Specifically what is a thyristor?
A thyristor is really a high-power semiconductor device, also known as a silicon-controlled rectifier. Its structure contains four quantities of semiconductor elements, including three PN junctions corresponding towards the Anode, Cathode, and control electrode Gate. These three poles are the critical parts in 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 popular in various electronic circuits, including controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency alteration.
The graphical symbol of a Thyristor is usually represented from the text symbol “V” or “VT” (in older standards, the letters “SCR”). Furthermore, derivatives of thyristors also have fast thyristors, bidirectional thyristors, reverse conduction thyristors, and light-weight-controlled thyristors. The working condition in the thyristor is that each time a forward voltage is used, the gate will need to have a trigger current.
Characteristics of thyristor
- Forward blocking
As shown in Figure a above, when an ahead voltage can be used between the anode and cathode (the anode is linked to the favorable pole in the power supply, and also the cathode is connected to the negative pole in the power supply). But no forward voltage is used towards the control pole (i.e., K is disconnected), and also the indicator light does not light up. This demonstrates that the thyristor will not be 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 used towards the control electrode (referred to as a trigger, and also the applied voltage is called trigger voltage), the indicator light turns on. Which means that the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, right after the thyristor is switched on, even when the voltage in the control electrode is taken away (that is, K is switched on again), the indicator light still glows. This demonstrates that the thyristor can continue to conduct. At this time, to be able to stop the conductive thyristor, the power supply Ea should be stop 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 between the anode and cathode, and also the indicator light does not light up at the moment. This demonstrates that the thyristor will not be conducting and can reverse blocking.
- In summary
1) If the thyristor is put through a reverse anode voltage, the thyristor is within a reverse blocking state no matter what voltage the gate is put through.
2) If the thyristor is put through a forward anode voltage, the thyristor will only conduct if the gate is put through a forward voltage. At this time, the thyristor is incorporated in the forward conduction state, the thyristor characteristic, that is, the controllable characteristic.
3) If the thyristor is switched on, so long as there is a specific forward anode voltage, the thyristor will always be switched on whatever the gate voltage. That is, right after the thyristor is switched on, the gate will lose its function. The gate only serves 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 that a forward voltage needs to be applied between the anode and also the cathode, and an appropriate forward voltage also need to be applied between the gate and also the cathode. To change off a conducting thyristor, the forward voltage between the anode and cathode should be stop, or the voltage should be reversed.
Working principle of thyristor
A thyristor is essentially a distinctive triode made up of three PN junctions. It can be equivalently regarded as comprising a PNP transistor (BG2) and an NPN transistor (BG1).
- If a forward voltage is used between the anode and cathode in the thyristor without applying a forward voltage towards 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 used towards the control electrode at the moment, BG1 is triggered to generate 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 is going to 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 vital positive feedback, causing both BG1 and BG2 to get in a saturated conduction state quickly. A large current appears inside the emitters of these two transistors, that is, the anode and cathode in the thyristor (the size of the current is actually determined by the size of the stress and the size of Ea), and so the thyristor is totally switched on. This conduction process is done in an exceedingly short period of time.
- Right after the thyristor is switched on, its conductive state is going to be maintained from the positive feedback effect in the tube itself. Whether or not the forward voltage in the control electrode disappears, it really is still inside the conductive state. Therefore, the function of the control electrode is only to trigger the thyristor to change on. When the thyristor is switched on, the control electrode loses its function.
- The only way to turn off the turned-on thyristor would be to decrease the anode current that it is insufficient to maintain the positive feedback process. The best way to decrease the anode current would be to stop the forward power supply Ea or reverse the link of Ea. The minimum anode current required to keep the thyristor inside the conducting state is called the holding current in the thyristor. Therefore, as it happens, so long as the anode current is under the holding current, the thyristor could be turned off.
Exactly 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 made up of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The work of a transistor relies on 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 change on or off.
Transistors are popular in amplification, switches, oscillators, along with other elements of electronic circuits.
Thyristors are mainly found in electronic circuits including 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 switched on or off by controlling the trigger voltage in the control electrode to comprehend the switching function.
The circuit parameters of thyristors are related to stability and reliability and often 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 the different structures and working principles, they have noticeable differences in performance and use occasions.
Application scope of thyristor
- In power electronic equipment, thyristors may be used in frequency converters, motor controllers, welding machines, power supplies, etc.
- In the lighting field, thyristors may be used in dimmers and light-weight 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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