So what is a thyristor?
A thyristor is a high-power semiconductor device, also known as a silicon-controlled rectifier. Its structure contains 4 quantities of semiconductor materials, including 3 PN junctions corresponding to the Anode, Cathode, and control electrode Gate. These 3 poles are definitely the critical parts of 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 working status. Therefore, thyristors are popular in a variety of electronic circuits, such as controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversion.
The graphical symbol of a silicon-controlled rectifier is generally represented by 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 light-weight-controlled thyristors. The working condition of the thyristor is the fact that each time 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 between the anode and cathode (the anode is linked to the favorable pole of the power supply, and also the cathode is attached 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 will 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 to the control electrode (known as a trigger, and also the applied voltage is called trigger voltage), the indicator light switches on. This means that the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, following the thyristor is turned on, even when the voltage on the control electrode is taken off (which is, K is turned on again), the indicator light still glows. This demonstrates that the thyristor can continue to conduct. Currently, in order to shut down the conductive thyristor, the power supply Ea must 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 between the anode and cathode, and also the indicator light will not light up at the moment. This demonstrates that the thyristor is not really conducting and can reverse blocking.
- In conclusion
1) If the thyristor is put through a reverse anode voltage, the thyristor is within a reverse blocking state regardless of what voltage the gate is put through.
2) If the thyristor is put through a forward anode voltage, the thyristor will simply conduct once the gate is put through a forward voltage. Currently, the thyristor is within the forward conduction state, which is the thyristor characteristic, which is, the controllable characteristic.
3) If the thyristor is turned on, as long as there is a specific forward anode voltage, the thyristor will always be turned on whatever the gate voltage. Which is, following the thyristor is turned on, the gate will lose its function. The gate only works as a trigger.
4) If the thyristor is on, and also the primary circuit voltage (or current) decreases to close to zero, the thyristor turns off.
5) The problem for that thyristor to conduct is the fact that a forward voltage ought to be applied between the anode and also the cathode, as well as an appropriate forward voltage should also be applied between the gate and also the cathode. To transform off a conducting thyristor, the forward voltage between the anode and cathode must be shut down, or 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 regarded as comprising a PNP transistor (BG2) as well as an NPN transistor (BG1).
- If a forward voltage is used 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 remains switched off because BG1 has no base current. If a forward voltage is used to 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 the collector. This current is precisely the base current of BG2. After amplification by BG2, a ß1ß2Ig current will 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 big current appears inside the emitters of these two transistors, which is, the anode and cathode of the thyristor (the size of the current is actually based on the size of the burden and the size of Ea), and so 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 will be maintained by the positive feedback effect of the tube itself. Even when 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 change on. When the thyristor is turned on, the control electrode loses its function.
- The only method to switch off the turned-on thyristor would be to decrease the anode current so that it is inadequate to keep up the positive feedback process. How you can decrease the anode current would be to shut down the forward power supply Ea or reverse the connection of Ea. The minimum anode current needed to keep your thyristor inside the conducting state is called the holding current of the thyristor. Therefore, strictly speaking, as long as the anode current is under the holding current, the thyristor may be switched off.
What exactly is the difference between a transistor and a thyristor?
Transistors usually contain 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 work of a transistor relies upon electrical signals to control its opening and closing, allowing fast switching operations.
The thyristor demands a forward voltage and a trigger current on the gate to change on or off.
Transistors are popular in amplification, switches, oscillators, and other aspects of electronic circuits.
Thyristors are mainly 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 achieve current amplification.
The thyristor is turned on or off by manipulating the trigger voltage of the control electrode to understand the switching function.
The circuit parameters of thyristors are related to stability and reliability and in most cases have higher turn-off voltage and larger on-current.
To summarize, although transistors and thyristors can be utilized in similar applications sometimes, because of the different structures and working 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.
- In the lighting field, thyristors can be utilized in dimmers and light-weight 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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