What is a thyristor?
A thyristor is really a high-power semiconductor device, also called a silicon-controlled rectifier. Its structure consists of 4 levels of semiconductor elements, including three PN junctions corresponding to the Anode, Cathode, and control electrode Gate. These three poles would be the critical parts in 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 operating status. Therefore, thyristors are popular in various electronic circuits, including controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversion.
The graphical symbol of a Thyristor is generally represented by the text symbol “V” or “VT” (in older standards, the letters “SCR”). In addition, derivatives of thyristors also include fast thyristors, bidirectional thyristors, reverse conduction thyristors, and light-controlled thyristors. The operating condition in the thyristor is the fact whenever a forward voltage is applied, the gate should have a trigger current.
Characteristics of thyristor
- Forward blocking
As shown in Figure a above, when an ahead voltage is used in 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 does not glow. This shows that the thyristor is not really conducting and has 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 switches on. This means that the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, after the thyristor is excited, even if the voltage in the control electrode is taken off (that is certainly, K is excited again), the indicator light still glows. This shows that the thyristor can still conduct. Currently, to be able to cut off the conductive thyristor, the power supply Ea has to 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 in between the anode and cathode, and also the indicator light does not glow currently. This shows that the thyristor is not really conducting and can 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. Currently, the thyristor is incorporated in the forward conduction state, which is the thyristor characteristic, that is certainly, the controllable characteristic.
3) When the thyristor is excited, provided that there is a specific forward anode voltage, the thyristor will remain excited regardless of the gate voltage. That is certainly, after the thyristor is excited, the gate will lose its function. The gate only serves as a trigger.
4) When the thyristor is on, and also the primary circuit voltage (or current) decreases to seal to zero, the thyristor turns off.
5) The condition for that thyristor to conduct is the fact a forward voltage should be applied in between the anode and also the cathode, plus an appropriate forward voltage ought to be applied in between the gate and also the cathode. To transform off a conducting thyristor, the forward voltage in between the anode and cathode has to be cut off, or perhaps the voltage has to be reversed.
Working principle of thyristor
A thyristor is essentially an exclusive triode composed of three PN junctions. It may be equivalently viewed as composed of a PNP transistor (BG2) plus an NPN transistor (BG1).
- If a forward voltage is applied in 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 continues to be switched off because BG1 has no base current. If a forward voltage is applied to the control electrode currently, BG1 is triggered to produce basics current Ig. BG1 amplifies this current, as well as 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 introduced the collector of BG2. This current is delivered to BG1 for amplification then delivered 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 large current appears inside the emitters of these two transistors, that is certainly, the anode and cathode in the thyristor (how big the current is really based on how big the burden and how big Ea), and so the thyristor is completely excited. This conduction process is completed in a very short period of time.
- Following the thyristor is excited, its conductive state is going to be maintained by the positive feedback effect in the tube itself. Even when the forward voltage in the control electrode disappears, it is actually still inside the conductive state. Therefore, the function of the control electrode is simply to trigger the thyristor to change on. Once the thyristor is excited, the control electrode loses its function.
- The only way to turn off the turned-on thyristor would be to reduce the anode current that it is not enough to keep up the positive feedback process. The best way to reduce the anode current would be to cut off the forward power supply Ea or reverse the bond of Ea. The minimum anode current needed to maintain the thyristor inside the conducting state is referred to as the holding current in the thyristor. Therefore, as it happens, provided that the anode current is under the holding current, the thyristor may be switched off.
Exactly what is the difference between a transistor as well as a thyristor?
Transistors usually consist of 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 task of a transistor relies on electrical signals to control its closing and opening, allowing fast switching operations.
The thyristor needs a forward voltage as well as a trigger current in the gate to change on or off.
Transistors are popular in amplification, switches, oscillators, along with 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 accomplish current amplification.
The thyristor is excited or off by manipulating the trigger voltage in the control electrode to realize 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 may be used in similar applications in some instances, because of their different structures and operating principles, they have 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.
- In the lighting field, thyristors may be used 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 may be used in motor controllers.
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