What exactly is a thyristor?
A thyristor is a high-power semiconductor device, also known as a silicon-controlled rectifier. Its structure includes 4 quantities of semiconductor components, including 3 PN junctions corresponding towards the Anode, Cathode, and control electrode Gate. These 3 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 functioning status. Therefore, thyristors are commonly used in different electronic circuits, such as controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency alteration.
The graphical symbol of a Thyristor 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 lightweight-controlled thyristors. The functioning condition from the thyristor is the fact that each time 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 is utilized in between the anode and cathode (the anode is linked to the favorable pole from the power supply, as well as the cathode is attached to the negative pole from the power supply). But no forward voltage is applied towards the control pole (i.e., K is disconnected), as well as the indicator light does not glow. This demonstrates that the thyristor is not really conducting and has forward blocking capability.
- Controllable conduction
As shown in Figure b above, when K is closed, and a forward voltage is applied towards the control electrode (referred to as a trigger, as well as the applied voltage is referred to as trigger voltage), the indicator light switches on. Which means that the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, after the thyristor is excited, even if the voltage in the control electrode is removed (that is certainly, K is excited again), the indicator light still glows. This demonstrates that the thyristor can still conduct. Currently, to be able to cut off the conductive thyristor, the power supply Ea should 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, as well as the indicator light does not glow currently. This demonstrates that the thyristor is not really conducting and will reverse blocking.
- To sum up
1) When the thyristor is put through a reverse anode voltage, the thyristor is at a reverse blocking state whatever voltage the gate is put through.
2) When the thyristor is put through a forward anode voltage, the thyristor is only going to conduct when the gate is put through a forward voltage. Currently, the thyristor is incorporated in the forward conduction state, the thyristor characteristic, that is certainly, the controllable characteristic.
3) When the thyristor is excited, as long as you will find a specific forward anode voltage, the thyristor will stay excited whatever the gate voltage. That 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, as well as the primary circuit voltage (or current) decreases to close to zero, the thyristor turns off.
5) The condition for the thyristor to conduct is the fact that a forward voltage ought to be applied in between the anode as well as the cathode, as well as an appropriate forward voltage should also be applied in between the gate as well as the cathode. To change off a conducting thyristor, the forward voltage in between the anode and cathode should be cut off, or the voltage should be reversed.
Working principle of thyristor
A thyristor is basically a unique triode made up of three PN junctions. It may be equivalently viewed as composed of a PNP transistor (BG2) as well as an NPN transistor (BG1).
- If a forward voltage is applied 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 turned off because BG1 has no base current. If a forward voltage is applied towards the control electrode currently, BG1 is triggered to produce 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 is going to be brought in the collector of BG2. This current is delivered to BG1 for amplification and after that delivered to BG2 for amplification again. Such repeated amplification forms a crucial positive feedback, causing both BG1 and BG2 to get in a saturated conduction state quickly. A sizable current appears in the emitters of these two transistors, that is certainly, the anode and cathode from the thyristor (the size of the current is really based on the size of the stress and the size of Ea), so the thyristor is totally excited. This conduction process is done in a really short period of time.
- After the thyristor is excited, its conductive state is going to be maintained through the positive feedback effect from the tube itself. Whether or not the forward voltage from the control electrode disappears, it really is still in the conductive state. Therefore, the function of the control electrode is only to trigger the thyristor to turn on. When the thyristor is excited, the control electrode loses its function.
- The only way to switch off the turned-on thyristor would be to reduce the anode current that it is not enough to keep up the positive feedback process. The best way to reduce the anode current would be to cut off the forward power supply Ea or reverse the link of Ea. The minimum anode current necessary to keep the thyristor in the conducting state is referred to as the holding current from the thyristor. Therefore, as it happens, as long as the anode current is less than the holding current, the thyristor could be turned off.
What is the difference between a transistor and a thyristor?
Transistors usually consist of a PNP or NPN structure made up of three semiconductor materials.
The thyristor consists of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The job of a transistor depends on electrical signals to control its opening and closing, allowing fast switching operations.
The thyristor needs a forward voltage and a trigger current in the gate to turn on or off.
Transistors are commonly used in amplification, switches, oscillators, as well as other elements of electronic circuits.
Thyristors are mostly found in electronic circuits such as 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 attain current amplification.
The thyristor is excited or off by controlling the trigger voltage from the control electrode to comprehend the switching function.
The circuit parameters of thyristors are related to stability and reliability and often have higher turn-off voltage and larger on-current.
To summarize, although transistors and thyristors may be used in similar applications in some cases, due to their different structures and functioning principles, they may have noticeable variations in performance and utilize occasions.
Application scope of thyristor
- In power electronic equipment, thyristors may be used in frequency converters, motor controllers, welding machines, power supplies, etc.
- Inside the lighting field, thyristors may be used in dimmers and lightweight 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 may be used in motor controllers.
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