What is a thyristor?
A thyristor is actually a high-power semiconductor device, also referred to as a silicon-controlled rectifier. Its structure consists of 4 quantities of semiconductor elements, including 3 PN junctions corresponding to the Anode, Cathode, and control electrode Gate. These 3 poles are the critical parts in 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 working status. Therefore, thyristors are commonly used in different electronic circuits, including controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency alteration.
The graphical symbol of the semiconductor device is generally represented from the text symbol “V” or “VT” (in older standards, the letters “SCR”). In addition, derivatives of thyristors also include fast thyristors, bidirectional thyristors, reverse conduction thyristors, and light-controlled thyristors. The working condition in the thyristor is the fact each time 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 utilized in between the anode and cathode (the anode is attached to the favorable pole in the power supply, and the cathode is connected to the negative pole in the power supply). But no forward voltage is used to the control pole (i.e., K is disconnected), and the indicator light will not illuminate. This implies that the thyristor will not be conducting and contains forward blocking capability.
- Controllable conduction
As shown in Figure b above, when K is closed, as well as a forward voltage is used to the control electrode (referred to as a trigger, and the applied voltage is known as 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 when the voltage on the control electrode is taken away (which is, K is switched on again), the indicator light still glows. This implies that the thyristor can still conduct. At this time, to be able to stop the conductive thyristor, the power supply Ea should be stop 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 in between the anode and cathode, and the indicator light will not illuminate at the moment. This implies that the thyristor will not be conducting and may reverse blocking.
- In conclusion
1) If the thyristor is put through a reverse anode voltage, the thyristor is at a reverse blocking state no matter what voltage the gate is put through.
2) If the thyristor is put through a forward anode voltage, the thyristor will simply conduct if the gate is put through a forward voltage. At this time, the thyristor is incorporated in the forward conduction state, the thyristor characteristic, which is, the controllable characteristic.
3) If the thyristor is switched on, so long as there is a specific forward anode voltage, the thyristor will stay switched on no matter the gate voltage. Which is, right after the thyristor is switched on, the gate will lose its function. The gate only functions as a trigger.
4) If the thyristor is on, and the primary circuit voltage (or current) decreases to seal to zero, the thyristor turns off.
5) The disorder for that thyristor to conduct is the fact a forward voltage should be applied in between the anode and the cathode, plus an appropriate forward voltage should also be applied in between the gate and 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 made from three PN junctions. It could be equivalently viewed as composed of a PNP transistor (BG2) plus an NPN transistor (BG1).
- When a forward voltage is used in between the anode and cathode in 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. When a forward voltage is used to the control electrode at the moment, BG1 is triggered to create a base current Ig. BG1 amplifies this current, as well as 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 introduced the collector of BG2. This current is sent to BG1 for amplification then sent to BG2 for amplification again. Such repeated amplification forms a vital positive feedback, causing both BG1 and BG2 to enter a saturated conduction state quickly. A sizable current appears in the emitters of these two transistors, which is, the anode and cathode in the thyristor (how big the current is actually dependant on how big the burden and how big Ea), and so the thyristor is entirely switched on. This conduction process is finished in a really short time.
- Right after the thyristor is switched on, its conductive state will be maintained from the positive feedback effect in the tube itself. Even when the forward voltage in the control electrode disappears, it really 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 switched on, the control electrode loses its function.
- The only way to turn off the turned-on thyristor is always to reduce the anode current so that it is inadequate to keep up the positive feedback process. How you can reduce the anode current is always to stop the forward power supply Ea or reverse the link of Ea. The minimum anode current required to keep the thyristor in the conducting state is known as the holding current in the thyristor. Therefore, strictly speaking, so long as the anode current is less than the holding current, the thyristor may be switched off.
What is the distinction between a transistor as well as a thyristor?
Transistors usually include a PNP or NPN structure made from three semiconductor materials.
The thyristor is made up of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The job of the transistor relies upon electrical signals to control its opening and closing, allowing fast switching operations.
The thyristor requires a forward voltage as well as a trigger current in the gate to turn on or off.
Transistors are commonly used in amplification, switches, oscillators, as well as other aspects of electronic circuits.
Thyristors are mainly found in electronic circuits including 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 accomplish current amplification.
The thyristor is switched on or off by managing the trigger voltage in the control electrode to understand 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 summarize, although transistors and thyristors can be used in similar applications sometimes, because of their different structures and working principles, they have got noticeable variations in performance and make use of occasions.
Application scope of thyristor
- In power electronic equipment, thyristors can be used in frequency converters, motor controllers, welding machines, power supplies, etc.
- Within the lighting field, thyristors can 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 can be used in motor controllers.
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