What is a thyristor?

A thyristor is a high-power semiconductor device, also known as a silicon-controlled rectifier. Its structure contains four levels of semiconductor materials, including 3 PN junctions corresponding to the Anode, Cathode, and control electrode Gate. These 3 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 widely used in various electronic circuits, such as controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency alteration.

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

Characteristics of thyristor

1. Forward blocking

As shown in Figure a above, when an ahead voltage can be used in between the anode and cathode (the anode is connected to the favorable pole of the power supply, as well as the cathode is connected to the negative pole of the power supply). But no forward voltage is applied to the control pole (i.e., K is disconnected), as well as the indicator light fails to light up. This demonstrates that the thyristor will not be conducting and it has forward blocking capability.

2. Controllable conduction

As shown in Figure b above, when K is closed, as well as a forward voltage is applied to the control electrode (called a trigger, as well as the applied voltage is called trigger voltage), the indicator light turns on. Which means that the transistor can control conduction.

3. Continuous conduction

As shown in Figure c above, after the thyristor is turned on, even if the voltage in the control electrode is taken off (which is, K is turned on again), the indicator light still glows. This demonstrates that the thyristor can continue to conduct. At this time, so that you can stop the conductive thyristor, the power supply Ea should be stop or reversed.

4. Reverse blocking

As shown in Figure d above, although a forward voltage is applied to the control electrode, a reverse voltage is applied in between the anode and cathode, as well as the indicator light fails to light up at the moment. This demonstrates that the thyristor will not be conducting and can reverse blocking.

5. In conclusion

1) Once the thyristor is subjected to a reverse anode voltage, the thyristor is at a reverse blocking state no matter what voltage the gate is subjected to.

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

3) Once the thyristor is turned on, provided that there exists a specific forward anode voltage, the thyristor will always be turned on whatever the gate voltage. That is, after the thyristor is turned on, the gate will lose its function. The gate only works as a trigger.

4) Once the thyristor is on, as well as 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 needs to be applied in between the anode as well as the cathode, and an appropriate forward voltage ought to be applied in between the gate as well as the cathode. To change off a conducting thyristor, the forward voltage in between the anode and cathode should be stop, or even the voltage should be reversed.

Working principle of thyristor

A thyristor is actually a unique triode composed of three PN junctions. It may be equivalently viewed as comprising a PNP transistor (BG2) and an NPN transistor (BG1).

1. In case a forward voltage is applied in between the anode and cathode of the thyristor without applying a forward voltage to the control electrode, although both BG1 and BG2 have forward voltage applied, the thyristor continues to be switched off because BG1 has no base current. In case a forward voltage is applied to the control electrode at the moment, BG1 is triggered to produce basics current Ig. BG1 amplifies this current, as well as a ß1Ig current is obtained in the collector. This current is precisely the base current of BG2. After amplification by BG2, a ß1ß2Ig current is going to be introduced the collector of BG2. This current is delivered to BG1 for amplification and after that delivered to BG2 for amplification again. Such repeated amplification forms a vital positive feedback, causing both BG1 and BG2 to get in a saturated conduction state quickly. A big current appears in the emitters of the two transistors, which is, the anode and cathode of the thyristor (the size of the current is actually based on the size of the burden and the size of Ea), therefore the thyristor is completely turned on. This conduction process is finished in a really short time.
2. Following the thyristor is turned on, its conductive state is going to be maintained by the positive feedback effect of the tube itself. Whether or not the forward voltage of the control electrode disappears, it is still in the conductive state. Therefore, the purpose of the control electrode is just to trigger the thyristor to turn on. When the thyristor is turned on, the control electrode loses its function.
3. The only method to shut off the turned-on thyristor is to reduce the anode current that it is not enough to keep up the positive feedback process. The best way to reduce the anode current is to stop the forward power supply Ea or reverse the bond of Ea. The minimum anode current required to keep your thyristor in the conducting state is called the holding current of the thyristor. Therefore, strictly speaking, provided that the anode current is under the holding current, the thyristor may be switched off.

What is the difference between a transistor as well as a thyristor?

Structure

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

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

Functioning conditions:

The job of the transistor depends on electrical signals to control its closing and opening, allowing fast switching operations.

The thyristor requires a forward voltage as well as a trigger current in the gate to turn on or off.

Application areas

Transistors are widely used in amplification, switches, oscillators, and other aspects of electronic circuits.

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

Means of working

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

The thyristor is turned on or off by managing the trigger voltage of the control electrode to comprehend the switching function.

Circuit parameters

The circuit parameters of thyristors are related to stability and reliability and in most cases have higher turn-off voltage and larger on-current.

To summarize, although transistors and thyristors may be used in similar applications sometimes, because of the different structures and working principles, they have noticeable differences in performance and make use of occasions.

Application scope of thyristor

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

Supplier

PDDN Photoelectron Technology Co., Ltd is a wonderful thyristor supplier. It is one of the leading enterprises in the Home Accessory & Solar Power System, which can be fully working in the development of power industry, intelligent operation and maintenance handling 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 searching for high-quality thyristor, please feel free to contact us and send an inquiry.