Specifically what is a thyristor?
A thyristor is actually a high-power semiconductor device, also called a silicon-controlled rectifier. Its structure consists of 4 levels of semiconductor components, including three PN junctions corresponding to the Anode, Cathode, and control electrode Gate. These three poles are definitely 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 functioning status. Therefore, thyristors are popular in different electronic circuits, including controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency alteration.
The graphical symbol of the silicon-controlled rectifier is usually represented through the text symbol “V” or “VT” (in older standards, the letters “SCR”). Additionally, derivatives of thyristors also include fast thyristors, bidirectional thyristors, reverse conduction thyristors, and light-controlled thyristors. The functioning condition of 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 utilized in between the anode and cathode (the anode is attached to the favorable pole of the power supply, and also the cathode is linked to the negative pole of the power supply). But no forward voltage is used to the control pole (i.e., K is disconnected), and also the indicator light does not light up. This shows that the thyristor is not 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 (known as a trigger, and also 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 switched on, whether or not the voltage on the control electrode is taken off (which is, K is switched on again), the indicator light still glows. This shows that the thyristor can carry on and conduct. At this time, to be able 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 in between the anode and cathode, and also the indicator light does not light up currently. This shows that the thyristor is not conducting and will reverse blocking.
- In summary
1) If the thyristor is subjected to a reverse anode voltage, the thyristor is within a reverse blocking state no matter what voltage the gate is subjected to.
2) If the thyristor is subjected to a forward anode voltage, the thyristor will simply conduct if the gate is subjected to a forward voltage. At this time, the thyristor is incorporated in the forward conduction state, the thyristor characteristic, which is, the controllable characteristic.
3) If the thyristor is switched on, as long as you will find a specific forward anode voltage, the thyristor will stay switched on whatever the gate voltage. Which is, after the thyristor is switched on, the gate will lose its function. The gate only functions as a trigger.
4) If the thyristor is on, and also the primary circuit voltage (or current) decreases to seal to zero, the thyristor turns off.
5) The disorder for the thyristor to conduct is the fact that a forward voltage needs to be applied in between the anode and also the cathode, plus 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 shut down, or the voltage has to be reversed.
Working principle of thyristor
A thyristor is basically an exclusive triode made from three PN junctions. It could be equivalently regarded as consisting of a PNP transistor (BG2) plus an NPN transistor (BG1).
- When a forward voltage is used in between the anode and cathode of the thyristor without applying a forward voltage to the control electrode, although both BG1 and BG2 have forward voltage applied, the thyristor is still turned off because BG1 has no base current. When a forward voltage is used to the control electrode currently, BG1 is triggered to produce a base current Ig. BG1 amplifies this current, and 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 be introduced the collector of BG2. This current is sent to BG1 for amplification and then 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 (the size of the current is really based on the size of the burden and the size of Ea), therefore the thyristor is completely switched on. This conduction process is completed in a very short time.
- After the thyristor is switched on, its conductive state will be maintained through the positive feedback effect of the tube itself. Even if the forward voltage of the control electrode disappears, it is actually still inside the conductive state. Therefore, the purpose of the control electrode is simply to trigger the thyristor to transform on. When the thyristor is switched on, the control electrode loses its function.
- The only method to switch off the turned-on thyristor is always to lessen the anode current that it is inadequate to keep the positive feedback process. How you can lessen the anode current is always to shut down the forward power supply Ea or reverse the link of Ea. The minimum anode current required to keep the thyristor inside the conducting state is referred to as the holding current of the thyristor. Therefore, strictly speaking, as long as the anode current is under the holding current, the thyristor could be turned off.
What exactly is the distinction between a transistor and a thyristor?
Transistors usually include a PNP or NPN structure made from three semiconductor materials.
The thyristor consists of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The work of the transistor relies on electrical signals to control its opening and closing, allowing fast switching operations.
The thyristor requires a forward voltage and a trigger current at the gate to transform on or off.
Transistors are popular in amplification, switches, oscillators, as well as other aspects of electronic circuits.
Thyristors are mainly found in electronic circuits including controlled rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversions.
Way 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 managing the trigger voltage of the control electrode to understand the switching function.
The circuit parameters of thyristors are based on stability and reliability and usually have higher turn-off voltage and larger on-current.
To sum up, although transistors and thyristors can be utilized in similar applications sometimes, because of their different structures and functioning principles, they have noticeable differences in performance and utilize 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 could be used to control the current flow to the heating element.
- In electric vehicles, transistors can be utilized in motor controllers.
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