Just what is a thyristor?
A thyristor is actually a high-power semiconductor device, also referred to as a silicon-controlled rectifier. Its structure contains four levels of semiconductor materials, including three PN junctions corresponding to the Anode, Cathode, and control electrode Gate. These three poles are definitely the critical parts of the thyristor, allowing it to control current and perform high-frequency switching operations. Thyristors can operate under high voltage and high current conditions, and external signals can maintain their functioning status. Therefore, thyristors are popular in different electronic circuits, such as controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency alteration.
The graphical symbol of any silicon-controlled rectifier is usually represented from the text symbol “V” or “VT” (in older standards, the letters “SCR”). Furthermore, derivatives of thyristors also have fast thyristors, bidirectional thyristors, reverse conduction thyristors, and light-controlled thyristors. The functioning condition of the thyristor is the fact when a forward voltage is used, the gate will need to have a trigger current.
Characteristics of thyristor
- Forward blocking
As shown in Figure a above, when an ahead voltage is used between the anode and cathode (the anode is attached to the favorable pole of the power supply, and also the cathode is linked to the negative pole of the power supply). But no forward voltage is used to the control pole (i.e., K is disconnected), and also the indicator light will not glow. This shows that the thyristor will not be conducting and has forward blocking capability.
- Controllable conduction
As shown in Figure b above, when K is closed, and a forward voltage is used to the control electrode (referred to as a trigger, and also the applied voltage is called trigger voltage), the indicator light switches on. This means that the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, right after the thyristor is switched on, even if the voltage on the control electrode is taken away (that is, K is switched on again), the indicator light still glows. This shows that the thyristor can still conduct. Currently, so that you can shut down the conductive thyristor, the power supply Ea must be shut down or reversed.
- Reverse blocking
As shown in Figure d above, although a forward voltage is used to the control electrode, a reverse voltage is used between the anode and cathode, and also the indicator light will not glow at the moment. This shows that the thyristor will not be conducting and may reverse blocking.
- In summary
1) When the thyristor is exposed to a reverse anode voltage, the thyristor is in a reverse blocking state no matter what voltage the gate is exposed to.
2) When the thyristor is exposed to a forward anode voltage, the thyristor will only conduct when the gate is exposed to a forward voltage. Currently, the thyristor is incorporated in the forward conduction state, the thyristor characteristic, that is, the controllable characteristic.
3) When the thyristor is switched on, provided that there is a specific forward anode voltage, the thyristor will remain switched on whatever the gate voltage. That is, right after the thyristor is switched on, the gate will lose its function. The gate only serves as a trigger.
4) When the thyristor is on, and also the primary circuit voltage (or current) decreases to close to zero, the thyristor turns off.
5) The condition for the thyristor to conduct is the fact a forward voltage ought to be applied between the anode and also the cathode, plus an appropriate forward voltage ought to be applied between the gate and also the cathode. To turn off a conducting thyristor, the forward voltage between the anode and cathode must be shut down, or even the voltage must be reversed.
Working principle of thyristor
A thyristor is actually a unique triode made up of three PN junctions. It can be equivalently thought to be comprising a PNP transistor (BG2) plus an NPN transistor (BG1).
- In case a forward voltage is used 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 is still turned off because BG1 has no base current. In case a forward voltage is used to the control electrode at the moment, BG1 is triggered to produce a base current Ig. BG1 amplifies this current, and a ß1Ig current is obtained in its collector. This current is precisely the base current of BG2. After amplification by BG2, a ß1ß2Ig current will be brought in the collector of BG2. This current is sent to BG1 for amplification then sent to BG2 for amplification again. Such repeated amplification forms an essential positive feedback, causing both BG1 and BG2 to get in a saturated conduction state quickly. A sizable current appears within the emitters of the two transistors, that is, the anode and cathode of the thyristor (the size of the current is really based on the size of the burden and the size of Ea), so the thyristor is completely switched on. This conduction process is done in a very limited time.
- Following the thyristor is switched on, its conductive state will be maintained from the positive feedback effect of the tube itself. Even if the forward voltage of the control electrode disappears, it really is still within the conductive state. Therefore, the purpose of the control electrode is only to trigger the thyristor to transform on. When the thyristor is switched on, the control electrode loses its function.
- The only method to switch off the turned-on thyristor is to lessen the anode current that it is insufficient to keep the positive feedback process. The way to lessen the anode current is to shut down the forward power supply Ea or reverse the link of Ea. The minimum anode current needed to keep the thyristor within the conducting state is called the holding current of the thyristor. Therefore, strictly speaking, provided that the anode current is lower than the holding current, the thyristor may be turned off.
What is the difference between a transistor and a thyristor?
Transistors usually contain a PNP or NPN structure made up of three semiconductor materials.
The thyristor is composed of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The task of any transistor relies on electrical signals to control its closing and opening, allowing fast switching operations.
The thyristor needs a forward voltage and a trigger current on the gate to transform on or off.
Transistors are popular in amplification, switches, oscillators, along with other facets of electronic circuits.
Thyristors are mainly found in electronic circuits such as controlled rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversions.
Method of working
The transistor controls the collector current by holding the base current to attain current amplification.
The thyristor is switched on or off by controlling the trigger voltage of the control electrode to realize the switching function.
The circuit parameters of thyristors are based on stability and reliability and usually have higher turn-off voltage and larger on-current.
To sum up, although transistors and thyristors can be utilized in similar applications sometimes, because of their different structures and functioning principles, they have noticeable variations in performance and make use of occasions.
Application scope of thyristor
- In power electronic equipment, thyristors can be utilized in frequency converters, motor controllers, welding machines, power supplies, etc.
- Within the lighting field, thyristors can be utilized in dimmers and light control devices.
- In induction cookers and electric water heaters, thyristors can be used to control the current flow to the heating element.
- In electric vehicles, transistors can be utilized in motor controllers.
PDDN Photoelectron Technology Co., Ltd is a wonderful thyristor supplier. It really is one of the leading enterprises in the Home Accessory & Solar Power System, that is fully involved in the growth and development of power industry, intelligent operation and maintenance handling of power plants, solar panel 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. Should you be looking for high-quality thyristor, please feel free to contact us and send an inquiry.