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The Best Guide to 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 solid-state semiconductor device made up of four layers of P- and N-type materials that alternate. 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, each with a different mechanism for triggering the conducting state. 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 


In 1956, the first commercially available thyristor devices were released. Thyristors are widely used in electric power management, ranging from light dimmers to electric motor speed control to high-voltage direct-current power transmission, due to their ability to control a huge quantity of power and voltage with a small device. 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 difficult to use in direct current applications; however, newer device types may be turned on and off using the control gate signal. A gate turn-off thyristor, or GTO thyristor, is the latter. Thyristors, unlike transistors, have a two-valued switching characteristic, which means 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 yet effective as a switch.


Ⅱ How Does a Thyristor Work?

The P-N-P-N structure of a thyristor has three junctions: PN, NP, and PN. The exterior junctions, PN and PN, are forward-biased when the anode is a positive terminal concerning the cathode, whereas the middle NP junction is reverse-biased. As a result, the NP junction prevents a positive current from flowing from the anode to the cathode. In a forward blocking state, the thyristor is said to be. Similarly, the outer PN junctions impede the flow of a negative current. The thyristor is in a state of reverse blocking.

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 after the thyristor is turned "ON" and passing current in the forward direction (anode positive) due to the regenerative latching action of the two internal transistors. Because the thyristor is already conducting and fully-ON, any gate signals or pulses applied after regeneration has begun would have no effect.


The SCR, unlike the transistor, cannot be biased to remain in an active zone along a load line between blocking and saturation states. Because conduction is controlled internally, the magnitude and length of the gate "turn-on" pulse have no impact on the device's operation. The gadget will conduct if a transient gate pulse is applied to it, and it will remain permanently "ON" even if the gate signal is removed.


As a result, the thyristor can be viewed as a Bistable Latch with two stable states: "OFF" and "ON." This is because a silicon-controlled rectifier stops current in both directions of an AC waveform when no gate signal is supplied, and once it is triggered into conduction, the regenerative latching mechanism prevents it from being turned "OFF" simply by utilizing its Gate.


3.2 Thyristor turn-off

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


Before a forward voltage is given to the device without it immediately self-conducting, the anode current must be lowered below this minimum holding level long enough for the thyristors internal latching pn-junctions to recover their blocking condition. For a thyristor to conduct in the first place, the Anode current, also known as the load current, IL, must be greater than the holding current value. That is to say, IL > IH.


Because the thyristor can turn "OFF" whenever the Anode current is reduced below this minimum holding value, when used on a sinusoidal AC supply, the SCR will automatically turn "OFF" at some value near the half-cycle cross over point and will remain "OFF" until the next Gate trigger pulse is applied, as we now know.


Because the polarity of an AC sinusoidal voltage changes from positive to negative every half-cycle, the thyristor can turn "OFF" at the 180o zero point of the positive waveform. This is referred to as "natural commutation," and it is a key feature of the silicon-controlled rectifier.


Thyristors used in circuits fed from DC sources cannot experience this natural commutation because the DC supply voltage is constant, hence another method of turning the thyristor "OFF" at the right moment must be given because once triggered, it will continue to conduct.


Natural commutation happens 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. Even if a Gate signal is provided, this voltage reverse biases the thyristor, preventing it from conducting.


The thyristor can be triggered into conduction until the conclusion of the positive half cycle by applying a Gate signal at the right point during the positive half of an AC waveform. 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. The SCR is triggered into conduction when the gate pulse is applied, and it remains fully latched "ON" for the duration of the positive cycle. The load (a light) will be "ON" for the complete positive cycle of the AC waveform (half-wave rectified AC) at a high average voltage of 0.318 x Vp if the thyristor is triggered at the beginning of the half-cycle (Θ= 0°).


Thyristor Phase Control


The lamp is lighted for a shorter duration when the gate trigger pulse is applied 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.


We've seen that, regardless of the Gate signal, a thyristor is simply a half-wave device that conducts only in the positive half of the cycle when the Anode is positive and stops current flow like a diode when the Anode is negative.


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.


"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) are just a few examples, with all of these devices available in a variety of voltage and current.


Ⅴ Applications of Thyristors

Thyristors are typically utilized where high currents and voltages are present, and they're frequently used to manage alternating currents, where a change in polarity causes the device to turn off automatically, 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, requiring no further gate modulation, i.e., the device is biased completely on. This is not to be confused with the asymmetrical operation, which is asymmetrical because the output is unidirectional, flowing exclusively from cathode to anode.


Phase angle triggered controllers, also known as phase fired controllers, can use thyristors as control elements.


They're also present in digital circuit power supplies, where they act as an "improved circuit breaker" to prevent downstream components from being damaged by a power supply failure. When a thyristor is used with a Zener diode coupled to its gate, the thyristor will conduct and short-circuit the power supply output to the ground if the output voltage of the supply increases over the Zener voltage (in general also tripping an upstream breaker or fuse). A crowbar is a type of safety circuit that, unlike a typical circuit breaker or fuse, generates a high-conductance path to ground for the hazardous supply voltage as well as any stored energy in the system being supplied.


In the early 1970s, a stable power supply in color television receivers was the first large-scale application of thyristors and their accompanying triggering diac in consumer devices. Moving the switching point of the thyristor device up and down the falling slope of the positive-going half of the AC supply input resulted in a steady high voltage DC supply for the receiver (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 load on the DC output supply, as well as AC input oscillations, controlled the precise switching point.


For decades, thyristors have been utilized as light dimmers in television, film, and theater, replacing inferior technology like autotransformers and rheostats. They've also been employed as a crucial component of flashes in photography (strobes).


Ⅵ Different Types of Thyristors and Their Uses

Thyristors are classed based on their voltage and current properties, as well as their turn-on and turn-off behavior.


6.1 Thyristors with turn-on capability (Unidirectional control)

1. Silicon controlled rectifier (SCR)

The most well-known thyristor is the SCR. An SCR remains latched on even when the gate current is released, as indicated in the general thyristor description above. The anode to cathode current must be removed or the anode must be reset to a negative voltage relative to the cathode to unlatch. This property is helpful for the controlling phase. The SCR stops conducting and blocks the reverse voltage when the anode current reaches zero.


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


2. Reverse conducting thyristor (RCT)

Thyristors normally only allow current to flow in one way while blocking current flow in the opposite direction. An RCT, on the other hand, is made up of an SCR plus a reverse diode, which avoids unwanted loop inductance and lowers reverse voltage transients. Electric conduction in the reverse direction is possible with the RCT, which improves commutation.


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


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

Light-triggered thyristors are another name for them (LTT). The number of electron-hole pairs in the thyristor grows when light particles impact the reverse-biased junction in these devices. The thyristor will turn on if the light intensity exceeds a specific point. Between the light source and the power converter's switching component, a LASCR provides total electrical isolation.


HVDC transmission equipment, reactive power compensators, and high-power pulse generators all use LASCRs.


6.2 Thyristors with turn-off capability (Unidirectional control)

When a sufficient gate pulse is supplied, traditional thyristors, such as SCRs, turn on. The main current must be cut to turn them off. In DC to AC and DC to DC conversion circuits, when the current does not normally become zero, this is inconvenient.


1. Gate turn-off thyristor (GTO)

A GTO varies from a traditional thyristor in that it may be turned off by applying a negative current (voltage) to the gate without removing the current from the anode and cathode (forced commutation). This means that a gate signal with a negative polarity can turn off the GTO, making it a fully controlled switch. A Gate-Controlled Switch, or GCS, is another name for it. A GTO's turn-off time is around ten times faster than that of an identical SCR.


Symmetric GTOs have reverse blocking abilities that are comparable to their forward voltage ratings. The reverse voltage blocking performance of asymmetric GTOs is limited. GTOs with reverse conductivity is made up of a GTO and an anti-parallel diode. The most common type of GTO on the market is the asymmetric GTO.


2.MOS turn–off thyristor (MTO)

An MTO combines a GTO and a MOSFET to improve the GTO's ability to turn off. GTOs necessitate a high gate turn-off current, with a peak amplitude of 20-35 percent of the anode to cathode current (current to be controlled). A turn-on gate and a turn-off gate, commonly known as the MOSFET gate, are the two control terminals on an MTO.


An applied gate pulse of sufficient magnitude leads the thyristor to latch on to turn on an MTO (similar to SCR and GTO).


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


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


DC and AC motor drives, high-power inverters, and AC stabilizing power all require GTOs.


3.Emitter turn off thyristors (ETO)

The ETO has two terminals, a conventional gate, and a second gate connected in series with a MOSFET, just like an MTO.


Positive voltages are provided to both gates to turn on an ETO, causing 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 procedure is terminated, and the ETO is turned off.


High-power voltage source inverters, Flexible AC line Transmissions (FACTs), and Static Synchronous Compensators all use ETOs (STATCOM).


6.3 Bidirectional control

Until now, all of the thyristors discussed have been unidirectional and have been utilized as rectifiers, DC-DC converters, and inverters. Two thyristors must be connected in anti-parallel to use these thyristors for AC voltage control, resulting in two independent control circuits with extra wire connections. To solve this problem, bidirectional thyristors were invented, which may conduct current in both directions when activated.


1. Triode for alternating current (TRIAC)

After SCRs, TRIACS are the most extensively utilized thyristors. They can regulate both halves of the alternating waveform, allowing them to make better use of the available power. TRIACs, on the other hand, are typically employed for low-power applications due to their non-symmetrical design. When switching at various gate voltages throughout each half-cycle, TRIACs have some drawbacks in high-power applications. This adds more harmonics to the system, causing it to become unbalanced and affecting EMC performance.


Light dimmers, speed controllers for electric fans and other electric motors, and computerized control circuits for domestic appliances all use low-power TRIACs.


2. Diode for alternating current (DIAC)

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


Because TRIACS are inherently asymmetrical, a DIAC prohibits any current from flowing through the gate until the DIAC reaches its trigger voltage in either direction. This guarantees that TRIACS used in AC switches fire in both directions evenly.


Light bulb dimmers contain DIACs.


3. Silicon Diode for Alternating Current (SIDAC)

A SIDAC has the same electrical characteristics as a DIAC. SIDACs have a higher break-over voltage and stronger power handling capabilities than DIACs, which is the fundamental difference between the two. A SIDAC is a five-layer device that can act 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 when the applied voltage matches or surpasses the break-over voltage. Even if the applied voltage changes, it remains in this conducting state until the current can be decreased below the rated holding current. To repeat the cycle, the SIDAC returns to its nonconductive condition.


Relaxation oscillators and other special-purpose devices use SIDACs.


Ⅶ Thyristor VS Transistors

Both a thyristor and a transistor 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 is measured in watts. In this case, a Thyristor is analyzed 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 simply requires a single trigger to stay on. Transistors can't be used in applications like alarm circuits that need to trigger once and stay on forever. As a result, we use a Thyristor to solve these issues.


More differences 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

Thyristors, or Silicon Controlled Rectifiers, are three-junction PNPN semiconductor devices that can be thought of as two interconnected transistors that can be used to switch high electrical loads. A single pulse of positive current applied to their Gate terminal can latch them "ON," and they will stay "ON" eternally 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 many circuits since they don't have any moving components, don't cause contact arcing, and don't get dirty or corroded. Thyristors can be used to adjust the mean value of an AC load current without dissipating significant quantities of power, in addition to simply switching large currents "ON" and "OFF." Electric lighting, heaters, and motor speed control are all examples of thyristor power control.


Ⅸ Frequently Asked Questions about Thyristor

1. What is the difference between SCR and thyristor?

A thyristor is a four semiconductor layer or three PN junction device. It is also known as “SCR” (Silicon Control Rectifier). The term “Thyristor” is derived from the words of thyratron (a gas fluid tube which works as SCR) and Transistor. Thyristors are also known as PN PN Devices.


2. Why SCR is called thyristor?

Silicon Controlled Rectifier (SCR) is a unidirectional semiconductor device made of silicon. This device is the solid state equivalent of thyratron and hence it is also referred to as thyristor or thyroid transistor.


3. Is thyristor a semiconductor device?

A thyristor is a four-layer semiconductor device, consisting of alternating P-type and N-type materials (PNPN). A thyristor usually has 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 for circuit diagrams or circuits seeks to emphasize its rectifier characteristics while also showing the control gate. As a result, the thyristor symbol shows the traditional diode symbol with a control gate entering near the junction.


5. What is the difference between diode and thyristor?

The main difference between diode and thyristor is that diode has 2 terminals and used as a rectifier for converting AC to DC and as a switch. While thyristor has 2 terminals and operates as a switch. Both diode and thyristor are semiconductor devices and constructed with a combination of p and n types of materials.


6. How is thyristor measured?

The multimeter is generally used to measure the DC resistance between anode and cathode of thyristors and diodes and also the gate to the cathode on thyristors. These measurements are of the “off state” or blocking voltage of the device. The only valid readings are “open circuit” and “short circuit”.


7. How to Check a Thyristor?

1)Connect the anode (entry terminal) on the thyristor to the positive (red) lead on the multimeter;

2)Set the multimeter to high resistance mode;

3)Return the leads to their original positions, this time adding the gate terminal to the positive lead.


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

Connect the negative lead of your ohmmeter to the anode of the SCR and the positive lead to the cathode of the SCR. Read the resistance value that is displayed on the ohmmeter. It should read a very high value of resistance. If it reads a very low value, then the SCR is shorted and should be replaced.


9. Which is better IGBT or thyristor?

IGBTs are much faster than the traditional thyristor and can be controlled by simply toggling an on/off gate signal using a digital signal processor and a field-programmable gate array as opposed to waiting for a zero crossing. For the IGBT, the two primary losses are the conduction losses and the switching losses.


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

The primary function of a thyristor is to control electric power and current by acting as a switch. For such a small and lightweight component, it offers adequate protection to circuits with large voltages and currents (up to 6000 V, 4500 A).