So what is a thyristor?
A thyristor is actually a high-power semiconductor device, also called a silicon-controlled rectifier. Its structure includes 4 levels of semiconductor elements, including three PN junctions corresponding to the Anode, Cathode, and control electrode Gate. These three 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 functioning status. Therefore, thyristors are widely used in different electronic circuits, such as controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversion.
The graphical symbol of the Thyristor is generally represented by 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 light-weight-controlled thyristors. The functioning condition from the thyristor is the fact that each time a forward voltage is used, the gate needs to have a trigger current.
Characteristics of thyristor
- Forward blocking
As shown in Figure a above, when an ahead voltage can be used in between the anode and cathode (the anode is attached to the favorable pole from the power supply, as well as the cathode is connected to the negative pole from the power supply). But no forward voltage is used to the control pole (i.e., K is disconnected), as well as the indicator light does not glow. This shows that the thyristor will not be 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 to the control electrode (called a trigger, as well as the applied voltage is known as trigger voltage), the indicator light switches on. This means that the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, right after the thyristor is switched on, even if the voltage around the control electrode is taken off (which is, K is switched on again), the indicator light still glows. This shows that the thyristor can still conduct. At this time, to be able to shut down the conductive thyristor, the power supply Ea has to be shut down or reversed.
- Reverse blocking
As shown in Figure d above, although a forward voltage is used to the control electrode, a reverse voltage is used in between the anode and cathode, as well as the indicator light does not glow at the moment. This shows that the thyristor will not be conducting and can reverse blocking.
- In conclusion
1) If 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) If the thyristor is subjected to a forward anode voltage, the thyristor will simply conduct when the gate is subjected to a forward voltage. At this time, the thyristor is within the forward conduction state, the thyristor characteristic, which is, the controllable characteristic.
3) If the thyristor is switched on, so long as there exists a specific forward anode voltage, the thyristor will always be switched on regardless of the gate voltage. That is, right after the thyristor is switched on, the gate will lose its function. The gate only functions as a trigger.
4) If the thyristor is on, as well as 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 that a forward voltage ought to be applied in between the anode as well as the cathode, and an appropriate forward voltage also need to be applied in between the gate as well as the cathode. To turn off a conducting thyristor, the forward voltage in between the anode and cathode has to be shut down, or even the voltage has to be reversed.
Working principle of thyristor
A thyristor is basically a unique triode made up of three PN junctions. It may be equivalently thought to be consisting of a PNP transistor (BG2) and an NPN transistor (BG1).
- When a forward voltage is used in between the anode and cathode from the thyristor without applying a forward voltage to the control electrode, although both BG1 and BG2 have forward voltage applied, the thyristor is still turned off because BG1 has no base current. When a forward voltage is used to the control electrode at the moment, BG1 is triggered to create a base 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 sent to BG1 for amplification and after that sent 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 sizable current appears inside the emitters of the two transistors, which is, the anode and cathode from the thyristor (the size of the current is really based on the size of the stress and the size of Ea), so the thyristor is entirely switched on. This conduction process is completed in an exceedingly short period of time.
- Following the thyristor is switched on, its conductive state will likely be maintained by the positive feedback effect from the tube itself. Whether or not the forward voltage from the control electrode disappears, it is still inside the conductive state. Therefore, the function of the control electrode is simply to trigger the thyristor to change 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 lessen the anode current that it is insufficient to keep up the positive feedback process. The way to lessen the anode current is always to shut down the forward power supply Ea or reverse the connection of Ea. The minimum anode current required to keep your thyristor inside the conducting state is known as the holding current from the thyristor. Therefore, strictly speaking, so long as the anode current is lower than the holding current, the thyristor could be turned off.
Exactly what is the difference between a transistor along with a thyristor?
Transistors usually contain 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 the 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 on the gate to change on or off.
Transistors are widely used in amplification, switches, oscillators, and other elements of electronic circuits.
Thyristors are mainly utilized in electronic circuits such as 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 achieve current amplification.
The thyristor is switched on or off by managing 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 sum up, although transistors and thyristors may be used in similar applications in some instances, because of the different structures and functioning 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 could be used to control the current flow to the heating element.
- In electric vehicles, transistors may be used in motor controllers.
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