What exactly is a thyristor?
A thyristor is really a high-power semiconductor device, also referred to as a silicon-controlled rectifier. Its structure contains 4 quantities of semiconductor components, including 3 PN junctions corresponding for the Anode, Cathode, and control electrode Gate. These 3 poles would be the critical parts of 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 popular in various electronic circuits, including controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversion.
The graphical symbol of any semiconductor device is usually represented from the text symbol “V” or “VT” (in older standards, the letters “SCR”). Additionally, derivatives of thyristors include fast thyristors, bidirectional thyristors, reverse conduction thyristors, and light-controlled thyristors. The working condition of the thyristor is the fact when a forward voltage is applied, the gate needs to have a trigger current.
Characteristics of thyristor
- Forward blocking
As shown in Figure a above, when an ahead voltage is utilized involving the anode and cathode (the anode is linked 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 applied for the control pole (i.e., K is disconnected), and also the indicator light will not illuminate. This implies that the thyristor is not really conducting and it has forward blocking capability.
- Controllable conduction
As shown in Figure b above, when K is closed, along with a forward voltage is applied for the control electrode (called a trigger, and also the applied voltage is known as trigger voltage), the indicator light switches on. Because of this the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, following the thyristor is excited, even when the voltage in the control electrode is taken off (which is, K is excited again), the indicator light still glows. This implies that the thyristor can still conduct. Currently, to be able 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 applied for the control electrode, a reverse voltage is applied involving the anode and cathode, and also the indicator light will not illuminate currently. This implies that the thyristor is not really conducting and can reverse blocking.
- In conclusion
1) If the thyristor is subjected to a reverse anode voltage, the thyristor is at a reverse blocking state whatever voltage the gate is subjected to.
2) If 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, which is, the controllable characteristic.
3) If the thyristor is excited, provided that there is a specific forward anode voltage, the thyristor will remain excited no matter the gate voltage. That is, following the thyristor is excited, the gate will lose its function. The gate only functions as a trigger.
4) If the thyristor is on, and also the primary circuit voltage (or current) decreases to seal to zero, the thyristor turns off.
5) The condition for your thyristor to conduct is the fact a forward voltage ought to be applied involving the anode and also the cathode, and an appropriate forward voltage should also be applied involving the gate and also the cathode. To transform off a conducting thyristor, the forward voltage involving the anode and cathode has to be stop, or the voltage has to be reversed.
Working principle of thyristor
A thyristor is actually a distinctive triode composed of three PN junctions. It could be equivalently viewed as comprising a PNP transistor (BG2) and an NPN transistor (BG1).
- If a forward voltage is applied involving the anode and cathode of 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. If a forward voltage is applied for the control electrode currently, BG1 is triggered to generate basics current Ig. BG1 amplifies this current, along with a ß1Ig current is obtained in its collector. This current is precisely the base current of BG2. After amplification by BG2, a ß1ß2Ig current is going to 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 a crucial positive feedback, causing both BG1 and BG2 to get into a saturated conduction state quickly. A large current appears inside the emitters of these two transistors, which is, the anode and cathode of the thyristor (the size of the current is in fact dependant on the size of the stress and the size of Ea), so the thyristor is entirely excited. This conduction process is done in an exceedingly short period of time.
- Following the thyristor is excited, its conductive state is going to be maintained from the positive feedback effect of the tube itself. Whether or not the forward voltage of the control electrode disappears, it is actually still inside the conductive state. Therefore, the purpose of the control electrode is just to trigger the thyristor to turn on. After the thyristor is excited, the control electrode loses its function.
- The only method to switch off the turned-on thyristor is to decrease the anode current that it is not enough to maintain the positive feedback process. The best way to decrease the anode current is to stop the forward power supply Ea or reverse the connection of Ea. The minimum anode current necessary to keep your thyristor inside the conducting state is known as the holding current of the thyristor. Therefore, strictly speaking, provided that the anode current is lower than the holding current, the thyristor could be turned off.
What is the difference between a transistor along with a thyristor?
Transistors usually contain a PNP or NPN structure composed of three semiconductor materials.
The thyristor consists of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The job of any transistor depends on electrical signals to control its closing and opening, allowing fast switching operations.
The thyristor requires a forward voltage along with a trigger current on the gate to turn on or off.
Transistors are popular in amplification, switches, oscillators, as well as other facets of electronic circuits.
Thyristors are mostly found in electronic circuits including 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 accomplish current amplification.
The thyristor is excited or off by manipulating 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 the different structures and working 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.
- Inside 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 for the heating element.
- In electric vehicles, transistors can be utilized in motor controllers.
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