Just what is a thyristor?
A thyristor is actually a high-power semiconductor device, also called a silicon-controlled rectifier. Its structure consists of 4 levels of semiconductor materials, including 3 PN junctions corresponding for the Anode, Cathode, and control electrode Gate. These 3 poles are the critical parts in 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 functioning 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 a Thyristor is generally represented from the text symbol “V” or “VT” (in older standards, the letters “SCR”). Additionally, derivatives of thyristors include fast thyristors, bidirectional thyristors, reverse conduction thyristors, and light-controlled thyristors. The functioning condition in the thyristor is that each time 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 can be used involving the anode and cathode (the anode is attached to the favorable pole in the power supply, as well as the cathode is connected to the negative pole in the power supply). But no forward voltage is applied for the control pole (i.e., K is disconnected), as well as the indicator light will not illuminate. This demonstrates that the thyristor is not really conducting and contains forward blocking capability.
- Controllable conduction
As shown in Figure b above, when K is closed, as well as a forward voltage is applied for the control electrode (called a trigger, as well as the applied voltage is referred to as trigger voltage), the indicator light switches on. Because of this the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, 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 continue to conduct. At this time, so that you can stop the conductive thyristor, the power supply Ea should be stop 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, as well as 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) If the thyristor is subjected to a reverse anode voltage, the thyristor is in a reverse blocking state whatever voltage the gate is subjected to.
2) If the thyristor is subjected to a forward anode voltage, the thyristor will simply conduct once the gate is subjected to a forward voltage. At this time, the thyristor is in the forward conduction state, the thyristor characteristic, that is certainly, the controllable characteristic.
3) If the thyristor is turned on, provided that there exists a specific forward anode voltage, the thyristor will always be turned on no matter the gate voltage. That is, after the thyristor is turned on, the gate will lose its function. The gate only serves as a trigger.
4) If the thyristor is on, as well as the primary circuit voltage (or current) decreases to close to zero, the thyristor turns off.
5) The disorder for that thyristor to conduct is that a forward voltage needs to be applied involving the anode as well as the cathode, as well as an appropriate forward voltage should also be applied involving the gate as well as the cathode. To change off a conducting thyristor, the forward voltage involving the anode and cathode should be stop, or the voltage should be reversed.
Working principle of thyristor
A thyristor is basically a unique triode made from three PN junctions. It could be equivalently viewed as consisting of a PNP transistor (BG2) as well as an NPN transistor (BG1).
- In case a forward voltage is applied involving 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. In case a forward voltage is applied for the control electrode at this time, BG1 is triggered to generate a base current Ig. BG1 amplifies this current, as well as a ß1Ig current is obtained in their collector. This current is precisely the base current of BG2. After amplification by BG2, a ß1ß2Ig current will be introduced the collector of BG2. This current is sent to BG1 for amplification and then sent to BG2 for amplification again. Such repeated amplification forms a crucial 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 certainly, the anode and cathode in the thyristor (the size of the current is really determined by the size of the burden and the size of Ea), so the thyristor is entirely turned on. This conduction process is done in an exceedingly short time.
- After the thyristor is turned on, its conductive state will be maintained from the positive feedback effect in the tube itself. Whether or not the forward voltage in the control electrode disappears, it is still inside the conductive state. Therefore, the function of the control electrode is only to trigger the thyristor to turn on. Once the thyristor is turned on, the control electrode loses its function.
- The only method to switch off the turned-on thyristor would be to reduce the anode current that it is inadequate to maintain the positive feedback process. The way to reduce the anode current would be to stop the forward power supply Ea or reverse the bond of Ea. The minimum anode current needed to maintain the thyristor inside the conducting state is referred to as the holding current in the thyristor. Therefore, as it happens, provided that the anode current is less than the holding current, the thyristor may be turned off.
Exactly what is the distinction between a transistor as well as a thyristor?
Transistors usually consist of a PNP or NPN structure made from three semiconductor materials.
The thyristor consists of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The job of a transistor relies upon electrical signals to control its opening and closing, allowing fast switching operations.
The thyristor needs a forward voltage as well as a trigger current in the gate to turn on or off.
Transistors are widely used in amplification, switches, oscillators, and other facets of electronic circuits.
Thyristors are mostly used in electronic circuits like controlled rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversions.
Means of working
The transistor controls the collector current by holding the base current to achieve current amplification.
The thyristor is turned on or off by controlling the trigger voltage in the control electrode to realize 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 may be used in similar applications sometimes, because of the different structures and functioning principles, they may have noticeable differences in performance and make use of 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 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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