Specifically what is a thyristor?
A thyristor is actually 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 to the Anode, Cathode, and control electrode Gate. These 3 poles are definitely 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 commonly used in various electronic circuits, such as controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency alteration.
The graphical symbol of any Thyristor is usually 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 light-controlled thyristors. The functioning condition from the thyristor is the fact that whenever a forward voltage is applied, the gate needs to have a trigger current.
Characteristics of thyristor
- Forward blocking
As shown in Figure a above, when an ahead voltage is utilized between the anode and cathode (the anode is connected to the favorable pole from the power supply, and the cathode is attached to the negative pole from the power supply). But no forward voltage is applied to the control pole (i.e., K is disconnected), and the indicator light does not glow. This demonstrates that the thyristor will not be conducting and contains forward blocking capability.
- Controllable conduction
As shown in Figure b above, when K is closed, along with a forward voltage is applied to the control electrode (known as a trigger, and the applied voltage is called trigger voltage), the indicator light switches on. Which means that the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, following the thyristor is turned on, whether or not the voltage around the control electrode is taken away (that is certainly, K is turned on again), the indicator light still glows. This demonstrates that the thyristor can continue to conduct. At the moment, in order 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 to the control electrode, a reverse voltage is applied between the anode and cathode, and the indicator light does not glow at the moment. This demonstrates that the thyristor will not be conducting and will reverse blocking.
- In conclusion
1) If the thyristor is exposed to a reverse anode voltage, the thyristor is within a reverse blocking state no matter what voltage the gate is exposed to.
2) If the thyristor is exposed to a forward anode voltage, the thyristor is only going to conduct when the gate is exposed to a forward voltage. At the moment, the thyristor is in the forward conduction state, the thyristor characteristic, that is certainly, the controllable characteristic.
3) If the thyristor is turned on, as long as you will find a specific forward anode voltage, the thyristor will always be turned on regardless of the gate voltage. Which is, following the thyristor is turned on, the gate will lose its function. The gate only functions 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 the thyristor to conduct is the fact that a forward voltage needs to be applied between the anode and the cathode, and an appropriate forward voltage also need to be applied between the gate and the cathode. To turn off a conducting thyristor, the forward voltage between the anode and cathode should be cut off, or perhaps the voltage should be reversed.
Working principle of thyristor
A thyristor is essentially a unique triode made from three PN junctions. It may be equivalently viewed as consisting of a PNP transistor (BG2) and an NPN transistor (BG1).
- When a forward voltage is applied between the anode and cathode from the thyristor without applying a forward voltage to the control electrode, although both BG1 and BG2 have forward voltage applied, the thyristor remains turned off because BG1 has no base current. When a forward voltage is applied to the control electrode at the moment, BG1 is triggered to produce basics current Ig. BG1 amplifies this current, along with a ß1Ig current is obtained in the 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 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 sizable current appears inside the emitters of these two transistors, that is certainly, the anode and cathode from the thyristor (the size of the current is in fact based on the size of the load and the size of Ea), therefore the thyristor is entirely turned on. This conduction process is completed in a very short period of time.
- Right after the thyristor is turned on, its conductive state is going to be maintained by the positive feedback effect from the tube itself. Even if the forward voltage from the control electrode disappears, it really is still inside the conductive state. Therefore, the purpose of the control electrode is only to trigger the thyristor to change on. Once the thyristor is turned 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 up the positive feedback process. The way to lessen the anode current is always to cut off the forward power supply Ea or reverse the link of Ea. The minimum anode current required to keep your thyristor inside the conducting state is called 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 along with a thyristor?
Transistors usually contain a PNP or NPN structure made from 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 needs a forward voltage along with a trigger current at the gate to change 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.
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 controlling the trigger voltage from the control electrode to realize 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 in some instances, because of their different structures and functioning principles, they have got noticeable differences in performance and make use of 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 light control devices.
- In induction cookers and electric water heaters, thyristors may be used to control the current flow to the heating element.
- In electric vehicles, transistors may be used in motor controllers.
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