So what is a thyristor?
A thyristor is actually 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 are definitely the critical parts in 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 commonly used in various electronic circuits, such as controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversion.
The graphical symbol of a silicon-controlled rectifier is normally 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 light-controlled thyristors. The operating condition in the thyristor is the fact whenever 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 in between the anode and cathode (the anode is connected to the favorable pole in the power supply, as well as the cathode is linked to the negative pole in the power supply). But no forward voltage is used for the control pole (i.e., K is disconnected), as well as the indicator light fails to light up. This demonstrates that the thyristor is not conducting and contains forward blocking capability.
- Controllable conduction
As shown in Figure b above, when K is closed, along with a forward voltage is used for the control electrode (known as a trigger, as well as the applied voltage is called trigger voltage), the indicator light switches on. Which means that the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, right after the thyristor is turned on, even if the voltage in the control electrode is taken away (that is certainly, K is turned on again), the indicator light still glows. This demonstrates that the thyristor can carry on and conduct. At the moment, to be able to 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 for the control electrode, a reverse voltage is used in between the anode and cathode, as well as the indicator light fails to light up currently. This demonstrates that the thyristor is not conducting and will reverse blocking.
- In conclusion
1) When the thyristor is put through a reverse anode voltage, the thyristor is in a reverse blocking state whatever voltage the gate is put through.
2) When the thyristor is put through a forward anode voltage, the thyristor is only going to conduct when the gate is put through a forward voltage. At the moment, the thyristor is within the forward conduction state, the thyristor characteristic, that is certainly, the controllable characteristic.
3) When the thyristor is turned on, so long as there is a specific forward anode voltage, the thyristor will always be turned on regardless of the gate voltage. Which is, right after the thyristor is turned on, the gate will lose its function. The gate only functions as a trigger.
4) When the thyristor is on, as well as 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 should be applied in between the anode as well as the cathode, as well as an appropriate forward voltage should also be applied in between the gate as well as the cathode. To transform off a conducting thyristor, the forward voltage in between the anode and cathode must be shut down, or even the voltage must be reversed.
Working principle of thyristor
A thyristor is actually a distinctive triode made from three PN junctions. It can be equivalently viewed as comprising a PNP transistor (BG2) as well as an NPN transistor (BG1).
- If a forward voltage is used in between the anode and cathode in 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 used for the control electrode currently, BG1 is triggered to produce 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 will likely be brought in the collector of BG2. This current is delivered to BG1 for amplification and then delivered to BG2 for amplification again. Such repeated amplification forms an essential positive feedback, causing both BG1 and BG2 to get in a saturated conduction state quickly. A large current appears in the emitters of the two transistors, that is certainly, the anode and cathode in the thyristor (how big the current is in fact based on how big the load and how big Ea), and so the thyristor is completely turned on. This conduction process is finished in a very short time.
- Right after the thyristor is turned on, its conductive state will likely be maintained from the positive feedback effect in the tube itself. Even when the forward voltage in the control electrode disappears, it is still in the conductive state. Therefore, the function of the control electrode is only to trigger the thyristor to transform on. Once the thyristor is turned on, the control electrode loses its function.
- The only method to turn off the turned-on thyristor is always to reduce the anode current that it is insufficient to maintain 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 bond of Ea. The minimum anode current needed to keep your thyristor in 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 can be turned off.
Exactly what is the distinction between a transistor along with a thyristor?
Transistors usually include 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 relies on electrical signals to control its closing and opening, allowing fast switching operations.
The thyristor requires a forward voltage along with a trigger current at the gate to transform on or off.
Transistors are commonly used in amplification, switches, oscillators, and other aspects of electronic circuits.
Thyristors are mostly used in electronic circuits such as 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 turned on or off by controlling the trigger voltage in the control electrode to understand the switching function.
The circuit parameters of thyristors are related to stability and reliability and in most cases have higher turn-off voltage and larger on-current.
To summarize, although transistors and thyristors can be used in similar applications in some instances, because of their different structures and operating principles, they have noticeable variations 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.
- In the lighting field, thyristors can be used 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 used in motor controllers.
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