What is a thyristor?

A thyristor is actually a high-power semiconductor device, also known as a silicon-controlled rectifier. Its structure includes four quantities of semiconductor elements, including three PN junctions corresponding towards the Anode, Cathode, and control electrode Gate. These three poles are the critical parts of the thyristor, letting it control current and perform high-frequency switching operations. Thyristors can operate under high voltage and high current conditions, and external signals can maintain their working status. Therefore, thyristors are popular in different electronic circuits, including controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversion.

The graphical symbol of a Thyristor is usually represented by the text symbol “V” or “VT” (in older standards, the letters “SCR”). Additionally, derivatives of thyristors also include fast thyristors, bidirectional thyristors, reverse conduction thyristors, and light-weight-controlled thyristors. The working condition of the thyristor is the fact whenever a forward voltage is applied, the gate should have a trigger current.

Characteristics of thyristor

1. Forward blocking

As shown in Figure a above, when an ahead voltage is used involving the anode and cathode (the anode is attached to the favorable pole of the power supply, and the cathode is attached to the negative pole of the power supply). But no forward voltage is applied towards the control pole (i.e., K is disconnected), and the indicator light will not illuminate. This implies that the thyristor is not really conducting and has forward blocking capability.

2. Controllable conduction

As shown in Figure b above, when K is closed, along with a forward voltage is applied towards the control electrode (referred to as a trigger, and the applied voltage is referred to as trigger voltage), the indicator light turns on. This means that the transistor can control conduction.

3. Continuous conduction

As shown in Figure c above, after the thyristor is switched on, even if the voltage in the control electrode is taken away (that is, K is switched on again), the indicator light still glows. This implies that the thyristor can carry on and conduct. At this time, to be able to cut off the conductive thyristor, the power supply Ea has to be cut off or reversed.

4. Reverse blocking

As shown in Figure d above, although a forward voltage is applied towards the control electrode, a reverse voltage is applied involving the anode and cathode, and the indicator light will not illuminate at the moment. This implies that the thyristor is not really conducting and can reverse blocking.

5. In summary

1) Once the thyristor is exposed to a reverse anode voltage, the thyristor is within a reverse blocking state regardless of what voltage the gate is exposed to.

2) Once the thyristor is exposed to a forward anode voltage, the thyristor is only going to conduct once the gate is exposed to a forward voltage. At this time, the thyristor is in the forward conduction state, which is the thyristor characteristic, that is, the controllable characteristic.

3) Once the thyristor is switched on, so long as there exists a specific forward anode voltage, the thyristor will remain switched on no matter the gate voltage. That is, after the thyristor is switched on, the gate will lose its function. The gate only works as a trigger.

4) Once the thyristor is on, and the primary circuit voltage (or current) decreases to seal to zero, the thyristor turns off.

5) The problem for the thyristor to conduct is the fact a forward voltage should be applied involving the anode and the cathode, as well as an appropriate forward voltage also need to be applied involving the gate and the cathode. To change off a conducting thyristor, the forward voltage involving the anode and cathode has to be cut off, or perhaps the voltage has to be reversed.

Working principle of thyristor

A thyristor is basically an exclusive triode composed of three PN junctions. It may be equivalently thought to be composed of a PNP transistor (BG2) as well as an NPN transistor (BG1).

1. When a forward voltage is applied involving the anode and cathode of the thyristor without applying a forward voltage towards the control electrode, although both BG1 and BG2 have forward voltage applied, the thyristor is still turned off because BG1 has no base current. When a forward voltage is applied towards the control electrode at the moment, BG1 is triggered to produce a base current Ig. BG1 amplifies this current, along with a ß1Ig current is obtained in the collector. This current is precisely the base current of BG2. After amplification by BG2, a ß1ß2Ig current will likely be introduced the collector of BG2. This current is sent to BG1 for amplification and after that sent to BG2 for amplification again. Such repeated amplification forms a crucial positive feedback, causing both BG1 and BG2 to get in a saturated conduction state quickly. A sizable current appears in the emitters of these two transistors, that is, the anode and cathode of the thyristor (how big the current is actually dependant on how big the load and how big Ea), and so the thyristor is completely switched on. This conduction process is completed in an exceedingly short period of time.
2. Following the thyristor is switched on, its conductive state will likely be maintained by the positive feedback effect of the tube itself. Even when the forward voltage of the control electrode disappears, it really is still in the conductive state. Therefore, the function of the control electrode is only to trigger the thyristor to turn on. After the thyristor is switched on, the control electrode loses its function.
3. The best way to turn off the turned-on thyristor is always to reduce the anode current that it is not enough to maintain the positive feedback process. The best way to reduce the anode current is always to cut off the forward power supply Ea or reverse the connection of Ea. The minimum anode current required to keep the thyristor in the conducting state is referred to as the holding current of the thyristor. Therefore, as it happens, so long as the anode current is under the holding current, the thyristor can be turned off.

What is the distinction between a transistor along with a thyristor?

Structure

Transistors usually consist of a PNP or NPN structure composed of three semiconductor materials.

The thyristor consists of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.

Working conditions:

The work of a transistor relies on electrical signals to control its closing and opening, allowing fast switching operations.

The thyristor requires a forward voltage along with a trigger current at the gate to turn on or off.

Application areas

Transistors are popular in amplification, switches, oscillators, and other elements of electronic circuits.

Thyristors are mostly used in electronic circuits including controlled rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversions.

Way of working

The transistor controls the collector current by holding the base current to achieve current amplification.

The thyristor is switched on or off by controlling the trigger voltage of the control electrode to realize the switching function.

Circuit parameters

The circuit parameters of thyristors are based on stability and reliability and usually have higher turn-off voltage and larger on-current.

To summarize, although transistors and thyristors may be used in similar applications sometimes, due to their different structures and working principles, they may have noticeable differences in performance and use occasions.

Application scope of thyristor

• In power electronic equipment, thyristors may be used in frequency converters, motor controllers, welding machines, power supplies, etc.
• Within the lighting field, thyristors may be used in dimmers and light-weight control devices.
• In induction cookers and electric water heaters, thyristors may be used to control the current flow towards the heating element.
• In electric vehicles, transistors may be used in motor controllers.

Supplier

PDDN Photoelectron Technology Co., Ltd is an excellent thyristor supplier. It really is one of the leading enterprises in the Home Accessory & Solar Power System, that is fully involved in the development of power industry, intelligent operation and maintenance management of power plants, solar power and related solar products manufacturing.

It accepts payment via Credit Card, T/T, West Union and Paypal. PDDN will ship the goods to customers overseas through FedEx, DHL, by air, or by sea. If you are looking for high-quality thyristor, please feel free to contact us and send an inquiry.