by the requirement that the maximum permissible gate power (P G ), specified for a given trigger pulse duration, is not exceeded. As well as the usual failure modes due to exceeding voltage, current or power ratings, thyristors have their own particular modes of failure, including: Thyristors are mainly used where high currents and voltages are involved, and are often used to control alternating currents , where

2584-408: Is reverse biased. As J 2 is reverse biased, no conduction takes place (Off state). Now if V AK is increased beyond the breakdown voltage V BO of the thyristor, avalanche breakdown of J 2 takes place and the thyristor starts conducting (On state). If a positive potential V G is applied at the gate terminal with respect to the cathode, the breakdown of the junction J 2 occurs at

2660-406: Is similar to triggering in quadrant III. The process uses a remote gate control and is illustrated in Figure 7. As current flows from the p-layer under the gate into the n-layer under MT1, minority carriers in the form of free electrons are injected into the p-region and some of them are collected by the underlying n-p junction and pass into the adjoining n-region without recombining. As in the case of

2736-407: Is that they are not fully controllable switches. The GTO thyristor and IGCT are two devices related to the thyristor that address this problem. In high-frequency applications, thyristors are poor candidates due to long switching times arising from bipolar conduction. MOSFETs, on the other hand, have much faster switching capability because of their unipolar conduction (only majority carriers carry

2812-437: Is the reason why a TRIAC needs more gate current to turn on than a comparably rated SCR. Generally, this quadrant is the most sensitive of the four. This is because it is the only quadrant where gate current is injected directly into the base of one of the main device transistors. Quadrant 2 operation occurs when the gate is negative and MT2 is positive with respect to MT1. Figure 5 shows the triggering process. The turn-on of

2888-418: Is usually connected to positive voltage (e.g. +5V) and gate is pulled down to 0V (ground). Quadrant 1 operation occurs when the gate and MT2 are positive with respect to MT1. The mechanism is illustrated in Figure 3. The gate current makes an equivalent NPN transistor switch on, which in turn draws current from the base of an equivalent PNP transistor, turning it on also. Part of the gate current (dotted line)

2964-529: Is very important when the TRIAC is used to drive a load with a phase shift between current and voltage, such as an inductive load. Suppose one wants to turn the inductor off: when the current goes to zero, if the gate is not fed, the TRIAC attempts to turn off, but this causes a step in the voltage across it due to the aforementioned phase shift. If the commutating d v /d t rating is exceeded, the device will not turn off. When used to control reactive ( inductive or capacitive) loads, care must be taken to ensure that

3040-607: The SCR and diode never conduct at the same time they do not produce heat simultaneously and can easily be integrated and cooled together. Reverse conducting thyristors are often used in frequency changers and inverters . Photothyristors are activated by light. The advantage of photothyristors is their insensitivity to electrical signals, which can cause faulty operation in electrically noisy environments. A light-triggered thyristor (LTT) has an optically sensitive region in its gate, into which electromagnetic radiation (usually infrared )

3116-413: The TRIAC has completed the triggering process when the gate signal is discontinued and the latching current reaches a minimum level called holding current . Holding current is the minimum required current flowing between the two main terminals that keeps the device on after it has achieved commutation in every part of its internal structure. In datasheets, the latching current is indicated as I L , while

TRIAC - Misplaced Pages Continue

3192-804: The TRIAC to assure that it will turn off with each half-cycle of mains power. Inverse parallel SCRs can also be used in place of the triac; because each SCR in the pair has an entire half-cycle of reverse polarity applied to it, the SCRs, unlike TRIACs, are sure to turn off. The "price" to be paid for this arrangement, however, is the added complexity of two separate, but essentially identical gating circuits. Although thyristors are heavily used in megawatt-scale rectification of AC to DC, in low- and medium-power (from few tens of watts to few tens of kilowatts) applications they have virtually been replaced by other devices with superior switching characteristics like power MOSFETs or IGBTs . One major problem associated with SCRs

3268-479: The TRIAC turns off correctly at the end of each half-cycle of the AC in the main circuit. TRIACs can be sensitive to fast voltage changes (dv/dt) between MT1 and MT2, so a phase shift between current and voltage caused by reactive loads can lead to a voltage step that can turn the thyristor on erroneously. An electric motor is typically an inductive load and off-line power supplies—as used in most TVs and computers—are capacitive. Unwanted turn-ons can be avoided by using

3344-502: The TRIAC's gate to trigger it. This ensures that the TRIAC is triggered in quadrants II and III and avoids quadrant IV where TRIACs are typically insensitive. Three-quadrant TRIACs only operate in quadrants 1 through 3 and cannot be triggered in quadrant 4. These devices are made specifically for improved commutation and can often control reactive loads without the use of a snubber circuit. The first TRIACs of this type were marketed by Thomson Semiconductors (now ST Microelectronics ) under

3420-491: The TRIAC. In datasheets, this parameter is usually indicated as d ⁡ i d ⁡ t {\displaystyle {\frac {\operatorname {d} i}{\operatorname {d} t}}} and is typically in the order of the tens of ampere per microsecond. The commutating d v /d t rating applies when a TRIAC has been conducting and attempts to turn off with a partially reactive load, such as an inductor. The current and voltage are out of phase, so when

3496-444: The advantage over a standard circuit breaker or fuse in that it creates a high-conductance path to ground from damaging supply voltage and potentially for stored energy (in the system being powered). The first large-scale application of thyristors, with associated triggering diac , in consumer products related to stabilized power supplies within color television receivers in the early 1970s. The stabilized high voltage DC supply for

3572-471: The capacitive current generated during the transient flows out of the device without activating it. A careful reading of the application notes provided by the manufacturer and testing of the particular device model to design the correct network is in order. Typical values for capacitors and resistors between the gate and MT1 may be up to 100 nF and 10 Ω to 1 kΩ. Normal TRIACs, except for low-power types marketed as sensitive gate , already have such

3648-399: The change of polarity of the current causes the device to switch off automatically, referred to as " zero cross " operation. The device can be said to operate synchronously ; being that, once the device is triggered, it conducts current in phase with the voltage applied over its cathode to anode junction with no further gate modulation being required, i.e., the device is biased fully on . This

3724-449: The collector of this PNP transistor and has its voltage heightened: this p-layer also acts as the base of an NPN transistor made up by the last three layers just over the MT2 terminal, which, in turn, gets activated. Therefore, the red arrow labeled with a "3" in Figure 6 shows the final conduction path of the current. Quadrant 4 operation occurs when the gate is positive and MT2 is negative with respect to MT1. Triggering in this quadrant

3800-507: The control gate signal. The latter is known as a gate turn-off thyristor , or GTO thyristor. Unlike transistors , thyristors have a two-valued switching characteristic, meaning that a thyristor can only be fully on or off, while a transistor can lie in between on and off states. This makes a thyristor unsuitable as an analog amplifier, but useful as a switch. The silicon controlled rectifier (SCR) or thyristor proposed by William Shockley in 1950 and championed by Moll and others at Bell Labs

3876-493: The current decreases below the holding value, the TRIAC attempts to turn off, but because of the phase shift between current and voltage, a sudden voltage step takes place between the two main terminals, which turns the device on again. In datasheets, this parameter is usually indicated as ( d ⁡ v d ⁡ t ) c {\displaystyle \left({\frac {\operatorname {d} v}{\operatorname {d} t}}\right)_{c}} and

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3952-422: The dV/dt (i.e., rate of voltage change over time). Snubbers are energy-absorbing circuits used to suppress the voltage spikes caused by the circuit's inductance when a switch, electrical or mechanical, opens. The most common snubber circuit is a capacitor and resistor connected in series across the switch (transistor). Since modern thyristors can switch power on the scale of megawatts , thyristor valves have become

4028-424: The device is three-fold and starts when the current from MT1 flows into the gate through the p-n junction under the gate. This switches on a structure composed by an NPN transistor and a PNP transistor, which has the gate as cathode (the turn-on of this structure is indicated by "1" in the figure). As current into the gate increases, the potential of the left side of the p-silicon under the gate rises towards MT1, since

4104-405: The difference in potential between the gate and MT2 tends to lower: this establishes a current between the left side and the right side of the p-silicon (indicated by "2" in the figure), which in turn switches on the NPN transistor under the MT1 terminal and as a consequence also the pnp transistor between MT2 and the right side of the upper p-silicon. So, in the end, the structure which is crossed by

4180-403: The fine-tuning should be done on the particular device model. For higher-powered, more-demanding loads, two SCRs in inverse parallel may be used instead of one TRIAC. Because each SCR will have an entire half-cycle of reverse polarity voltage applied to it, turn-off of the SCRs is assured, no matter what the character of the load. However, due to the separate gates, proper triggering of the SCRs

4256-425: The first thyristor. This method is called forced commutation. Once the current through the thyristor drops below the holding current, there must be a delay before the anode can be positively biased and retain the thyristor in the off-state. This minimum delay is called the circuit commutated turn off time ( t Q ). Attempting to positively bias the anode within this time causes the thyristor to be self-triggered by

4332-458: The gate (an SCR requires a positive voltage). Once triggered, SCRs and TRIACs continue to conduct, even if the gate current ceases, until the main current drops below a certain level called the holding current . Gate turn-off thyristors (GTOs) are similar to TRIACs but provide more control by turning off when the gate signal ceases. The bidirectionality of TRIACs makes them convenient switches for alternating-current (AC). In addition, applying

4408-511: The gate and MT1 to provide a low-impedance path to MT1 and further prevent false triggering. This, however, increases the required trigger current or adds latency due to capacitor charging. On the other hand, a resistor between the gate and MT1 helps draw leakage currents out of the device, thus improving the performance of the TRIAC at high temperature, where the maximum allowed d v /d t is lower. Values of resistors less than 1kΩ and capacitors of 100nF are generally suitable for this purpose, although

4484-444: The gate current is discontinued, if the current between the two main terminals is more than what is called the latching current , the device continues to conduct. Latching current is the minimum current that keeps the device internal structure latched in the absence of gate current. The value of this parameter varies with: In particular, if the pulse width of the gate current is sufficiently large (generally some tens of microseconds),

4560-406: The gate terminal, the device remains latched in the on-state (i.e. does not need a continuous supply of gate current to remain in the on state), providing the anode current has exceeded the latching current ( I L ). As long as the anode remains positively biased, it cannot be switched off unless the current drops below the holding current ( I H ). In normal working conditions the latching current

4636-416: The heart of high-voltage direct current (HVDC) conversion either to or from alternating current. In the realm of this and other very high-power applications, both electrically triggered (ETT) and light-triggered (LTT) thyristors are still the primary choice. Thyristors are arranged into a diode bridge circuit and to reduce harmonics are connected in series to form a 12-pulse converter . Each thyristor

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4712-404: The holding current is indicated as I H . They are typically in the order of some milliamperes. A high d ⁡ v d ⁡ t {\displaystyle \operatorname {d} v \over \operatorname {d} t} between MT2 and MT1 may turn on the TRIAC when it is off. Typical values of critical static d v /d t are in the terms of volts per microsecond. The turn-on

4788-458: The injection of minority carriers in the two layers joining the junction, electrons are injected in the p-layer under the gate. Some of these electrons do not recombine and escape to the underlying n-region (step 2). This in turn lowers the potential of the n-region, acting as the base of a pnp transistor which switches on (turning the transistor on without directly lowering the base potential is called remote gate control ). The lower p-layer works as

4864-423: The mains supply. In these situations it is normal to connect the neutral terminal to the positive rail of the microcontroller's power supply, together with A1 of the triac, with A2 connected to the live. The TRIAC's gate can be connected through an opto-isolated transistor, and sometimes a resistor to the microcontroller, so that bringing the voltage down to the microcontroller's logic zero pulls enough current through

4940-401: The major portion of the current is the same as quadrant-I operation ("3" in Figure 5). Quadrant 3 operation occurs when the gate and MT2 are negative with respect to MT1. The whole process is outlined in Figure 6. The process happens in different steps here too. In the first phase, the pn junction between the MT1 terminal and the gate becomes forward-biased (step 1). As forward-biasing implies

5016-419: The modern computerized control circuits of many household small and major appliances . When mains voltage TRIACs are triggered by microcontrollers, optoisolators are frequently used; for example optotriacs can be used to control the gate current. Alternatively, where safety allows and electrical isolation of the controller isn't necessary, one of the microcontroller's power rails may be connected to one of

5092-538: The name "Alternistor". Later versions are sold under the trademark "Snubberless" and "ACS" (AC Switch, though this type also incorporates a gate buffer, which further precludes Quadrant I operation). Littelfuse also uses the name "Alternistor". Philips Semiconductors (now NXP Semiconductors ) originated the trademark "Hi-Com" (High Commutation). Often these TRIACs can operate with smaller gate-current to be directly driven by logic level components. Thyristor A thyristor ( / θ aɪ ˈ r ɪ s t ər / , from

5168-503: The physical structure of a particular triac, but as a rule, quadrant I is the most sensitive (least gate current required), and quadrant 4 is the least sensitive (most gate current required). In quadrants 1 and 2, MT2 is positive, and current flows from MT2 to MT1 through P, N, P and N layers. The N region attached to MT2 does not participate significantly. In quadrants 3 and 4, MT2 is negative, and current flows from MT1 to MT2, also through P, N, P and N layers. The N region attached to MT2

5244-726: The receiver was obtained by moving the switching point of the thyristor device up and down the falling slope of the positive going half of the AC supply input (if the rising slope was used the output voltage would always rise towards the peak input voltage when the device was triggered and thus defeat the aim of regulation). The precise switching point was determined by the load on the DC output supply, as well as AC input fluctuations. Thyristors have been used for decades as light dimmers in television , motion pictures , and theater , where they replaced inferior technologies such as autotransformers and rheostats . They have also been used in photography as

5320-432: The red arrow labeled with a "3" in Figure 6 shows the final conduction path of the current. Generally, this quadrant is the least sensitive of the four. In addition, some models of TRIACs (three-quadrant high commutation triacs named by different suppliers as "logic level", "snubberless" or "Hi-Com" types) cannot be triggered in this quadrant but only in the other three. There are some limitations one should know when using

5396-494: The remaining charge carriers ( holes and electrons ) that have not yet recombined . For applications with frequencies higher than the domestic AC mains supply (e.g. 50 Hz or 60 Hz), thyristors with lower values of t Q are required. Such fast thyristors can be made by diffusing heavy metal ions such as gold or platinum which act as charge combination centers into the silicon. Today, fast thyristors are more usually made by electron or proton irradiation of

5472-416: The silicon, or by ion implantation . Irradiation is more versatile than heavy metal doping because it permits the dosage to be adjusted in fine steps, even at quite a late stage in the processing of the silicon. A reverse conducting thyristor (RCT) has an integrated reverse diode , so is not capable of reverse blocking. These devices are advantageous where a reverse or freewheel diode must be used. Because

5548-402: The simplification they can bring to the electronics of an HVDC valve, light-triggered thyristors may still require some simple monitoring electronics and are only available from a few manufacturers. Two common photothyristors include the light-activated SCR (LASCR) and the light-activated TRIAC . A LASCR acts as a switch that turns on when exposed to light. Following light exposure, when light

5624-420: The triggering scheme is the same as an SCR. The equivalent circuit is depicted in Figure 4. However, the structure is different from SCRs. In particular, TRIAC always has a small current flowing directly from the gate to MT1 through the p-silicon without passing through the p-n junction between the base and the emitter of the equivalent NPN transistor. This current is indicated in Figure 3 by a dotted red line and

5700-477: The voltage across the device is reverse-biased or the voltage is removed (by some other means), or through the control gate signal on newer types. Some sources define " silicon-controlled rectifier " (SCR) and "thyristor" as synonymous. Other sources define thyristors as more complex devices that incorporate at least four layers of alternating N-type and P-type substrate. The first thyristor devices were released commercially in 1956. Because thyristors can control

5776-452: Was developed in 1956 by power engineers at General Electric (GE), led by Gordon Hall and commercialized by GE's Frank W. "Bill" Gutzwiller. The Institute of Electrical and Electronics Engineers recognized the invention by placing a plaque at the invention site in Clyde, New York , and declaring it an IEEE Historic Milestone. An earlier gas-filled tube device called a thyratron provided

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