What exactly is a thyristor?
A thyristor is a high-power semiconductor device, also called a silicon-controlled rectifier. Its structure includes four levels of semiconductor materials, including 3 PN junctions corresponding towards the Anode, Cathode, and control electrode Gate. These 3 poles are the critical parts of 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 working status. Therefore, thyristors are widely used in various electronic circuits, like controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency alteration.
The graphical symbol of a semiconductor device is usually represented through the text symbol “V” or “VT” (in older standards, the letters “SCR”). Furthermore, derivatives of thyristors also have fast thyristors, bidirectional thyristors, reverse conduction thyristors, and light-controlled thyristors. The working condition of the thyristor is the fact whenever a forward voltage is applied, the gate should 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 connected to the negative pole of the power supply). But no forward voltage is applied towards the control pole (i.e., K is disconnected), and the indicator light will not illuminate. This shows that the thyristor is not really 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 applied towards the control electrode (referred to as a trigger, and the applied voltage is known as trigger voltage), the indicator light turns on. Because of this the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, after the thyristor is turned on, even if the voltage in the control electrode is removed (which is, K is turned on again), the indicator light still glows. This shows that the thyristor can carry on and conduct. Currently, 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 applied towards the control electrode, a reverse voltage is applied in between the anode and cathode, and the indicator light will not illuminate at the moment. This shows that the thyristor is not really conducting and may reverse blocking.
- In summary
1) If the thyristor is put through a reverse anode voltage, the thyristor is within a reverse blocking state regardless of what voltage the gate is put through.
2) If the thyristor is put through a forward anode voltage, the thyristor will only conduct if the gate is put through a forward voltage. Currently, the thyristor is within the forward conduction state, the thyristor characteristic, which is, the controllable characteristic.
3) If the thyristor is turned on, as long as there is a specific forward anode voltage, the thyristor will remain turned on no matter the gate voltage. That is certainly, after the thyristor is turned on, the gate will lose its function. The gate only works as a trigger.
4) If the thyristor is on, and the primary circuit voltage (or current) decreases to close to zero, the thyristor turns off.
5) The problem for your thyristor to conduct is the fact a forward voltage needs to be applied in between the anode and the cathode, and an appropriate forward voltage ought to be applied in between the gate and the cathode. To change off a conducting thyristor, the forward voltage in between the anode and cathode has to be shut down, or even the voltage has to be reversed.
Working principle of thyristor
A thyristor is actually an exclusive triode made up of three PN junctions. It may be equivalently viewed as composed of a PNP transistor (BG2) and an NPN transistor (BG1).
- In case a forward voltage is applied in between the anode and cathode of the thyristor without applying a forward voltage towards the control electrode, although both BG1 and BG2 have forward voltage applied, the thyristor continues to be turned off because BG1 has no base current. In case a forward voltage is applied towards the control electrode at the moment, BG1 is triggered to produce 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 is going to 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 vital positive feedback, causing both BG1 and BG2 to enter a saturated conduction state quickly. A large current appears inside the emitters of these two transistors, which is, the anode and cathode of the thyristor (how big the current is really based on how big the load and how big 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 is going to be maintained through the positive feedback effect of the tube itself. Whether or not the forward voltage of the control electrode disappears, it really is still inside the conductive state. Therefore, the function of the control electrode is just to trigger the thyristor to transform on. When the thyristor is turned on, the control electrode loses its function.
- The best way to turn off the turned-on thyristor is always to lessen the anode current that it is not enough to keep the positive feedback process. The way to lessen the anode current is always to shut down the forward power supply Ea or reverse the bond of Ea. The minimum anode current needed to keep the thyristor inside the conducting state is known as the holding current of the thyristor. Therefore, as it happens, as long as the anode current is lower than the holding current, the thyristor could be turned off.
What is the difference between a transistor as well as a thyristor?
Transistors usually consist of a PNP or NPN structure made up of three semiconductor materials.
The thyristor is made up 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 opening and closing, allowing fast switching operations.
The thyristor needs a forward voltage as well as a trigger current on the gate to transform on or off.
Transistors are widely used in amplification, switches, oscillators, and other elements of electronic circuits.
Thyristors are mostly found 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 attain current amplification.
The thyristor is turned on or off by manipulating the trigger voltage of the control electrode to understand 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 cases, because of 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.
- In the lighting field, thyristors can be utilized in dimmers and light control devices.
- In induction cookers and electric water heaters, thyristors could 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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