What exactly is a thyristor?
A thyristor is a high-power semiconductor device, also called a silicon-controlled rectifier. Its structure consists of four levels of semiconductor materials, including three PN junctions corresponding for the Anode, Cathode, and control electrode Gate. These three poles would be the critical parts from 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 operating status. Therefore, thyristors are widely used in different electronic circuits, such as controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency alteration.
The graphical symbol of a silicon-controlled rectifier is usually 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 lightweight-controlled thyristors. The operating condition from the thyristor is the fact each time 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 in between the anode and cathode (the anode is attached to the favorable pole from the power supply, and also the cathode is attached to the negative pole from the power supply). But no forward voltage is used for the control pole (i.e., K is disconnected), and also the indicator light fails to light up. This shows that the thyristor is not conducting and has forward blocking capability.
- Controllable conduction
As shown in Figure b above, when K is closed, and a forward voltage is used for the control electrode (called a trigger, and also the applied voltage is known as trigger voltage), the indicator light switches on. Which means that the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, following the thyristor is turned on, even if the voltage in the control electrode is taken away (that is certainly, K is turned on again), the indicator light still glows. This shows that the thyristor can still conduct. Currently, in order to stop the conductive thyristor, the power supply Ea has to be stop or reversed.
- Reverse blocking
As shown in Figure d above, although a forward voltage is used for the control electrode, a reverse voltage is used in between the anode and cathode, and also the indicator light fails to light up at this time. This shows that the thyristor is not conducting and can reverse blocking.
- In conclusion
1) Once the thyristor is subjected to a reverse anode voltage, the thyristor is within 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 will simply conduct when the gate is subjected to a forward voltage. Currently, the thyristor is incorporated in the forward conduction state, the thyristor characteristic, that is certainly, 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 no matter the gate voltage. Which is, following the thyristor is turned on, the gate will lose its function. The gate only functions 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 the thyristor to conduct is the fact a forward voltage needs to be applied in between the anode and also the cathode, and an appropriate forward voltage ought to be applied in between the gate and also the cathode. To transform off a conducting thyristor, the forward voltage in between the anode and cathode has to be stop, or even the voltage has to be reversed.
Working principle of thyristor
A thyristor is essentially a unique triode made up of three PN junctions. It could be equivalently viewed as consisting of a PNP transistor (BG2) and an NPN transistor (BG1).
- In case a forward voltage is used in between the anode and cathode from the thyristor without applying a forward voltage for 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 for the control electrode at this time, BG1 is triggered to generate basics 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 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 big current appears within the emitters of the two transistors, that is certainly, the anode and cathode from the thyristor (the size of the current is really determined by the size of the burden and the size of Ea), so the thyristor is totally turned on. This conduction process is completed in a very short time.
- After the thyristor is turned on, its conductive state will be maintained from the positive feedback effect from the tube itself. Even when the forward voltage from the control electrode disappears, it really is still within the conductive state. Therefore, the purpose of the control electrode is simply to trigger the thyristor to transform on. Once the thyristor is turned on, the control electrode loses its function.
- The only way to switch off the turned-on thyristor would be to lessen the anode current so that it is insufficient to maintain the positive feedback process. The best way to lessen the anode current would be to stop the forward power supply Ea or reverse the connection of Ea. The minimum anode current needed to keep your thyristor within the conducting state is known as the holding current from the thyristor. Therefore, as it happens, provided that the anode current is lower than the holding current, the thyristor may be turned off.
What exactly is the difference between a transistor and a thyristor?
Transistors usually consist of 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 task of a transistor depends on electrical signals to control its closing and opening, allowing fast switching operations.
The thyristor demands a forward voltage and a trigger current at the gate to transform on or off.
Transistors are widely used in amplification, switches, oscillators, as well as other facets of electronic circuits.
Thyristors are mostly utilized in electronic circuits such as 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 attain current amplification.
The thyristor is turned on or off by managing the trigger voltage from the control electrode to realize the switching function.
The circuit parameters of thyristors are related to 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 operating principles, they have got noticeable differences in performance and use occasions.
Application scope of thyristor
- In power electronic equipment, thyristors can be utilized in frequency converters, motor controllers, welding machines, power supplies, etc.
- In the lighting field, thyristors can be utilized in dimmers and lightweight control devices.
- In induction cookers and electric water heaters, thyristors could be used to control the current flow for the heating element.
- In electric vehicles, transistors can be utilized in motor controllers.
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