So what is a thyristor?
A thyristor is actually a high-power semiconductor device, also known as a silicon-controlled rectifier. Its structure consists of four levels of semiconductor components, including 3 PN junctions corresponding towards the Anode, Cathode, and control electrode Gate. These 3 poles would be 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 working status. Therefore, thyristors are commonly used in various electronic circuits, like controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversion.
The graphical symbol of the Thyristor is generally represented by the text symbol “V” or “VT” (in older standards, the letters “SCR”). Furthermore, derivatives of thyristors include fast thyristors, bidirectional thyristors, reverse conduction thyristors, and lightweight-controlled thyristors. The working condition of the thyristor is that each time a forward voltage is applied, the gate should have a trigger current.
Characteristics of thyristor
- Forward blocking
As shown in Figure a above, when an ahead voltage can be used in between the anode and cathode (the anode is linked 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 towards the control pole (i.e., K is disconnected), as well as the indicator light will not illuminate. This implies that the thyristor is not conducting and contains forward blocking capability.
- Controllable conduction
As shown in Figure b above, when K is closed, along with a forward voltage is applied towards the control electrode (referred to as a trigger, as well as the applied voltage is referred to as trigger voltage), the indicator light turns on. Because of this the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, right after the thyristor is switched on, whether or not the voltage on the control electrode is removed (that is certainly, K is switched on again), the indicator light still glows. This implies that the thyristor can continue to conduct. Currently, so that you can shut down the conductive thyristor, the power supply Ea has to be shut down or reversed.
- Reverse blocking
As shown in Figure d above, although a forward voltage is applied towards the control electrode, a reverse voltage is applied in between the anode and cathode, as well as the indicator light will not illuminate at the moment. This implies that the thyristor is not conducting and will reverse blocking.
- In summary
1) Once the thyristor is put through a reverse anode voltage, the thyristor is within a reverse blocking state regardless of what voltage the gate is put through.
2) Once the thyristor is put through a forward anode voltage, the thyristor will only conduct once the gate is put through a forward voltage. Currently, the thyristor is incorporated in the forward conduction state, which is the thyristor characteristic, that is certainly, the controllable characteristic.
3) Once the thyristor is switched on, so long as there is a specific forward anode voltage, the thyristor will remain switched on regardless of the gate voltage. That is certainly, right after the thyristor is switched on, the gate will lose its function. The gate only serves as a trigger.
4) Once the thyristor is on, as well as the primary circuit voltage (or current) decreases to close to zero, the thyristor turns off.
5) The condition for that thyristor to conduct is that 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 transform off a conducting thyristor, the forward voltage in between the anode and cathode has to be shut down, or the voltage has to be reversed.
Working principle of thyristor
A thyristor is basically an exclusive triode composed of three PN junctions. It could be equivalently thought to be comprising a PNP transistor (BG2) and an NPN transistor (BG1).
- When a forward voltage is applied in between the anode and cathode of the thyristor without applying a forward voltage towards 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 applied towards the control electrode at the moment, BG1 is triggered to create basics current Ig. BG1 amplifies this current, along with a ß1Ig current is obtained in their 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 brought to BG1 for amplification then brought 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 big current appears within the emitters of the two transistors, that is certainly, the anode and cathode of the thyristor (the size of the current is in fact determined by the size of the stress and the size of Ea), and so the thyristor is entirely switched on. This conduction process is finished in an exceedingly short period of time.
- After the thyristor is switched on, its conductive state will be maintained by the positive feedback effect of the tube itself. Even when the forward voltage of 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 turn on. After the thyristor is switched on, the control electrode loses its function.
- The only way to shut off the turned-on thyristor is to lessen the anode current so that it is insufficient to keep up the positive feedback process. How you can lessen the anode current is to shut down the forward power supply Ea or reverse the connection of Ea. The minimum anode current required to keep your thyristor within the conducting state is referred to as the holding current of the thyristor. Therefore, as it happens, so long as the anode current is less than the holding current, the thyristor can be switched off.
What exactly is the distinction between a transistor along with a thyristor?
Transistors usually consist of a PNP or NPN structure composed 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 demands a forward voltage along with a trigger current at the gate to turn on or off.
Transistors are commonly used in amplification, switches, oscillators, and other elements of electronic circuits.
Thyristors are mostly found 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 accomplish current amplification.
The thyristor is switched on or off by manipulating the trigger voltage of 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 sum up, although transistors and thyristors can be used in similar applications sometimes, due to their different structures and working principles, they may have noticeable variations in performance and utilize occasions.
Application scope of thyristor
- In power electronic equipment, thyristors can be used in frequency converters, motor controllers, welding machines, power supplies, etc.
- In the lighting field, thyristors can be used in dimmers and lightweight control devices.
- In induction cookers and electric water heaters, thyristors may be used to control the current flow towards the heating element.
- In electric vehicles, transistors can be used in motor controllers.
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