Just what is a thyristor?
A thyristor is really a high-power semiconductor device, also referred to as a silicon-controlled rectifier. Its structure includes 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 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 various electronic circuits, including controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversion.
The graphical symbol of a semiconductor device is generally represented through 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-weight-controlled thyristors. The working condition from 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 attached 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 applied for the control pole (i.e., K is disconnected), and also the indicator light fails to light up. This implies that the thyristor is not 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 (known as a trigger, and also the applied voltage is referred to as trigger voltage), the indicator light switches on. Because of this the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, after the thyristor is switched on, even if the voltage around the control electrode is taken away (which is, K is switched on again), the indicator light still glows. This implies that the thyristor can continue to conduct. Currently, so that you can 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 fails to light up currently. This implies that the thyristor is not conducting and may 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 once 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) Once the thyristor is switched on, as long as you will find a specific forward anode voltage, the thyristor will remain switched on no matter the gate voltage. That is certainly, 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, and also the primary circuit voltage (or current) decreases to seal to zero, the thyristor turns off.
5) The problem for the thyristor to conduct is the fact a forward voltage ought to be applied involving the anode and also the cathode, plus 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 has to be stop, or the voltage has to be reversed.
Working principle of thyristor
A thyristor is basically a unique triode made from three PN junctions. It may be equivalently regarded as comprising a PNP transistor (BG2) plus an NPN transistor (BG1).
- If a forward voltage is applied involving 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 remains turned off because BG1 has no base current. If a forward voltage is applied for the control electrode currently, BG1 is triggered to create 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 then brought to BG2 for amplification again. Such repeated amplification forms an essential positive feedback, causing both BG1 and BG2 to get into a saturated conduction state quickly. A big current appears within the emitters of these two transistors, which is, the anode and cathode from the thyristor (the size of the current is actually based on the size of the burden and the size of Ea), so the thyristor is entirely switched on. This conduction process is completed in a very short time.
- Right after the thyristor is switched on, its conductive state is going to be maintained through 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 just to trigger the thyristor to turn on. When the thyristor is switched on, the control electrode loses its function.
- The only method to shut off the turned-on thyristor would be to reduce the anode current that it is insufficient to maintain the positive feedback process. The way to reduce 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 referred to as the holding current from the thyristor. Therefore, as it happens, as long as the anode current is lower than the holding current, the thyristor may be turned off.
What is the distinction between a transistor along with a thyristor?
Transistors usually contain a PNP or NPN structure made from three semiconductor materials.
The thyristor is made up of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The work of a 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 widely used in amplification, switches, oscillators, along with other facets of electronic circuits.
Thyristors are mainly used in electronic circuits including 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 controlling the trigger voltage from 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 summarize, although transistors and thyristors can be used in similar applications sometimes, due to their 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 used in frequency converters, motor controllers, welding machines, power supplies, etc.
- Inside the lighting field, thyristors can be used in dimmers and light-weight 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 used in motor controllers.
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