Just what is a thyristor?
A thyristor is a high-power semiconductor device, also known as a silicon-controlled rectifier. Its structure includes four quantities of semiconductor elements, including three PN junctions corresponding for the Anode, Cathode, and control electrode Gate. These three poles are 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 working status. Therefore, thyristors are commonly used in a variety of electronic circuits, such as controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency alteration.
The graphical symbol of the silicon-controlled rectifier is normally represented through the text symbol “V” or “VT” (in older standards, the letters “SCR”). Additionally, derivatives of thyristors also include fast thyristors, bidirectional thyristors, reverse conduction thyristors, and lightweight-controlled thyristors. The working condition in the thyristor is the fact when a forward voltage is used, 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 linked to the favorable pole in the power supply, and also the cathode is linked to the negative pole in the power supply). But no forward voltage is used for the control pole (i.e., K is disconnected), and also the indicator light will not light up. This implies that the thyristor will not be 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 for the control electrode (known 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 excited, whether or not the voltage on the control electrode is removed (that is, K is excited again), the indicator light still glows. This implies that the thyristor can carry on and conduct. At the moment, to be able to stop the conductive thyristor, the power supply Ea has to be stop or reversed.
- Reverse blocking
As shown in Figure d above, although a forward voltage is used for the control electrode, a reverse voltage is used involving the anode and cathode, and also the indicator light will not light up currently. This implies that the thyristor will not be conducting and may reverse blocking.
- To sum up
1) Once 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) Once the thyristor is subjected to a forward anode voltage, the thyristor will only conduct when the gate is subjected to a forward voltage. At the moment, the thyristor is within the forward conduction state, the thyristor characteristic, that is, the controllable characteristic.
3) Once the thyristor is excited, as long as there is a specific forward anode voltage, the thyristor will stay excited no matter the gate voltage. That is certainly, following the thyristor is excited, the gate will lose its function. The gate only works as a trigger.
4) Once the thyristor is on, and also the primary circuit voltage (or current) decreases to close to zero, the thyristor turns off.
5) The condition for the thyristor to conduct is the fact a forward voltage needs to be applied involving the anode and also the cathode, and an appropriate forward voltage should also be applied involving the gate and also the cathode. To turn off a conducting thyristor, the forward voltage involving the anode and cathode has to be stop, or the voltage has to be reversed.
Working principle of thyristor
A thyristor is basically a unique triode made from three PN junctions. It could be equivalently regarded as consisting of a PNP transistor (BG2) and an NPN transistor (BG1).
- When a forward voltage is used involving the anode and cathode in the thyristor without applying a forward voltage for the control electrode, although both BG1 and BG2 have forward voltage applied, the thyristor continues to be turned off because BG1 has no base current. When a forward voltage is used for the control electrode currently, BG1 is triggered to create basics current Ig. BG1 amplifies this current, and a ß1Ig current is obtained in its 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 sizable current appears inside the emitters of the two transistors, that is, the anode and cathode in the thyristor (the dimensions of the current is actually dependant on the dimensions of the load and the dimensions of Ea), and so the thyristor is entirely excited. This conduction process is finished in a very short time.
- Following the thyristor is excited, its conductive state will be maintained through the positive feedback effect in the tube itself. Whether or not the forward voltage in the control electrode disappears, it is actually still inside the conductive state. Therefore, the purpose of the control electrode is just to trigger the thyristor to turn on. After the thyristor is excited, the control electrode loses its function.
- The only way to shut off the turned-on thyristor is to lessen the anode current that it is inadequate to keep the positive feedback process. The best way to lessen the anode current is to stop the forward power supply Ea or reverse the link of Ea. The minimum anode current needed to keep the thyristor inside the conducting state is referred to as the holding current in the thyristor. Therefore, as it happens, as long as the anode current is lower than the holding current, the thyristor may be turned off.
Exactly what is the distinction between a transistor and a thyristor?
Transistors usually consist of 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 job of the transistor relies upon 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 turn on or off.
Transistors are commonly used in amplification, switches, oscillators, as well as 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.
Way of working
The transistor controls the collector current by holding the base current to achieve current amplification.
The thyristor is excited or off by manipulating the trigger voltage in the control electrode to comprehend 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 in some cases, because of the different structures and working principles, they 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 lightweight control devices.
- In induction cookers and electric water heaters, thyristors could be used to control the current flow for the heating element.
- In electric vehicles, transistors can be utilized in motor controllers.
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