Specifically what is a thyristor?
A thyristor is a high-power semiconductor device, also known as a silicon-controlled rectifier. Its structure contains 4 quantities of semiconductor materials, 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 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 any Thyristor is generally represented from the text symbol “V” or “VT” (in older standards, the letters “SCR”). In addition, derivatives of thyristors include fast thyristors, bidirectional thyristors, reverse conduction thyristors, and lightweight-controlled thyristors. The operating condition of the thyristor is the fact 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 is used involving the anode and cathode (the anode is linked to the favorable pole of the power supply, and 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 the indicator light will not light up. This shows 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 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 away (which is, K is excited again), the indicator light still glows. This shows that the thyristor can continue to conduct. At the moment, so that you can shut down the conductive thyristor, the power supply Ea should be shut down 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 the indicator light will not light up at this time. This shows that the thyristor is not really conducting and will reverse blocking.
- In summary
1) If the thyristor is exposed to a reverse anode voltage, the thyristor is in a reverse blocking state whatever voltage the gate is exposed to.
2) If the thyristor is exposed to a forward anode voltage, the thyristor will only conduct if the gate is exposed to a forward voltage. At the moment, the thyristor is incorporated in the forward conduction state, the thyristor characteristic, which is, the controllable characteristic.
3) If the thyristor is excited, so long as there is a specific forward anode voltage, the thyristor will stay excited whatever 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 the primary circuit voltage (or current) decreases to seal to zero, the thyristor turns off.
5) The condition for the thyristor to conduct is the fact that a forward voltage should be applied involving the anode and the cathode, and an appropriate forward voltage should also be applied involving the gate and the cathode. To change off a conducting thyristor, the forward voltage involving the anode and cathode should be shut down, or even the voltage should be reversed.
Working principle of thyristor
A thyristor is essentially an exclusive triode made up 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 remains turned off because BG1 has no base current. If a forward voltage is applied for the control electrode at this time, BG1 is triggered to generate basics current Ig. BG1 amplifies this current, along with a ß1Ig current is obtained in the 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 brought to BG1 for amplification and after that brought 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 inside the emitters of these two transistors, which is, the anode and cathode of the thyristor (the size of the current is actually based on the size of the load and the size of Ea), so the thyristor is entirely excited. This conduction process is finished in a very limited time.
- Right after 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 still inside the conductive state. Therefore, the purpose of the control electrode is simply to trigger the thyristor to transform on. After the thyristor is excited, the control electrode loses its function.
- The only way to switch off the turned-on thyristor is always to reduce the anode current that it is not enough to keep up the positive feedback process. The best way to reduce the anode current is always to shut down the forward power supply Ea or reverse the link of Ea. The minimum anode current required to keep your thyristor inside the conducting state is known as the holding current of the thyristor. Therefore, strictly speaking, so long as the anode current is under the holding current, the thyristor can be turned off.
What is the distinction between a transistor along with a thyristor?
Transistors usually include a PNP or NPN structure made up of three semiconductor materials.
The thyristor is composed of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The work of any transistor relies upon electrical signals to control its opening and closing, allowing fast switching operations.
The thyristor needs a forward voltage along with a trigger current at the gate to transform on or off.
Transistors are widely used in amplification, switches, oscillators, along with other elements of electronic circuits.
Thyristors are mostly found in electronic circuits such as controlled rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversions.
Means of working
The transistor controls the collector current by holding the base current to accomplish current amplification.
The thyristor is excited or off by controlling the trigger voltage of the control electrode to comprehend 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 may be used in similar applications sometimes, because of their different structures and operating principles, they have got noticeable differences in performance and utilize occasions.
Application scope of thyristor
- In power electronic equipment, thyristors may be used in frequency converters, motor controllers, welding machines, power supplies, etc.
- Inside the lighting field, thyristors may be used in dimmers and lightweight control devices.
- In induction cookers and electric water heaters, thyristors may be used to control the current flow for the heating element.
- In electric vehicles, transistors may be used in motor controllers.
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