So what is a thyristor?
A thyristor is actually a high-power semiconductor device, also called a silicon-controlled rectifier. Its structure contains 4 quantities of semiconductor elements, including 3 PN junctions corresponding to the Anode, Cathode, and control electrode Gate. These 3 poles are definitely the critical parts from 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 a variety of electronic circuits, like controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency alteration.
The graphical symbol of the silicon-controlled rectifier is generally represented from the text symbol “V” or “VT” (in older standards, the letters “SCR”). Furthermore, derivatives of thyristors also include fast thyristors, bidirectional thyristors, reverse conduction thyristors, and light-controlled thyristors. The working condition from the thyristor is the fact that whenever a forward voltage is used, the gate needs to have a trigger current.
Characteristics of thyristor
- Forward blocking
As shown in Figure a above, when an ahead voltage is used involving the anode and cathode (the anode is linked to the favorable pole from the power supply, and also the cathode is linked to the negative pole from 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 it 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 (called a trigger, and also the applied voltage is referred to as trigger voltage), the indicator light turns on. This means that the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, following the thyristor is turned on, whether or not the voltage around the control electrode is removed (that is, K is turned on again), the indicator light still glows. This shows that the thyristor can carry on and conduct. At the moment, to be able to 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 involving the anode and cathode, and also the indicator light will not glow at this time. This shows that the thyristor will not be conducting and may reverse blocking.
- In conclusion
1) Once 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) Once the thyristor is put through a forward anode voltage, the thyristor is only going to conduct once the gate is put through a forward voltage. At the moment, the thyristor is within the forward conduction state, the thyristor characteristic, that is, the controllable characteristic.
3) Once the thyristor is turned on, so long as you will find a specific forward anode voltage, the thyristor will remain turned on no matter the gate voltage. That is, following the thyristor is turned on, the gate will lose its function. The gate only works as a trigger.
4) Once 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 that thyristor to conduct is the fact that a forward voltage needs to be applied involving the anode and also the cathode, as well as an appropriate forward voltage ought to be applied involving the gate and also the cathode. To turn off a conducting thyristor, the forward voltage involving the anode and cathode must be shut down, or the voltage must be reversed.
Working principle of thyristor
A thyristor is actually a distinctive triode made up of three PN junctions. It may be equivalently thought to be composed of a PNP transistor (BG2) as well as an NPN transistor (BG1).
- When a forward voltage is used involving the anode and cathode from 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 this time, BG1 is triggered to produce a base current Ig. BG1 amplifies this current, and 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 brought in 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 large current appears within the emitters of these two transistors, that is, the anode and cathode from the thyristor (the dimensions of the current is really determined by the dimensions of the stress and the dimensions of Ea), so the thyristor is entirely turned on. This conduction process is finished in a really short period of time.
- Following the thyristor is turned on, its conductive state will likely be maintained from the positive feedback effect from the tube itself. Whether or not the forward voltage from the control electrode disappears, it is actually still within the conductive state. Therefore, the purpose of the control electrode is only to trigger the thyristor to transform on. Once the thyristor is turned on, the control electrode loses its function.
- The best way to turn off the turned-on thyristor would be to decrease the anode current that it is insufficient to maintain the positive feedback process. The best way to decrease the anode current would be to shut down the forward power supply Ea or reverse the link of Ea. The minimum anode current required to keep your thyristor within the conducting state is referred to as the holding current from 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 and a thyristor?
Transistors usually include a PNP or NPN structure made up of three semiconductor materials.
The thyristor is made up of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The work of the transistor relies upon electrical signals to control its opening and closing, allowing fast switching operations.
The thyristor requires a forward voltage and a trigger current at the gate to transform on or off.
Transistors are widely used in amplification, switches, oscillators, and other aspects of electronic circuits.
Thyristors are mainly used in electronic circuits like 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 achieve current amplification.
The thyristor is turned on or off by managing the trigger voltage from the control electrode to understand the switching function.
The circuit parameters of thyristors are based on stability and reliability and often have higher turn-off voltage and larger on-current.
To summarize, although transistors and thyristors can be utilized in similar applications in some instances, because of the different structures and working principles, they have got noticeable differences 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 may be used to control the current flow to the heating element.
- In electric vehicles, transistors can be utilized in motor controllers.
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