So what is a thyristor?
A thyristor is really a high-power semiconductor device, also referred to as a silicon-controlled rectifier. Its structure consists of 4 levels 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, 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 widely used in a variety of electronic circuits, including controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency alteration.
The graphical symbol of a silicon-controlled rectifier is generally represented by the text symbol “V” or “VT” (in older standards, the letters “SCR”). Additionally, 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 should have a trigger current.
Characteristics of thyristor
- Forward blocking
As shown in Figure a above, when an ahead voltage can be used involving the anode and cathode (the anode is connected to the favorable pole from the power supply, and the cathode is connected 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 fails to light up. This implies 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, along with a forward voltage is used for the control electrode (called a trigger, and the applied voltage is referred to 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 switched on, even if the voltage in the control electrode is removed (that is certainly, K is switched on again), the indicator light still glows. This implies that the thyristor can continue to conduct. At the moment, 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 used for the control electrode, a reverse voltage is used involving the anode and cathode, and the indicator light fails to light up at this time. This implies 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 no matter what voltage the gate is subjected to.
2) When the thyristor is subjected to a forward anode voltage, the thyristor is only going to conduct when the gate is subjected to a forward voltage. At the moment, the thyristor is incorporated in the forward conduction state, the thyristor characteristic, that is certainly, 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, after the thyristor is switched on, the gate will lose its function. The gate only works as a trigger.
4) When 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 that a forward voltage needs to be applied involving the anode and the cathode, plus an appropriate forward voltage ought to be applied involving the gate and the cathode. To change off a conducting thyristor, the forward voltage involving the anode and cathode has to be shut down, or perhaps the voltage has to be reversed.
Working principle of thyristor
A thyristor is basically an exclusive triode made up of three PN junctions. It could be equivalently thought to be consisting of a PNP transistor (BG2) plus 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 continues to be turned off because BG1 has no base current. In case a forward voltage is used for the control electrode at this time, BG1 is triggered to generate basics current Ig. BG1 amplifies this current, along with 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 introduced the collector of BG2. This current is brought to BG1 for amplification and then brought 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 in the emitters of these two transistors, that is certainly, the anode and cathode from the thyristor (the size of the current is really dependant on the size of the stress and the size of Ea), therefore the thyristor is completely switched on. This conduction process is finished in a really short time.
- Following the thyristor is switched on, its conductive state is going to be maintained by the positive feedback effect from the tube itself. Even when the forward voltage from the control electrode disappears, it is still in the conductive state. Therefore, the purpose of the control electrode is just to trigger the thyristor to transform on. Once the thyristor is switched on, the control electrode loses its function.
- The best way to turn off the turned-on thyristor is always to lessen the anode current so that it is not enough 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 necessary to maintain the thyristor in the conducting state is referred to as the holding current from the thyristor. Therefore, strictly speaking, so long as the anode current is lower than the holding current, the thyristor can be turned off.
What is the difference between a transistor along with a thyristor?
Transistors usually contain 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 work of a transistor relies upon electrical signals to control its closing and opening, allowing fast switching operations.
The thyristor demands a forward voltage along with a trigger current in the gate to transform on or off.
Transistors are widely used in amplification, switches, oscillators, as well as other aspects of electronic circuits.
Thyristors are mostly utilized in electronic circuits including 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 switched on or off by manipulating 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 sum up, although transistors and thyristors can be utilized in similar applications sometimes, due to their different structures and functioning principles, they have got noticeable variations 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 lightweight control devices.
- In induction cookers and electric water heaters, thyristors could be used to control the current flow for the heating element.
- In electric vehicles, transistors can be utilized in motor controllers.
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