What is a thyristor?
A thyristor is a high-power semiconductor device, also called a silicon-controlled rectifier. Its structure contains four quantities of semiconductor elements, including 3 PN junctions corresponding to the Anode, Cathode, and control electrode Gate. These 3 poles are 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 operating status. Therefore, thyristors are commonly used in various electronic circuits, like controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversion.
The graphical symbol of a semiconductor device is usually represented through the text symbol “V” or “VT” (in older standards, the letters “SCR”). Additionally, derivatives of thyristors also have fast thyristors, bidirectional thyristors, reverse conduction thyristors, and light-controlled thyristors. The operating condition of the thyristor is that whenever a forward voltage is applied, the gate will need to 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 of the power supply, and also the cathode is connected to the negative pole of the power supply). But no forward voltage is applied to the control pole (i.e., K is disconnected), and also the indicator light does not glow. This demonstrates that the thyristor is not really conducting and contains forward blocking capability.
- Controllable conduction
As shown in Figure b above, when K is closed, as well as a forward voltage is applied to the control electrode (referred to as a trigger, and also the applied voltage is referred to as trigger voltage), the indicator light turns on. Which means that the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, following the thyristor is turned on, even if the voltage on the control electrode is taken off (that is certainly, K is turned on again), the indicator light still glows. This demonstrates that the thyristor can still conduct. At the moment, so that you can cut off the conductive thyristor, the power supply Ea must 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 involving the anode and cathode, and also the indicator light does not glow at the moment. This demonstrates that the thyristor is not really conducting and can reverse blocking.
- In conclusion
1) If the thyristor is exposed to a reverse anode voltage, the thyristor is at 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 will simply conduct once the gate is exposed to a forward voltage. At the moment, the thyristor is incorporated in the forward conduction state, which is the thyristor characteristic, that is certainly, the controllable characteristic.
3) If the thyristor is turned on, provided that there exists a specific forward anode voltage, the thyristor will stay turned on regardless of the gate voltage. That 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 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 that a forward voltage should be applied involving the anode and also the cathode, plus an appropriate forward voltage ought to be applied involving the gate and also the cathode. To transform off a conducting thyristor, the forward voltage involving the anode and cathode must be cut off, or even the voltage must be reversed.
Working principle of thyristor
A thyristor is actually a unique triode composed of three PN junctions. It could be equivalently thought to be consisting of a PNP transistor (BG2) plus an NPN transistor (BG1).
- When a forward voltage is applied involving 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 switched 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, as well as 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 sent to BG1 for amplification and then 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 sizable current appears in the emitters of these two transistors, that is certainly, the anode and cathode of the thyristor (the dimensions of the current is in fact based on the dimensions of the burden and the dimensions of Ea), so the thyristor is completely turned on. This conduction process is done in an exceedingly limited time.
- Following the thyristor is turned on, its conductive state is going to be maintained through the positive feedback effect of the tube itself. Even when the forward voltage of the control electrode disappears, it is still in the conductive state. Therefore, the function of the control electrode is just to trigger the thyristor to change on. After the thyristor is turned on, the control electrode loses its function.
- The best way 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. How you can lessen the anode current is always to cut off the forward power supply Ea or reverse the link of Ea. The minimum anode current needed to keep your thyristor in the conducting state is referred to as the holding current of the thyristor. Therefore, as it happens, provided that the anode current is lower than the holding current, the thyristor could be switched off.
What is the difference between a transistor as well as a thyristor?
Transistors usually contain a PNP or NPN structure composed of three semiconductor materials.
The thyristor is made up of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The job of a transistor depends on electrical signals to control its opening and closing, allowing fast switching operations.
The thyristor demands a forward voltage as well as a trigger current at the gate to change on or off.
Transistors are commonly used in amplification, switches, oscillators, along with other aspects of electronic circuits.
Thyristors are mainly used in electronic circuits like 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 achieve current amplification.
The thyristor is turned on or off by managing the trigger voltage of the control electrode to understand the switching function.
The circuit parameters of thyristors are related to 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, because of their different structures and operating principles, they may have noticeable differences in performance and use 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 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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