Just what is a thyristor?
A thyristor is actually a high-power semiconductor device, also referred to as a silicon-controlled rectifier. Its structure contains four quantities of semiconductor materials, including three PN junctions corresponding towards 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 popular in a variety of electronic circuits, including controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency alteration.
The graphical symbol of any Thyristor is usually represented from the text symbol “V” or “VT” (in older standards, the letters “SCR”). Additionally, derivatives of thyristors also have fast thyristors, bidirectional thyristors, reverse conduction thyristors, and lightweight-controlled thyristors. The operating condition from the thyristor is that when a forward voltage is used, 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 connected 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 used towards the control pole (i.e., K is disconnected), and also the indicator light will not illuminate. This demonstrates that the thyristor is not really conducting and has forward blocking capability.
- Controllable conduction
As shown in Figure b above, when K is closed, as well as a forward voltage is used towards the control electrode (referred to as a trigger, and also the applied voltage is called trigger voltage), the indicator light turns on. This means that the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, following the thyristor is switched on, whether or not the voltage around the control electrode is taken off (which is, K is switched on again), the indicator light still glows. This demonstrates that the thyristor can continue to conduct. At the moment, so that you can shut down the conductive thyristor, the power supply Ea must be shut down or reversed.
- Reverse blocking
As shown in Figure d above, although a forward voltage is used towards the control electrode, a reverse voltage is used involving the anode and cathode, and also the indicator light will not illuminate at this time. This demonstrates that the thyristor is not really conducting and may reverse blocking.
- In conclusion
1) When the thyristor is exposed to a reverse anode voltage, the thyristor is within a reverse blocking state no matter what voltage the gate is exposed to.
2) When the thyristor is exposed to a forward anode voltage, the thyristor will simply conduct once the gate is exposed to a forward voltage. At the moment, the thyristor is in the forward conduction state, which is the thyristor characteristic, which is, the controllable characteristic.
3) When the thyristor is switched on, as long as there is a specific forward anode voltage, the thyristor will remain switched on whatever the gate voltage. That is, following the thyristor is switched on, the gate will lose its function. The gate only functions as a trigger.
4) When the thyristor is on, and also the primary circuit voltage (or current) decreases to close to zero, the thyristor turns off.
5) The problem for your thyristor to conduct is that a forward voltage ought to be applied involving the anode and also the cathode, and an appropriate forward voltage also need to be applied involving the gate and also the cathode. To turn off a conducting thyristor, the forward voltage involving the anode and cathode must be shut down, or even the voltage must be reversed.
Working principle of thyristor
A thyristor is actually a unique triode made up of three PN junctions. It could be equivalently thought to be consisting of a PNP transistor (BG2) and an NPN transistor (BG1).
- If a forward voltage is used involving the anode and cathode from the thyristor without applying a forward voltage towards the control electrode, although both BG1 and BG2 have forward voltage applied, the thyristor is still switched off because BG1 has no base current. If a forward voltage is used towards the control electrode at this time, BG1 is triggered to generate basics current Ig. BG1 amplifies this current, as well as 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 delivered to BG1 for amplification and after that delivered to BG2 for amplification again. Such repeated amplification forms a crucial positive feedback, causing both BG1 and BG2 to enter 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 in fact determined by the size of the stress and the size of Ea), therefore the thyristor is entirely switched on. This conduction process is done in a really limited time.
- After the thyristor is switched on, its conductive state will be maintained from the positive feedback effect from the tube itself. Whether or not 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 turn on. After the thyristor is switched on, the control electrode loses its function.
- The only way to switch off the turned-on thyristor is always to reduce the anode current so that it is insufficient to keep the positive feedback process. The way to reduce the anode current is always to shut down the forward power supply Ea or reverse the connection of Ea. The minimum anode current necessary to keep the thyristor within the conducting state is called 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 switched off.
What is the distinction between a transistor as well as 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 any transistor relies on electrical signals to control its opening and closing, allowing fast switching operations.
The thyristor needs a forward voltage as well as a trigger current in 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 utilized 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 attain current amplification.
The thyristor is switched on or off by managing the trigger voltage from the control electrode to understand 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 may be used in similar applications sometimes, due to their different structures and operating principles, they may have 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.
- Within 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 towards the heating element.
- In electric vehicles, transistors may be used in motor controllers.
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