So what is a thyristor?
A thyristor is really a high-power semiconductor device, also known as a silicon-controlled rectifier. Its structure consists of 4 quantities of semiconductor components, including 3 PN junctions corresponding to the Anode, Cathode, and control electrode Gate. These 3 poles are 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 widely used in various electronic circuits, like controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency alteration.
The graphical symbol of the silicon-controlled rectifier is usually represented from 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 operating condition from the thyristor is the fact that whenever 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 in between the anode and cathode (the anode is linked to the favorable pole from the power supply, and also the cathode is attached to the negative pole from the power supply). But no forward voltage is applied to the control pole (i.e., K is disconnected), and also the indicator light will not 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, as well as a forward voltage is applied to the control electrode (called a trigger, and also the applied voltage is known as 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, even if the voltage in the control electrode is taken off (that is certainly, K is switched on again), the indicator light still glows. This implies that the thyristor can continue to conduct. Currently, 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 applied to the control electrode, a reverse voltage is applied in between the anode and cathode, and also the indicator light will not light up at this time. This implies 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 no matter what voltage the gate is put through.
2) When the thyristor is put through a forward anode voltage, the thyristor is only going to conduct once the gate is put through a forward voltage. Currently, the thyristor is within the forward conduction state, the thyristor characteristic, that is certainly, the controllable characteristic.
3) When the thyristor is switched on, provided that there exists a specific forward anode voltage, the thyristor will remain switched on regardless of the gate voltage. Which is, following the thyristor is switched on, the gate will lose its function. The gate only works as a trigger.
4) When the thyristor is on, and also the primary circuit voltage (or current) decreases to seal to zero, the thyristor turns off.
5) The disorder for your thyristor to conduct is the fact that a forward voltage needs to be applied in between the anode and also the cathode, plus an appropriate forward voltage should also be applied in between the gate and also the cathode. To turn off a conducting thyristor, the forward voltage in between the anode and cathode must be shut down, or perhaps the voltage must be reversed.
Working principle of thyristor
A thyristor is actually a distinctive triode made up of three PN junctions. It may be equivalently viewed as composed of a PNP transistor (BG2) plus an NPN transistor (BG1).
- When a forward voltage is applied 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 continues to be switched off because BG1 has no base current. When a forward voltage is applied to the control electrode at this time, BG1 is triggered to produce a base current Ig. BG1 amplifies this current, as well as a ß1Ig current is obtained in its collector. This current is precisely the base current of BG2. After amplification by BG2, a ß1ß2Ig current will likely be introduced 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 in the emitters of these two transistors, that is certainly, the anode and cathode from the thyristor (the dimensions of the current is really dependant on the dimensions of the burden and the dimensions of Ea), so the thyristor is entirely switched on. This conduction process is completed in a really short time.
- After the thyristor is switched on, its conductive state will likely be maintained from the positive feedback effect from the tube itself. Even if the forward voltage from the control electrode disappears, it is actually still in the conductive state. Therefore, the function of the control electrode is just to trigger the thyristor to turn on. After the thyristor is switched on, the control electrode loses its function.
- The best way to turn off the turned-on thyristor is always to decrease the anode current so that it is inadequate to maintain the positive feedback process. The way to decrease the anode current is always to shut down the forward power supply Ea or reverse the link of Ea. The minimum anode current required to keep your thyristor in the conducting state is known as the holding current from the thyristor. Therefore, strictly speaking, provided that the anode current is less than the holding current, the thyristor may be switched off.
What exactly is the distinction between a transistor as well as 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 job of the transistor relies on electrical signals to control its closing and opening, allowing fast switching operations.
The thyristor needs a forward voltage as well as a trigger current at the gate to turn on or off.
Transistors are widely used in amplification, switches, oscillators, as well as other aspects of electronic circuits.
Thyristors are mostly used in electronic circuits like 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 manipulating the trigger voltage from the control electrode to understand the switching function.
The circuit parameters of thyristors are based on 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 sometimes, because of their different structures and operating principles, they may have noticeable variations in performance and make use of 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 can be used to control the current flow to the heating element.
- In electric vehicles, transistors can be used in motor controllers.
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