Specifically what is a thyristor?
A thyristor is actually a high-power semiconductor device, also known as a silicon-controlled rectifier. Its structure includes 4 levels of semiconductor materials, including three PN junctions corresponding for the Anode, Cathode, and control electrode Gate. These three poles are definitely the critical parts of 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 popular in various electronic circuits, such as controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency alteration.
The graphical symbol of any semiconductor device is generally 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 lightweight-controlled thyristors. The functioning condition of the thyristor is the fact when a forward voltage is applied, the gate should have a trigger current.
Characteristics of thyristor
- Forward blocking
As shown in Figure a above, when an ahead voltage is used involving 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 applied for the control pole (i.e., K is disconnected), and the indicator light will not light up. This implies 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 applied for the control electrode (called a trigger, and the applied voltage is known as trigger voltage), the indicator light switches on. Which means that the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, following the thyristor is switched on, even when 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 carry on and conduct. At the moment, to be able to cut off the conductive thyristor, the power supply Ea should be cut off or reversed.
- Reverse blocking
As shown in Figure d above, although a forward voltage is applied for the control electrode, a reverse voltage is applied involving the anode and cathode, and the indicator light will not light up currently. This implies that the thyristor is not really conducting and will reverse blocking.
- In summary
1) When the thyristor is put through a reverse anode voltage, the thyristor is within 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 if the gate is put through a forward voltage. At the moment, the thyristor is in the forward conduction state, the thyristor characteristic, that is certainly, the controllable characteristic.
3) When the thyristor is switched on, provided that there is a specific forward anode voltage, the thyristor will remain switched on no matter the gate voltage. That 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 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 ought to be applied involving the anode and the cathode, and an appropriate forward voltage also need to be applied involving the gate and the cathode. To change off a conducting thyristor, the forward voltage involving the anode and cathode should be cut off, or even the voltage should be reversed.
Working principle of thyristor
A thyristor is basically a distinctive triode composed of three PN junctions. It could be equivalently regarded as comprising a PNP transistor (BG2) and an NPN transistor (BG1).
- If a forward voltage is applied involving 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 remains turned off because BG1 has no base current. If a forward voltage is applied for the control electrode currently, BG1 is triggered to produce a base current Ig. BG1 amplifies this current, and 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 brought in the collector of BG2. This current is sent to BG1 for amplification then sent 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 inside the emitters of these two transistors, that is certainly, the anode and cathode of the thyristor (the dimensions of the current is actually determined by the dimensions of the stress and the dimensions of Ea), therefore the thyristor is completely switched on. This conduction process is completed in a very short period of time.
- Right after the thyristor is switched on, its conductive state will likely be maintained from 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 purpose 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 method to shut off the turned-on thyristor is to decrease the anode current so that it is not enough to maintain the positive feedback process. The way to decrease the anode current is to cut off the forward power supply Ea or reverse the link of Ea. The minimum anode current required to keep your thyristor inside the conducting state is known as the holding current of the thyristor. Therefore, strictly speaking, provided that the anode current is less than the holding current, the thyristor can be turned off.
What is the distinction between a transistor and a thyristor?
Transistors usually include a PNP or NPN structure composed of three semiconductor materials.
The thyristor is composed of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The work of any transistor depends on electrical signals to control its closing and opening, allowing fast switching operations.
The thyristor demands a forward voltage and a trigger current in the gate to change on or off.
Transistors are popular in amplification, switches, oscillators, and other aspects of electronic circuits.
Thyristors are mainly found 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 accomplish current amplification.
The thyristor is switched on or off by controlling the trigger voltage of 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 sum up, although transistors and thyristors may be used in similar applications in some instances, because of their different structures and functioning principles, they have noticeable variations 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 lightweight 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 may be used in motor controllers.
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