Just what is a thyristor?
A thyristor is a high-power semiconductor device, also known as a silicon-controlled rectifier. Its structure contains 4 levels of semiconductor elements, 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 functioning 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 the semiconductor device is usually represented by 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-weight-controlled thyristors. The functioning condition of the thyristor is the fact 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 can be used in between the anode and cathode (the anode is connected to the favorable pole of the power supply, and the cathode is attached 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 fails to illuminate. This implies that the thyristor is not 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 used for the control electrode (referred to as a trigger, and the applied voltage is known as trigger voltage), the indicator light switches on. This means that the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, after the thyristor is excited, whether or not the voltage around the control electrode is taken away (that is certainly, K is excited again), the indicator light still glows. This implies that the thyristor can carry on and conduct. At the moment, in order to 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 for the control electrode, a reverse voltage is used in between the anode and cathode, and the indicator light fails to illuminate currently. This implies that the thyristor is not conducting and will reverse blocking.
- In conclusion
1) When 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) When the thyristor is subjected to a forward anode voltage, the thyristor is only going to conduct when the gate is subjected to a forward voltage. At the moment, the thyristor is incorporated in the forward conduction state, the thyristor characteristic, that is certainly, the controllable characteristic.
3) When the thyristor is excited, so long as there exists a specific forward anode voltage, the thyristor will always be excited no matter the gate voltage. Which is, after the thyristor is excited, 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 close to zero, the thyristor turns off.
5) The problem for the thyristor to conduct is the fact a forward voltage should be applied in between the anode and the cathode, as well as an appropriate forward voltage should also be applied in between the gate and the cathode. To transform off a conducting thyristor, the forward voltage in between the anode and cathode has to be cut off, or even the voltage has to be reversed.
Working principle of thyristor
A thyristor is basically a distinctive triode made from three PN junctions. It may be equivalently thought to be composed of a PNP transistor (BG2) as well as an NPN transistor (BG1).
- In case a forward voltage is used in 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. In case a forward voltage is used for the control electrode currently, BG1 is triggered to produce 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 brought in the collector of BG2. This current is brought to BG1 for amplification and after that brought to BG2 for amplification again. Such repeated amplification forms a vital positive feedback, causing both BG1 and BG2 to get into a saturated conduction state quickly. A large current appears within the emitters of the two transistors, that is certainly, the anode and cathode of the thyristor (how big the current is really dependant on how big the load and how big Ea), so the thyristor is completely excited. This conduction process is done in a very short period of time.
- Right after the thyristor is excited, its conductive state will be maintained by the positive feedback effect of the tube itself. Whether or not the forward voltage of the control electrode disappears, it is still within the conductive state. Therefore, the purpose of the control electrode is simply to trigger the thyristor to change on. When the thyristor is excited, the control electrode loses its function.
- The only way to turn off the turned-on thyristor is always to decrease the anode current that it is inadequate to keep the positive feedback process. The way to decrease the anode current is always to cut off the forward power supply Ea or reverse the connection of Ea. The minimum anode current required to keep your thyristor within the conducting state is known as the holding current of the thyristor. Therefore, as it happens, so long as the anode current is under the holding current, the thyristor could be switched off.
Exactly what is the difference between a transistor as well as a thyristor?
Transistors usually include a PNP or NPN structure made from three semiconductor materials.
The thyristor is made up of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The job of the transistor relies upon electrical signals to control its opening and closing, allowing fast switching operations.
The thyristor demands a forward voltage as well as a trigger current at the gate to change on or off.
Transistors are popular in amplification, switches, oscillators, as well as 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.
Way of working
The transistor controls the collector current by holding the base current to attain 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 functioning principles, they have got noticeable variations 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.
- In the lighting field, thyristors can be used in dimmers and light-weight 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 can be used in motor controllers.
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