What exactly is a thyristor?
A thyristor is actually a high-power semiconductor device, also referred to as a silicon-controlled rectifier. Its structure includes four quantities of semiconductor materials, including 3 PN junctions corresponding for the Anode, Cathode, and control electrode Gate. These 3 poles are the critical parts of 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 commonly used in various electronic circuits, like controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversion.
The graphical symbol of the semiconductor device is generally represented by the text symbol “V” or “VT” (in older standards, the letters “SCR”). In addition, derivatives of thyristors also include fast thyristors, bidirectional thyristors, reverse conduction thyristors, and lightweight-controlled thyristors. The working condition of the thyristor is that when 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 is utilized between the anode and cathode (the anode is linked to the favorable pole of the power supply, and the cathode is connected to the negative pole of the power supply). But no forward voltage is used for the control pole (i.e., K is disconnected), and 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, and a forward voltage is used for the control electrode (known as a trigger, and 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, after the thyristor is excited, even if the voltage on the control electrode is taken away (that is, K is excited again), the indicator light still glows. This demonstrates that the thyristor can continue to conduct. At the moment, so that you can stop the conductive thyristor, the power supply Ea has to be stop 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 between the anode and cathode, and the indicator light will not illuminate at this time. This demonstrates that the thyristor is not really conducting and can reverse blocking.
- To sum up
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 if the gate is put through a forward voltage. At the moment, the thyristor is within the forward conduction state, which is the thyristor characteristic, that is, the controllable characteristic.
3) When the thyristor is excited, provided that there exists a specific forward anode voltage, the thyristor will stay excited regardless of the gate voltage. That is certainly, after the thyristor is excited, the gate will lose its function. The gate only serves as a trigger.
4) When the thyristor is on, and the primary circuit voltage (or current) decreases to seal to zero, the thyristor turns off.
5) The disorder for that thyristor to conduct is that a forward voltage needs to be applied between the anode and the cathode, and an appropriate forward voltage should also be applied between the gate and the cathode. To change off a conducting thyristor, the forward voltage between the anode and cathode has to be stop, or perhaps the voltage has to be reversed.
Working principle of thyristor
A thyristor is basically an exclusive triode made up of three PN junctions. It may be equivalently viewed as comprising a PNP transistor (BG2) and an NPN transistor (BG1).
- When a forward voltage is used between the anode and cathode of the thyristor without applying a forward voltage for 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 used for the control electrode at this time, BG1 is triggered to create a base current Ig. BG1 amplifies this current, and 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 then sent to BG2 for amplification again. Such repeated amplification forms an essential positive feedback, causing both BG1 and BG2 to enter a saturated conduction state quickly. A large current appears inside the emitters of these two transistors, that is, the anode and cathode of the thyristor (how big the current is in fact dependant on how big the burden and how big Ea), therefore the thyristor is entirely excited. This conduction process is finished in an exceedingly short time.
- Right after the thyristor is excited, its conductive state will likely be maintained by the positive feedback effect of the tube itself. Even when the forward voltage of the control electrode disappears, it is still inside the conductive state. Therefore, the function of the control electrode is just to trigger the thyristor to turn on. When the thyristor is excited, the control electrode loses its function.
- The only method to shut off the turned-on thyristor is always to lessen the anode current that it is inadequate to keep up the positive feedback process. How you can lessen the anode current is always 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 of the thyristor. Therefore, as it happens, provided that the anode current is under the holding current, the thyristor may be switched off.
Exactly what is the distinction between a transistor and a thyristor?
Transistors usually include a PNP or NPN structure made up of three semiconductor materials.
The thyristor is composed of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The work of the transistor relies upon electrical signals to control its opening and closing, allowing fast switching operations.
The thyristor requires a forward voltage and a trigger current on the gate to turn on or off.
Transistors are commonly used in amplification, switches, oscillators, along with other aspects of electronic circuits.
Thyristors are mostly found 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 excited or off by managing the trigger voltage of the control electrode to realize the switching function.
The circuit parameters of thyristors are based on stability and reliability and usually have higher turn-off voltage and larger on-current.
To sum up, although transistors and thyristors can be used in similar applications in some cases, due to their different structures and working principles, they may have noticeable differences 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.
- Within the lighting field, thyristors can be used in dimmers and lightweight control devices.
- In induction cookers and electric water heaters, thyristors could 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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