What is a thyristor?
A thyristor is really a high-power semiconductor device, also called a silicon-controlled rectifier. Its structure contains four quantities of semiconductor components, including 3 PN junctions corresponding towards 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 working status. Therefore, thyristors are widely used in different electronic circuits, like controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversion.
The graphical symbol of the semiconductor device is generally represented through the text symbol “V” or “VT” (in older standards, the letters “SCR”). In addition, derivatives of thyristors also have fast thyristors, bidirectional thyristors, reverse conduction thyristors, and light-controlled thyristors. The working condition in the thyristor is that whenever a forward voltage is applied, the gate needs to have a trigger current.
Characteristics of thyristor
- Forward blocking
As shown in Figure a above, when an ahead voltage is used in between the anode and cathode (the anode is attached to the favorable pole in the power supply, and also the cathode is connected to the negative pole in the power supply). But no forward voltage is applied towards the control pole (i.e., K is disconnected), and also the indicator light will not light up. 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, along with a forward voltage is applied towards the control electrode (known as a trigger, and also the applied voltage is called trigger voltage), the indicator light turns on. Because of this the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, right after the thyristor is excited, even if the voltage around the control electrode is removed (that is, K is excited again), the indicator light still glows. This implies that the thyristor can carry on and conduct. Currently, to be able 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 applied towards the control electrode, a reverse voltage is applied in between the anode and cathode, and also the indicator light will not light up currently. This implies that the thyristor is not conducting and can reverse blocking.
- In conclusion
1) Once the thyristor is exposed to a reverse anode voltage, the thyristor is at 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 when the gate is exposed to a forward voltage. Currently, the thyristor is within the forward conduction state, the thyristor characteristic, that is, the controllable characteristic.
3) Once the thyristor is excited, as long as you will find a specific forward anode voltage, the thyristor will always be excited regardless of the gate voltage. Which 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, and also the primary circuit voltage (or current) decreases to seal to zero, the thyristor turns off.
5) The condition for the thyristor to conduct is that a forward voltage ought to be applied in between the anode and also the cathode, as well as an appropriate forward voltage ought to be applied in between the gate and also the cathode. To turn off a conducting thyristor, the forward voltage in between the anode and cathode has to be cut off, or the voltage has to be reversed.
Working principle of thyristor
A thyristor is actually an exclusive triode composed of three PN junctions. It can be equivalently viewed as comprising a PNP transistor (BG2) as well as an NPN transistor (BG1).
- When a forward voltage is applied in between 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 is still switched off because BG1 has no base current. When a forward voltage is applied towards the control electrode currently, BG1 is triggered to produce a base current Ig. BG1 amplifies this current, along with 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 likely be introduced 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 sizable current appears within the emitters of these two transistors, that is, the anode and cathode in the thyristor (the size of the current is in fact based on the size of the burden and the size of Ea), and so the thyristor is totally excited. This conduction process is finished in a really short period of time.
- After the thyristor is excited, its conductive state will likely be maintained through the positive feedback effect in the tube itself. Even if the forward voltage in the control electrode disappears, it really 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 excited, the control electrode loses its function.
- The best way to turn off the turned-on thyristor is always to decrease the anode current that it is inadequate to maintain the positive feedback process. The way to decrease the anode current is always to cut off the forward power supply Ea or reverse the link of Ea. The minimum anode current necessary to keep the thyristor within the conducting state is called the holding current in the thyristor. Therefore, as it happens, as long as the anode current is less than the holding current, the thyristor can be switched off.
What exactly is the distinction between a transistor along with a thyristor?
Transistors usually include 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 requires a forward voltage along with a trigger current on the gate to turn on or off.
Transistors are widely used in amplification, switches, oscillators, along with 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 manipulating the trigger voltage in the control electrode to comprehend 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 sum up, although transistors and thyristors can be utilized in similar applications in some instances, due to their different structures and working principles, they may have noticeable differences in performance and make use of occasions.
Application scope of thyristor
- In power electronic equipment, thyristors can be utilized in frequency converters, motor controllers, welding machines, power supplies, etc.
- Within the lighting field, thyristors can be utilized 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 utilized in motor controllers.
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