What is a thyristor?
A thyristor is a high-power semiconductor device, also referred to as a silicon-controlled rectifier. Its structure consists of four quantities of semiconductor elements, including three PN junctions corresponding for the Anode, Cathode, and control electrode Gate. These three poles are definitely 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 working status. Therefore, thyristors are popular in a variety of electronic circuits, like controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversion.
The graphical symbol of the Thyristor is usually represented through the text symbol “V” or “VT” (in older standards, the letters “SCR”). Furthermore, derivatives of thyristors include fast thyristors, bidirectional thyristors, reverse conduction thyristors, and light-controlled thyristors. The working condition in the thyristor is that when 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 utilized between the anode and cathode (the anode is connected to the favorable pole in the power supply, and also the cathode is attached to the negative pole in the power supply). But no forward voltage is applied for the control pole (i.e., K is disconnected), and also the indicator light does not light up. This shows that the thyristor will not be 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 for the control electrode (referred to as a trigger, and also the applied voltage is called trigger voltage), the indicator light turns 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 around the control electrode is taken away (which is, K is excited again), the indicator light still glows. This shows that the thyristor can continue to conduct. At this time, to be able to stop the conductive thyristor, the power supply Ea must be stop or reversed.
- Reverse blocking
As shown in Figure d above, although a forward voltage is applied for the control electrode, a reverse voltage is applied between the anode and cathode, and also the indicator light does not light up at the moment. This shows that the thyristor will not be conducting and will reverse blocking.
- To sum up
1) If the thyristor is subjected to a reverse anode voltage, the thyristor is in a reverse blocking state regardless of what voltage the gate is subjected to.
2) If the thyristor is subjected to a forward anode voltage, the thyristor will simply conduct when the gate is subjected to a forward voltage. At this time, the thyristor is incorporated in the forward conduction state, which is the thyristor characteristic, which is, the controllable characteristic.
3) If the thyristor is excited, provided that there exists a specific forward anode voltage, the thyristor will remain excited whatever 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) If the thyristor is on, and also the primary circuit voltage (or current) decreases to seal to zero, the thyristor turns off.
5) The problem for your thyristor to conduct is that a forward voltage ought to be applied between the anode and also the cathode, as well as 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 stop, or the voltage must be reversed.
Working principle of thyristor
A thyristor is essentially a distinctive triode made up of three PN junctions. It could be equivalently viewed as consisting of a PNP transistor (BG2) as well as an NPN transistor (BG1).
- If a forward voltage is applied between 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. If a forward voltage is applied for the control electrode at the moment, BG1 is triggered to generate basics current Ig. BG1 amplifies this current, as well as 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 sent to BG1 for amplification and then sent to BG2 for amplification again. Such repeated amplification forms a crucial positive feedback, causing both BG1 and BG2 to get into a saturated conduction state quickly. A large current appears inside the emitters of these two transistors, which is, the anode and cathode in the thyristor (the size of the current is actually based on the size of the load and the size of Ea), therefore the thyristor is completely excited. This conduction process is finished in a very short time.
- Right after 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 still inside the conductive state. Therefore, the purpose of the control electrode is just to trigger the thyristor to transform on. After the thyristor is excited, the control electrode loses its function.
- The only way to shut off the turned-on thyristor is to reduce the anode current that it is not enough to maintain the positive feedback process. The way to reduce the anode current is to stop the forward power supply Ea or reverse the connection of Ea. The minimum anode current required to keep the thyristor inside the conducting state is called the holding current in the thyristor. Therefore, strictly speaking, provided that the anode current is less than the holding current, the thyristor may be turned off.
What is the distinction between a transistor as well as a thyristor?
Transistors usually include 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 job of the transistor relies 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 on the gate to transform on or off.
Transistors are popular in amplification, switches, oscillators, as well as other elements of electronic circuits.
Thyristors are mostly utilized in electronic circuits like 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 accomplish current amplification.
The thyristor is excited or off by manipulating the trigger voltage in 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 used in similar applications in some instances, because of their different structures and working principles, they have got noticeable variations in performance and use occasions.
Application scope of thyristor
- In power electronic equipment, thyristors can be used in frequency converters, motor controllers, welding machines, power supplies, etc.
- Inside the lighting field, thyristors can be used in dimmers and light control devices.
- In induction cookers and electric water heaters, thyristors can be used to control the current flow for the heating element.
- In electric vehicles, transistors can be used in motor controllers.
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