So what is a thyristor?
A thyristor is really a high-power semiconductor device, also called a silicon-controlled rectifier. Its structure includes 4 quantities of semiconductor elements, including three PN junctions corresponding towards the Anode, Cathode, and control electrode Gate. These three poles are definitely the critical parts in 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 operating status. Therefore, thyristors are popular in different electronic circuits, including controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency alteration.
The graphical symbol of a semiconductor device is usually 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 lightweight-controlled thyristors. The operating condition in the thyristor is the fact 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 used involving the anode and cathode (the anode is connected to the favorable pole in the power supply, and the cathode is linked to the negative pole in the power supply). But no forward voltage is used towards the control pole (i.e., K is disconnected), and the indicator light will not light up. This demonstrates that the thyristor will not be 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 used towards the control electrode (referred to as a trigger, and the applied voltage is called trigger voltage), the indicator light turns on. Which means that the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, right after the thyristor is turned on, even if the voltage on the control electrode is removed (that is certainly, K is turned on again), the indicator light still glows. This demonstrates that the thyristor can still conduct. At this time, so that you can cut off the conductive thyristor, the power supply Ea has to be cut off or reversed.
- Reverse blocking
As shown in Figure d above, although a forward voltage is used towards the control electrode, a reverse voltage is used involving the anode and cathode, and the indicator light will not light up at this time. This demonstrates that the thyristor will not be conducting and will reverse blocking.
- In conclusion
1) Once the thyristor is exposed to a reverse anode voltage, the thyristor is in a reverse blocking state no matter what voltage the gate is exposed to.
2) Once the thyristor is exposed to a forward anode voltage, the thyristor will simply conduct if the gate is exposed to a forward voltage. At this time, the thyristor is within the forward conduction state, the thyristor characteristic, that is certainly, the controllable characteristic.
3) Once the thyristor is turned on, as long as you will find a specific forward anode voltage, the thyristor will remain turned on no matter the gate voltage. That is, right after the thyristor is turned on, the gate will lose its function. The gate only functions as a trigger.
4) Once the thyristor is on, and 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 should be applied involving the anode and the cathode, as well as an appropriate forward voltage ought to be applied involving the gate and the cathode. To transform off a conducting thyristor, the forward voltage involving the anode and cathode has to be cut off, or perhaps the voltage has to be reversed.
Working principle of thyristor
A thyristor is basically a distinctive triode composed of three PN junctions. It could be equivalently viewed as comprising a PNP transistor (BG2) as well as an NPN transistor (BG1).
- In case a forward voltage is used involving the anode and cathode in the thyristor without applying a forward voltage towards the control electrode, although both BG1 and BG2 have forward voltage applied, the thyristor remains turned off because BG1 has no base current. In case a forward voltage is used towards the control electrode at this time, BG1 is triggered to generate basics current Ig. BG1 amplifies this current, as well as 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 be introduced the collector of BG2. This current is delivered to BG1 for amplification and after that delivered 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 big current appears within the emitters of the two transistors, that is certainly, the anode and cathode in the thyristor (the size of the current is actually dependant on the size of the load and the size of Ea), and so the thyristor is completely turned on. This conduction process is completed in a really limited time.
- Following the thyristor is turned on, its conductive state will be maintained by the positive feedback effect in the tube itself. Whether or not the forward voltage in the control electrode disappears, it is still within the conductive state. Therefore, the function of the control electrode is simply to trigger the thyristor to turn on. After the thyristor is turned on, the control electrode loses its function.
- The best way to switch off the turned-on thyristor would be to reduce the anode current that it is not enough to keep the positive feedback process. The way to reduce the anode current would be to cut off the forward power supply Ea or reverse the bond of Ea. The minimum anode current needed to keep the thyristor within the conducting state is called the holding current in the thyristor. Therefore, strictly speaking, as long as the anode current is less than the holding current, the thyristor could be turned off.
Exactly what is the difference between a transistor as well as a thyristor?
Transistors usually consist of a PNP or NPN structure composed of three semiconductor materials.
The thyristor consists of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The task of a 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 on the gate to turn on or off.
Transistors are popular in amplification, switches, oscillators, as well as other facets of electronic circuits.
Thyristors are mostly utilized in electronic circuits including 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 turned on or off by controlling the trigger voltage in 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 summarize, although transistors and thyristors may be used in similar applications sometimes, because of their different structures and operating 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.
- Within 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 towards the heating element.
- In electric vehicles, transistors may be used in motor controllers.
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