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What is a Thyristor?

Author: Apogeeweb
Date: 9 Apr 2022
what is a thyristor


Ⅰ What Is a Thyristor?

Ⅱ How Does a Thyristor Work?

Ⅲ Thyristor I-V Characteristics Curves

3.1 Thyristor turn-on

3.2 Thyristor turn-off

Ⅳ Thyristor Phase Control

Ⅴ Applications of Thyristors

Ⅵ Different Types of Thyristors and Their Uses

6.1 Thyristors with turn-on capability (Unidirectional control)

6.2 Thyristors with turn-off capability (Unidirectional control)

6.3 Bidirectional control

Ⅶ Thyristor VS Transistors

Ⅷ Conclusion

Ⅸ Frequently Asked Questions about Thyristor


Ⅰ What Is a Thyristor?

A thyristor is a four-layer solid-state semiconductor device having alternating P- and N-type materials. It only functions as a bistable switch, conducting when the Gate gets a current trigger and continuing to conduct until the voltage across the device is reversed biased or removed (by some other means). There are two designs, which differ in what causes the conducting state to occur. A modest current on the Gate lead of a three-lead thyristor regulates the larger current of the Anode to Cathode circuit. Conduction begins in a two-lead thyristor when the potential difference between the Anode and Cathode is sufficiently large (breakdown voltage).


this video shows what a thyristor is 


The first thyristor devices were commercially available in 1956. Because thyristors can handle a relatively significant quantity of power and voltage with a compact device, they have a wide range of applications in power control, from light dimmers and electric motor speed control to high-voltage direct-current power transmission. Thyristors can be found 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, and many other applications.


Originally, thyristors could only be turned off by reversing the current, making them impractical to use for direct current; later device types can be turned on and off via the control gate signal. The latter is referred to as a gate turn-off thyristor (GTO thyristor). Thyristors, unlike transistors, have a two-valued switching characteristic, which means that they can only be fully on or off, whereas transistors can be in between on and off states. As a result, a thyristor is ineffective as an analog amplifier but beneficial as a switch.


Ⅱ How Does a Thyristor Work?

A P-N-P-N-P-N thyristor has three junctions: PN, NP, and PN. If the cathode is a positive terminal, the outer junctions, PN and PN, are forward-biased, while the center NP junction is reverse-biased. As a result, the NP junction prevents positive current from flowing from anode to cathode. In a forward blocking state, the thyristor is said to be. Similarly, the outer PN junctions prevent the flow of a negative current. The thyristor is currently in reverse blocking mode.


this video shows how a thyristor works


A thyristor can also be in the forward conducting condition, which occurs when it gets a sufficient signal to turn on and begin conducting.


Ⅲ Thyristor I-V Characteristics Curves


thyristor I-V characteristics curves

3.1 Thyristor turn-on

The gate signal loses all control once the thyristor is turned "ON" and passing current in the forward direction (anode positive). This is due to the regenerative latching action of the two internal transistors. Any gate signals or pulses applied after regeneration has begun will have no effect because the thyristor is already conducting and fully-ON.


The SCR, unlike the transistor, cannot be biased to remain in an active zone along a load line between its blocking and saturation states. Because conduction is controlled internally, the magnitude and duration of the gate "turn-on" pulse have no effect on the device's operation. Then, delivering a brief gate pulse to the device is enough to cause it to conduct, and it will remain permanently "ON" even if the gate signal is removed completely.


As a result, the thyristor can be thought of as a Bistable Latch with two stable states: "OFF" or "ON." This is because, in the absence of a gate signal, a silicon controlled rectifier blocks current in both directions of an alternating current waveform, and once triggered into conduction, the regenerative latching mechanism means that it cannot be turned "OFF" simply by using its Gate.


3.2 Thyristor turn-off

Once the thyristor has self-latched into its "ON" state and is passing a current, it can only be turned "OFF" by either completely removing the supply voltage and thus the Anode (IA) current, or by reducing its Anode to Cathode current by some external means (the opening of a switch, for example) to below a value commonly known as the "minimum holding current," IH.


The anode current must thus be lowered below this minimum holding level for the thyristors' internally latched pn-junctions to regain their blocking condition before a forward voltage is given to the device again without it instantly self-conducting. To conduct in the first place, a thyristor's anode current, which is also its load current, IL, must be greater than its holding current value. That would be IL > IH.


Since the thyristor has the ability to turn "OFF" whenever the Anode current is reduced below this minimum holding value, it follows that when used on a sinusoidal AC supply, the SCR will automatically turn "OFF" at some value near the cross over point of each half cycle, and will remain "OFF" until the next Gate trigger pulse is applied.


Because an alternating current sinusoidal voltage constantly switches polarity from positive to negative on every half-cycle, the thyristor can be turned "OFF" at the 180o zero point of the positive waveform. This effect is known as "natural commutation," and it is a crucial feature of the silicon controlled rectifier.


Thyristors used in circuits fed by DC sources cannot have this natural commutation condition since the DC supply voltage is continuous, hence another mechanism to turn "OFF" the thyristor at the proper moment must be given because once triggered, it will stay conducting.


Natural commutation, on the other hand, occurs every half cycle in AC sinusoidal circuits. The thyristor is thus forward biased (anode positive) during the positive half cycle of an AC sinusoidal waveform and can be triggered "ON" using a Gate signal or pulse. The Anode becomes negative throughout the negative half cycle, whereas the Cathode remains positive. This voltage reverse biases the thyristor, preventing it from conducting even while a Gate signal is present.


So, by applying a Gate signal at the proper point during the positive half of an AC waveform, the thyristor can be triggered into conduction until the positive half cycle is completed. Thus, phase control (as it is known) may be used to trigger the thyristor at any position along the positive half of the AC waveform, and power control of AC systems is one of the numerous applications of a Silicon Controlled Rectifier, as shown.


Ⅳ Thyristor Phase Control

The SCR is "OFF" at the start of each positive half-cycle. When the gate pulse is applied, the SCR enters conduction and remains fully latched "ON" for the duration of the positive cycle. If the thyristor is triggered at the half-cycle start ( Θ= 0°), the load (a light) will be "ON" throughout the entire positive cycle of the AC waveform (half-wave rectified AC) at a high average voltage of 0.318 x Vp.



Thyristor Phase Control


The lamp is lighted for less time as the application of the gate trigger pulse increases along the half cycle ( Θ= 0° to 90°), and the average voltage given to the lamp is proportionally smaller, diminishing its brightness.


A silicon controlled rectifier can thus be used as an AC light dimmer as well as in a range of other AC power applications such as AC motor-speed control, temperature control systems, and power regulator circuits, among others.


So far, we've learned that a thyristor is simply a half-wave device that conducts only in the positive half of the cycle when the Anode is positive and inhibits current flow like a diode when the Anode is negative, regardless of the Gate signal.


However, there are other semiconductor devices known as "Thyristors" that can conduct in both directions, are full-wave devices, or can be turned "OFF" by the Gate signal.


To name a few, these devices include "Gate Turn-OFF Thyristors" (GTO), "Static Induction Thyristors" (SITH), "MOS Controlled Thyristors" (MCT), "Silicon Controlled Switch" (SCS), "Triode Thyristors" (TRIAC), and "Light Activated Thyristors" (LASCR), with all of these devices available in a variety of voltage.


Ⅴ Applications of Thyristors

Thyristors are primarily used to regulate high currents and voltages, and are frequently used to control alternating currents, where a change in polarity of the current causes the device to automatically turn off, a process known as "zero cross" operation. The device is considered to work synchronously because, once triggered, it conducts current in phase with the voltage provided across its cathode to anode junction with no further gate modulation necessary, i.e., the device is fully biased on. This is not to be confused with asymmetrical operation because the output is unidirectional, flowing exclusively from cathode to anode, and hence asymmetrical.


Thyristors can be used to control phase angle triggered controllers, also known as phase fired controllers.


They can also be found in digital circuit power supplies, where they act as a form of "improved circuit breaker" to prevent a power supply failure from damaging downstream components. A thyristor is used in conjunction with a Zener diode coupled to its gate, and if the power supply output voltage exceeds 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).


In the early 1970s, the first large-scale application of thyristors, with associated triggering diac, in consumer devices linked to stable power supplies within color television sets. The stable high voltage DC supply for the receiver was generated by changing 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 decided by the load on the DC output supply as well as AC input fluctuations.


Thyristors have been utilized as light dimmers in television, film, and theater for decades, replacing inferior technology such as autotransformers and rheostats. They have also been utilized in photography as an important component of flashes (strobes).


Ⅵ Different Types of Thyristors and Their Uses

Thyristors are classified based on their voltage and current characteristics, as well as their on/off behavior.


6.1 Thyristors with turn-on capability (Unidirectional control)

1. Silicon controlled rectifier (SCR)

SCRs are the most well-known type of thyristor. An SCR remains latched on even when the gate current is released, as indicated in the general thyristor description above. To unlatch, either the anode to cathode current must be removed or the anode must be reset to a negative voltage relative to the cathode. This property is ideal for phase control. When the anode current reaches zero, the SCR stops conducting and the reverse voltage is blocked.


Switching circuits, DC motor drives, AC/DC static switches, and inverting circuits all require SCRs.


2. Reverse conducting thyristor (RCT)

Thyristors often allow current solely in one direction while blocking current in the other. An RCT, on the other hand, is made up of an SCR integrated with a reverse diode, which avoids unwanted loop inductance and lowers reverse voltage transients. The RCT enables electric conduction in the opposite direction, resulting in enhanced commutation.


RCTs are utilized in high-power choppers' inverters and DC drives.


3.Light-activated silicon-controlled rectifier (LASCR)

These are also referred to as light-triggered thyristors (LTT). When light particles reach the reverse-biased junction of these devices, the number of electron-hole pairs in the thyristor increases. The thyristor will turn on if the intensity of the light exceeds a specific value. An LASCR provides total electrical isolation between the light source and the power converter's switching component.


LASCRs are found in high-voltage direct current transmission equipment, reactive power compensators, and high-power pulse generators.


6.2 Thyristors with turn-off capability (Unidirectional control)

When a sufficient gate pulse is supplied, traditional thyristors, such as SCRs, turn on. To turn them off, the main current must be cut. This is troublesome in DC to AC and DC to DC conversion circuits where current does not naturally zero out.


1. Gate turn-off thyristor (GTO)

A GTO varies from a typical thyristor in that it can be turned off by applying a negative current (voltage) to the gate without requiring the current between the anode and cathode to be removed (forced commutation). This means that a gate signal with a negative polarity can turn off the GTO, making it a fully controlled switch. It is also known as a Gate-Controlled Switch, or GCS. A GTO's turn off time is approximately ten times faster than that of a similar SCR.


Symmetric GTOs have reverse blocking abilities that are comparable to their forward voltage ratings. Asymmetric GTOs lack significant reverse voltage blocking capacity. Reverse conducting GTOs are made up of a GTO and an anti-parallel diode. Asymmetric GTOs are the most common type on the market.


2.MOS turn–off thyristor (MTO)

An MTO is a combination of a GTO and a MOSFET that improves the turn-off capability of the GTO. GTOs require a high gate turn off current with a peak amplitude of 20-35 percent of the anode to cathode current (current to be controlled). An MTO contains two control terminals, one for the turn-on gate and one for the turn-off gate, also known as the MOSFET gate.


To activate an MTO, a sufficiently large gate pulse is given, causing the thyristor to latch on (similar to SCR and GTO).


A voltage pulse is applied to the MOSFET gate to turn off the MTO. When the MOSFET switches on, it shorts the NPN transistor's emitter and base, preventing latching. It's a considerably faster operation than a GTO (around 1-2 s), in which the huge negative pulse sent to the GTO's gate seeks to extract enough current from the NPN transistor's base. Furthermore, the shorter time (MTO) eliminates the losses associated with current transfer.


MTOs are employed in high voltage applications ranging from 20 MVA to motor drives, flexible AC line transmissions (FACTs), and high power voltage source inverters.


GTOs are utilized in DC and alternating current motor drives, high power inverters, and alternating current stabilizing power.


3.Emitter turn off thyristors (ETO)

The ETO, like the MTO, has two terminals, one for a regular gate and one for a second gate connected in series with a MOSFET.

Positive voltages are provided to both gates to turn on an ETO, which causes NMOS to turn on and PMOS to switch off. The ETO turns on when a positive current is introduced into the usual gate.

NMOS turns off and transfers all current away from the cathode when a negative voltage signal is supplied to the MOSFET gate. The latching process is terminated, and the ETO is turned off.

ETOs are used in high-power voltage source inverters, Flexible AC line Transmissions (FACTs), and Static Synchronous Compensators (STATCOM).


6.3 Bidirectional control

So far, the thyristors that have been discussed have been unidirectional and have been employed as rectifiers, DC-DC converters, and inverters. To use these thyristors for AC voltage control, two of them must be coupled in anti-parallel, resulting in two independent control circuits with extra wire connections. Bidirectional thyristors, which can conduct current in both directions when triggered, were created expressly to address this issue.


1. Triode for alternating current (TRIAC)

After SCRs, TRIACS are the most often utilized thyristors. They can regulate both half of the alternating waveform, allowing for more efficient use of available power. TRIACs, on the other hand, are normally only employed for low power applications due to their inherent non-symmetrical structure. When switching at various gate voltages throughout each half cycle, TRIACs have some drawbacks in high power applications. This generates more harmonics in the system, causing an imbalance and affecting EMC performance.

Low-power TRIACs are utilized in light dimmers, speed controllers for electric fans and other electric motors, and computerized control circuits for household appliances.


2. Diode for alternating current (DIAC)

DIACS are low-power devices that are typically used in tandem with TRIACS (placed in series with the gate terminal of a TRIAC).


Because TRIACS are inherently unsymmetrical, a DIAC stops any current from flowing through the TRIAC's gate until the DIAC reaches its trigger voltage in either direction. This guarantees that TRIACS used in AC switches trigger in both directions uniformly.


Light bulb dimmers contain DIACs.


3. Silicon Diode for Alternating Current (SIDAC)

Electrically, a SIDAC behaves similarly to a DIAC. SIDACs offer a higher breakover voltage and stronger power handling capabilities than DIACs. A SIDAC is a five-layer device that can be used as a switch on its own rather than as a trigger for another switching device (like DIACs are for TRIACS).


A SIDAC begins to conduct current if the applied voltage matches or exceeds the breakover voltage. Even if the applied voltage changes, it remains in this conducting state until the current can be decreased below the rated holding current. The SIDAC then returns to its nonconductive condition to begin the cycle again.


SIDACs are found in relaxation oscillators and other specialized devices.


Ⅶ Thyristor VS Transistors

Both thyristors and transistors are electrical switches, however thyristors have a much higher power handling capacity than transistors. Because of the Thyristor's high rating in kilowatts, whereas transistor power ranges in watts. In this analysis, a Thyristor is modeled as a closed couple pair of transistors. The major difference between a transistor and a thyristor is that a transistor requires constant switching power to stay on, but a thyristor requires only a single trigger to stay on. Transistors cannot be used in applications such as alarm circuits that must activate once and remain ON indefinitely. To address these issues, we employ the Thyristor.


More distinctions between Thyristor and Transistor are listed in the table below:


Property Thyristor Transistor
Layer Four Layers Three Layers
Terminals Anode, Cathode and Gate Emitter, Collector, and Base
Operation over-voltage and current Higher Lower than thyristor
Turning ON Just required a gate pulse to turn ON Required continuous supply of the controlling current
Internal power loss Lower than transistor higher


Ⅷ Conclusion

Silicon Controlled Rectifiers, also known as Thyristors, are three-junction PNPN semiconductor devices that can be thought of as two interconnected transistors capable of switching high electrical loads. They can be latched-"ON" with a single positive current pulse delivered to their Gate terminal and will remain "ON" endlessly until the Anode to Cathode current falls below their minimum latching level.


Thyristors are high-speed switches that can be used to replace electromechanical relays in a variety of circuits since they have no moving components, no contact arcing, and are not affected by corrosion or dirt. However, in addition to merely switching big currents "ON" and "OFF," thyristors can be used to adjust the mean value of an alternating current load current without dissipating large quantities of electricity. The regulation of electric lighting, heaters, and motor speed is a good example of thyristor power control.


Ⅸ Frequently Asked Questions about Thyristor

1. What is the difference between SCR and thyristor?

A thyristor is a four-layer semiconductor device with three PN junctions. It is also referred to as "SCR" (Silicon Control Rectifier). The phrase "Thyristor" is a combination of the words thyratron (a gas fluid tube that functions as an SCR) and transistor. Thyristors are also referred to as PN PN Devices.


2. Why SCR is called thyristor?

A silicon controlled rectifier (SCR) is a unidirectional silicon semiconductor device. Because this device is the solid-state analogue of a thyratron, it is also known as a thyristor or thyroid transistor.


3. Is thyristor a semiconductor device?

A thyristor is a four-layer semiconductor device that alternates between P-type and N-type materials (PNPN). A thyristor is typically composed of three electrodes: an anode, a cathode, and a gate (control electrode).


4. What is the symbol for a thyristor?

The silicon-controlled rectifier, SCR, or thyristor symbol used in circuit designs or circuits aims to highlight the rectifier properties while also displaying the control gate. As a result, the thyristor symbol resembles a typical diode with a control gate entering at the junction.


5. What is the difference between diode and thyristor?

The primary distinction between a diode and a thyristor is that a diode has two terminals and is employed as a rectifier for converting AC to DC as well as a switch. The thyristor, on the other hand, has two terminals and functions as a switch. Both a diode and a thyristor are semiconductor devices made of a combination of p and n materials.


6. How is thyristor measured?

In general, the multimeter is used to measure the DC resistance between the anode and cathode of thyristors and diodes, as well as the gate to the cathode on thyristors. These data are of the device's "off state" or blocking voltage. "Open circuit" and "short circuit" are the only valid readings.


7. How to Check a Thyristor?

1)Connect the anode (entry terminal) of the thyristor to the multimeter's positive (red) lead.

2)Place the multimeter in the high resistance mode.

3)Replace the leads in their original placements, adding the gate terminal to the positive lead this time.


8. How do I know if my thyristor is bad?

Connect the negative lead of your ohmmeter to the SCR's anode and the positive lead to the SCR's cathode. Take note of the resistance value displayed on the ohmmeter. It should display a very high resistance value. If it reads an extremely low value, the SCR is shorted and needs to be replaced.


9. Which is better IGBT or thyristor?

IGBTs are much faster than typical thyristors and can be controlled by toggling an on/off gate signal with a digital signal processor and a field-programmable gate array rather than waiting for a zero crossing. The conduction losses and switching losses are the two primary losses for the IGBT.


10. What is the purpose of a thyristor in a circuit?

A thyristor's principal function is to control electric power and current by acting as a switch. It provides adequate protection to circuits with high voltages and currents for such a compact and lightweight component (up to 6000 V, 4500 A).


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