What exactly 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 quantities of semiconductor materials, including three PN junctions corresponding to the Anode, Cathode, and control electrode Gate. These three poles would be the critical parts in 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 popular in different electronic circuits, including controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency alteration.
The graphical symbol of a semiconductor device is generally 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 in the thyristor is that each time 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 can be used between the anode and cathode (the anode is linked to the favorable pole in the power supply, and also the cathode is connected to the negative pole in the power supply). But no forward voltage is applied to the control pole (i.e., K is disconnected), and also the indicator light fails to light up. This demonstrates that the thyristor is not conducting and it has forward blocking capability.
- Controllable conduction
As shown in Figure b above, when K is closed, and a forward voltage is applied to the control electrode (known as a trigger, and also the applied voltage is called trigger voltage), the indicator light turns on. Which means that the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, right after the thyristor is switched on, whether or not the voltage around the control electrode is taken off (which is, K is switched on again), the indicator light still glows. This demonstrates that the thyristor can carry on and conduct. Currently, to be able 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 applied to the control electrode, a reverse voltage is applied between the anode and cathode, and also the indicator light fails to light up at this time. This demonstrates that the thyristor is not conducting and can reverse blocking.
- In summary
1) If the thyristor is exposed to a reverse anode voltage, the thyristor is within a reverse blocking state regardless of what voltage the gate is exposed to.
2) If the thyristor is exposed to a forward anode voltage, the thyristor will simply conduct if the gate is exposed to a forward voltage. Currently, the thyristor is in the forward conduction state, the thyristor characteristic, which is, the controllable characteristic.
3) If the thyristor is switched on, provided that there exists a specific forward anode voltage, the thyristor will stay switched on whatever the gate voltage. Which is, right 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 that thyristor to conduct is that a forward voltage should be applied between the anode and also the cathode, and an appropriate forward voltage ought to be applied between the gate and also the cathode. To change off a conducting thyristor, the forward voltage between the anode and cathode must be shut down, or even the voltage must be reversed.
Working principle of thyristor
A thyristor is actually a unique triode composed of three PN junctions. It can be equivalently regarded as comprising a PNP transistor (BG2) and an NPN transistor (BG1).
- When a forward voltage is applied between the anode and cathode in 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. When a forward voltage is applied to the control electrode at this time, BG1 is triggered to create basics 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 is going to be brought in the collector of BG2. This current is brought to BG1 for amplification and after that brought to BG2 for amplification again. Such repeated amplification forms an essential positive feedback, causing both BG1 and BG2 to get in a saturated conduction state quickly. A sizable current appears within the emitters of these two transistors, which is, the anode and cathode in the thyristor (how big the current is in fact determined by how big the load and how big Ea), and so the thyristor is totally switched on. This conduction process is finished in an exceedingly limited time.
- Right after the thyristor is switched on, its conductive state is going to be maintained through the positive feedback effect in the tube itself. Even if the forward voltage in the control electrode disappears, it is actually still within the conductive state. Therefore, the function of the control electrode is just to trigger the thyristor to change on. After the thyristor is switched on, the control electrode loses its function.
- The only method to turn off the turned-on thyristor is always to reduce the anode current that it is not enough to keep up the positive feedback process. The way to reduce 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 keep the thyristor within the conducting state is called the holding current in the thyristor. Therefore, strictly speaking, provided that the anode current is lower than the holding current, the thyristor could be switched off.
Exactly what is the difference between a transistor and a thyristor?
Transistors usually include a PNP or NPN structure composed of three semiconductor materials.
The thyristor is composed of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The job of a transistor relies on electrical signals to control its closing and opening, allowing fast switching operations.
The thyristor requires a forward voltage and a trigger current in the gate to change on or off.
Transistors are popular in amplification, switches, oscillators, as well as other elements of electronic circuits.
Thyristors are mostly found in electronic circuits including controlled rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversions.
Means 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 in 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 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 make use of 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 may 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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