What is a thyristor?
A thyristor is really a high-power semiconductor device, also known as a silicon-controlled rectifier. Its structure consists of 4 quantities of semiconductor components, including 3 PN junctions corresponding for the Anode, Cathode, and control electrode Gate. These 3 poles are definitely the critical parts from 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 functioning status. Therefore, thyristors are widely used in different electronic circuits, including controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversion.
The graphical symbol of a silicon-controlled rectifier is generally 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-weight-controlled thyristors. The functioning condition from the thyristor is the fact each time 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 involving the anode and cathode (the anode is attached to the favorable pole from the power supply, and the cathode is attached to the negative pole from the power supply). But no forward voltage is applied for the control pole (i.e., K is disconnected), and the indicator light will not illuminate. This demonstrates 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, as well as a forward voltage is applied for the control electrode (referred to as a trigger, and the applied voltage is known 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, whether or not the voltage on the control electrode is taken away (which is, K is excited again), the indicator light still glows. This demonstrates that the thyristor can carry on and conduct. At this time, in order 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 for the control electrode, a reverse voltage is applied involving the anode and cathode, and the indicator light will not illuminate at this time. This demonstrates that the thyristor will not be conducting and can reverse blocking.
- To sum up
1) When the thyristor is exposed to a reverse anode voltage, the thyristor is in a reverse blocking state regardless of what voltage the gate is exposed to.
2) When the thyristor is exposed to a forward anode voltage, the thyristor will only conduct when the gate is exposed to a forward voltage. At this time, the thyristor is incorporated in the forward conduction state, the thyristor characteristic, which is, the controllable characteristic.
3) When the thyristor is excited, provided that you will find a specific forward anode voltage, the thyristor will stay excited no matter the gate voltage. That is certainly, after the thyristor is excited, the gate will lose its function. The gate only functions as a trigger.
4) When the thyristor is on, and the primary circuit voltage (or current) decreases to close to zero, the thyristor turns off.
5) The problem for your thyristor to conduct is the fact a forward voltage needs to be applied involving the anode and the cathode, as well as an appropriate forward voltage ought to be applied involving the gate and the cathode. To change off a conducting thyristor, the forward voltage involving the anode and cathode has to be cut off, or the voltage has to be reversed.
Working principle of thyristor
A thyristor is basically a unique triode made up of three PN junctions. It could be equivalently thought to be consisting of a PNP transistor (BG2) as well as an NPN transistor (BG1).
- When a forward voltage is applied involving the anode and cathode from 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 applied for the control electrode at this time, BG1 is triggered to generate 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 will be introduced the collector of BG2. This current is sent to BG1 for amplification 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 the two transistors, which is, the anode and cathode from the thyristor (the dimensions of the current is actually dependant on the dimensions of the burden and the dimensions of Ea), and so the thyristor is totally excited. This conduction process is completed in a really short time.
- Right after the thyristor is excited, its conductive state will be maintained through the positive feedback effect from the tube itself. Even when the forward voltage from the control electrode disappears, it really is still inside the conductive state. Therefore, the purpose of the control electrode is just to trigger the thyristor to change on. Once the thyristor is excited, the control electrode loses its function.
- The best way to shut off the turned-on thyristor is to decrease the anode current so that it is insufficient to keep the positive feedback process. The best way to decrease the anode current is to cut off the forward power supply Ea or reverse the link of Ea. The minimum anode current necessary to keep the thyristor inside the conducting state is known as the holding current from the thyristor. Therefore, strictly speaking, provided that the anode current is lower than the holding current, the thyristor could be turned off.
Exactly 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 consists of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The work of a transistor relies on electrical signals to control its closing and opening, allowing fast switching operations.
The thyristor requires a forward voltage as well as a trigger current at the gate to change on or off.
Transistors are widely used in amplification, switches, oscillators, and other facets of electronic circuits.
Thyristors are mainly found in electronic circuits including 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 from the control electrode to comprehend the switching function.
The circuit parameters of thyristors are based on 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 instances, because of their different structures and functioning principles, they may have noticeable differences in performance and utilize 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-weight 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 utilized in motor controllers.
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