So what is a thyristor?
A thyristor is actually a high-power semiconductor device, also called a silicon-controlled rectifier. Its structure consists of four quantities of semiconductor components, including 3 PN junctions corresponding for the Anode, Cathode, and control electrode Gate. These 3 poles would be 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 conversion.
The graphical symbol of any silicon-controlled rectifier is usually represented by the text symbol “V” or “VT” (in older standards, the letters “SCR”). Furthermore, derivatives of thyristors include fast thyristors, bidirectional thyristors, reverse conduction thyristors, and lightweight-controlled thyristors. The functioning condition from the thyristor is that 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 is utilized involving the anode and cathode (the anode is attached to the favorable pole from the power supply, and the cathode is linked to the negative pole from the power supply). But no forward voltage is used for the control pole (i.e., K is disconnected), and the indicator light does not light up. This demonstrates 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, and a forward voltage is used for the control electrode (called a trigger, and the applied voltage is known as trigger voltage), the indicator light turns on. This means that the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, following the thyristor is switched on, even when the voltage on the control electrode is taken away (that is, K is switched on again), the indicator light still glows. This demonstrates that the thyristor can carry on and conduct. At this time, in order to stop the conductive thyristor, the power supply Ea must be stop 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 involving the anode and cathode, and the indicator light does not light up at the moment. This demonstrates that the thyristor is not really conducting and will reverse blocking.
- To sum up
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 will only conduct once the gate is subjected to a forward voltage. At this time, the thyristor is in the forward conduction state, the thyristor characteristic, that is, the controllable characteristic.
3) When the thyristor is switched on, so long as you will find a specific forward anode voltage, the thyristor will stay switched on regardless of the gate voltage. That is certainly, following the thyristor is switched on, the gate will lose its function. The gate only serves as a trigger.
4) When the thyristor is on, and the primary circuit voltage (or current) decreases to seal to zero, the thyristor turns off.
5) The disorder for your thyristor to conduct is that a forward voltage needs to be applied involving the anode and the cathode, as well as an appropriate forward voltage also need to be applied involving the gate and the cathode. To transform off a conducting thyristor, the forward voltage involving the anode and cathode must be stop, or perhaps the voltage must be reversed.
Working principle of thyristor
A thyristor is essentially an exclusive triode made up of three PN junctions. It could be equivalently viewed as consisting of a PNP transistor (BG2) as well as an NPN transistor (BG1).
- In case a forward voltage is used involving the anode and cathode from the thyristor without applying a forward voltage for the control electrode, although both BG1 and BG2 have forward voltage applied, the thyristor is still switched off because BG1 has no base current. In case a forward voltage is used for the control electrode at the moment, BG1 is triggered to generate a base 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 sent to BG1 for amplification and after that sent to BG2 for amplification again. Such repeated amplification forms an essential positive feedback, causing both BG1 and BG2 to enter a saturated conduction state quickly. A big current appears in the emitters of the two transistors, that is, the anode and cathode from the thyristor (how big the current is actually based on how big the burden and how big Ea), therefore the thyristor is entirely switched on. This conduction process is done in a really limited time.
- After the thyristor is switched on, its conductive state is going to be maintained by the positive feedback effect from the tube itself. Whether or not the forward voltage from the control electrode disappears, it is still in the conductive state. Therefore, the purpose of the control electrode is simply to trigger the thyristor to change on. When the thyristor is switched on, the control electrode loses its function.
- The only method to switch off the turned-on thyristor would be to decrease the anode current that it is insufficient to keep the positive feedback process. The way to decrease the anode current would be to stop the forward power supply Ea or reverse the connection of Ea. The minimum anode current needed to maintain the thyristor in the conducting state is known as the holding current from the thyristor. Therefore, strictly speaking, so long as the anode current is lower than the holding current, the thyristor could be switched off.
What exactly is the distinction between a transistor and a thyristor?
Transistors usually include a PNP or NPN structure made up of three semiconductor materials.
The thyristor is composed of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The task of any transistor depends on electrical signals to control its closing and opening, allowing fast switching operations.
The thyristor demands a forward voltage and a trigger current at the gate to change on or off.
Transistors are commonly used in amplification, switches, oscillators, as well as other facets of electronic circuits.
Thyristors are mainly utilized 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 accomplish current amplification.
The thyristor is switched on or off by controlling the trigger voltage from the control electrode to understand 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 may be used in similar applications sometimes, because of 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.
- Within 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 for the heating element.
- In electric vehicles, transistors may be used in motor controllers.
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