So what is a thyristor?
A thyristor is actually a high-power semiconductor device, also referred to as a silicon-controlled rectifier. Its structure contains 4 levels of semiconductor elements, including three PN junctions corresponding to 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 functioning status. Therefore, thyristors are commonly used in various electronic circuits, like controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversion.
The graphical symbol of the Thyristor is usually represented from the text symbol “V” or “VT” (in older standards, the letters “SCR”). Furthermore, derivatives of thyristors also include fast thyristors, bidirectional thyristors, reverse conduction thyristors, and light-weight-controlled thyristors. The functioning condition in the thyristor is the fact that when a forward voltage is used, 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 attached to the favorable pole in the power supply, as well as the cathode is linked to the negative pole in the power supply). But no forward voltage is used to the control pole (i.e., K is disconnected), as well as the indicator light fails to light up. This implies that the thyristor will not be conducting and contains forward blocking capability.
- Controllable conduction
As shown in Figure b above, when K is closed, and a forward voltage is used to the control electrode (referred to as a trigger, as well as the applied voltage is known as trigger voltage), the indicator light switches on. Because of this the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, right after the thyristor is excited, whether or not the voltage in the control electrode is removed (that is certainly, K is excited again), the indicator light still glows. This implies that the thyristor can continue to conduct. At this time, in order to shut down the conductive thyristor, the power supply Ea has to be shut down or reversed.
- Reverse blocking
As shown in Figure d above, although a forward voltage is used to the control electrode, a reverse voltage is used involving the anode and cathode, as well as the indicator light fails to light up at the moment. This implies that the thyristor will not be conducting and may 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 only conduct once the gate is exposed to a forward voltage. At this time, the thyristor is within the forward conduction state, which is the thyristor characteristic, that is certainly, the controllable characteristic.
3) Once the thyristor is excited, as long as there exists a specific forward anode voltage, the thyristor will always be excited regardless of the gate voltage. That is, right after the thyristor is excited, the gate will lose its function. The gate only functions as a trigger.
4) Once the thyristor is on, as well as 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 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 transform off a conducting thyristor, the forward voltage involving the anode and cathode has to be shut down, or perhaps the voltage has to be reversed.
Working principle of thyristor
A thyristor is actually a unique triode composed of three PN junctions. It could be equivalently regarded as consisting of a PNP transistor (BG2) as well as an NPN transistor (BG1).
- When a forward voltage is used involving the anode and cathode in the thyristor without applying a forward voltage to the control electrode, although both BG1 and BG2 have forward voltage applied, the thyristor remains turned off because BG1 has no base current. When a forward voltage is used to the control electrode at the moment, BG1 is triggered to generate basics current Ig. BG1 amplifies this current, and 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 likely be introduced the collector of BG2. This current is sent to BG1 for amplification then sent 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 in the thyristor (how big the current is in fact dependant on how big the stress and how big Ea), so the thyristor is totally excited. This conduction process is finished in a really limited time.
- Following the thyristor is excited, its conductive state will likely be maintained from the positive feedback effect in the tube itself. Whether or not the forward voltage in the control electrode disappears, it really is still within the conductive state. Therefore, the purpose of the control electrode is just to trigger the thyristor to transform on. After the thyristor is excited, the control electrode loses its function.
- The only way to shut off the turned-on thyristor would be to lessen the anode current so that it is insufficient to keep up the positive feedback process. How you can lessen the anode current would be to shut down the forward power supply Ea or reverse the link of Ea. The minimum anode current necessary to keep your thyristor within the conducting state is known as the holding current in the thyristor. Therefore, as it happens, as long as the anode current is under the holding current, the thyristor may be turned off.
What exactly is the distinction between a transistor and a thyristor?
Transistors usually consist of 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 task of the transistor relies upon electrical signals to control its closing and opening, allowing fast switching operations.
The thyristor requires a forward voltage and a trigger current in the gate to transform on or off.
Transistors are commonly used in amplification, switches, oscillators, and other facets of electronic circuits.
Thyristors are mainly utilized in electronic circuits like 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 excited or off by managing the trigger voltage in the control electrode to comprehend the switching function.
The circuit parameters of thyristors are based on stability and reliability and often have higher turn-off voltage and larger on-current.
To summarize, although transistors and thyristors can be used in similar applications sometimes, because of their different structures and functioning principles, they have noticeable differences 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 to the heating element.
- In electric vehicles, transistors can be used in motor controllers.
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