What is a thyristor?
A thyristor is actually a high-power semiconductor device, also called a silicon-controlled rectifier. Its structure includes 4 quantities of semiconductor components, including three PN junctions corresponding towards the Anode, Cathode, and control electrode Gate. These three poles are definitely 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 popular in a variety of electronic circuits, including controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversion.
The graphical symbol of the silicon-controlled rectifier is usually represented through 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 operating condition from the thyristor is the fact that when a forward voltage is used, 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 attached 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 used towards the control pole (i.e., K is disconnected), and also the indicator light will not light up. This implies 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 (known as 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 turned on, even if the voltage around the control electrode is removed (that is certainly, K is turned on again), the indicator light still glows. This implies that the thyristor can still conduct. At this time, to be able to shut down the conductive thyristor, the power supply Ea should be shut down 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 in between the anode and cathode, and also the indicator light will not light up at the moment. This implies that the thyristor will not be conducting and will reverse blocking.
- In conclusion
1) When the thyristor is put through a reverse anode voltage, the thyristor is within 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 this time, the thyristor is in the forward conduction state, the thyristor characteristic, that is certainly, the controllable characteristic.
3) When the thyristor is turned on, provided that you will find a specific forward anode voltage, the thyristor will always be turned on regardless of the gate voltage. That is certainly, following the thyristor is turned on, the gate will lose its function. The gate only serves 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 that thyristor to conduct is the fact that a forward voltage should be applied in between the anode and also the cathode, plus an appropriate forward voltage ought to be applied in between the gate and also the cathode. To transform off a conducting thyristor, the forward voltage in between the anode and cathode should be shut down, or the voltage should be reversed.
Working principle of thyristor
A thyristor is basically a distinctive triode composed of three PN junctions. It can be equivalently viewed as consisting of a PNP transistor (BG2) plus an NPN transistor (BG1).
- If a forward voltage is used in between the anode and cathode from the thyristor without applying a forward voltage towards the control electrode, although both BG1 and BG2 have forward voltage applied, the thyristor remains switched off because BG1 has no base current. If a forward voltage is used towards the control electrode at the moment, BG1 is triggered to create 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 brought in the collector of BG2. This current is sent to BG1 for amplification and after that sent to BG2 for amplification again. Such repeated amplification forms a crucial positive feedback, causing both BG1 and BG2 to get into a saturated conduction state quickly. A big current appears within the emitters of these two transistors, that is certainly, the anode and cathode from the thyristor (the dimensions of the current is actually based on the dimensions of the load and the dimensions of Ea), so the thyristor is totally turned on. This conduction process is finished in a very limited time.
- After the thyristor is turned on, its conductive state will likely be maintained through the positive feedback effect from the tube itself. Whether or not the forward voltage from the control electrode disappears, it is still within the conductive state. Therefore, the function of the control electrode is only to trigger the thyristor to turn on. When the thyristor is turned on, the control electrode loses its function.
- The best way to switch off the turned-on thyristor is to lessen the anode current so that it is not enough to keep the positive feedback process. The best way to lessen the anode current is to shut down the forward power supply Ea or reverse the connection of Ea. The minimum anode current needed to keep the thyristor within the conducting state is known as the holding current from the thyristor. Therefore, strictly speaking, provided that the anode current is under the holding current, the thyristor can be switched 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 job of the transistor relies on electrical signals to control its closing and opening, allowing fast switching operations.
The thyristor demands a forward voltage as well as a trigger current on the gate to turn on or off.
Transistors are popular in amplification, switches, oscillators, along with other facets of electronic circuits.
Thyristors are mainly found in electronic circuits including 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 managing the trigger voltage from 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 sum up, although transistors and thyristors can be used in similar applications in some cases, because of the different structures and operating principles, they have noticeable differences in performance and use 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 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 can be used in motor controllers.
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