What is a thyristor?
A thyristor is a high-power semiconductor device, also known as a silicon-controlled rectifier. Its structure includes four levels of semiconductor components, including 3 PN junctions corresponding towards the Anode, Cathode, and control electrode Gate. These 3 poles are the critical parts of 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 commonly used in different electronic circuits, like controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversion.
The graphical symbol of the silicon-controlled rectifier is normally represented from the text symbol “V” or “VT” (in older standards, the letters “SCR”). Furthermore, derivatives of thyristors also include fast thyristors, bidirectional thyristors, reverse conduction thyristors, and lightweight-controlled thyristors. The working condition of the thyristor is the fact when a forward voltage is used, the gate should 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 connected to the favorable pole of the power supply, as well as the cathode is linked to the negative pole of the power supply). But no forward voltage is used towards the control pole (i.e., K is disconnected), as well as the indicator light will not light up. This shows 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 used towards the control electrode (known as a trigger, as well as the applied voltage is referred to as 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 when the voltage around the control electrode is removed (which is, K is excited again), the indicator light still glows. This shows that the thyristor can continue to conduct. At the moment, so that you can 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 used towards the control electrode, a reverse voltage is used involving the anode and cathode, as well as the indicator light will not light up at the moment. This shows that the thyristor is not conducting and may reverse blocking.
- To sum up
1) Once the thyristor is put through a reverse anode voltage, the thyristor is in a reverse blocking state regardless of what voltage the gate is put through.
2) Once the thyristor is put through a forward anode voltage, the thyristor will only conduct if the gate is put through a forward voltage. At the moment, the thyristor is incorporated in the forward conduction state, which is the thyristor characteristic, which is, the controllable characteristic.
3) Once the thyristor is excited, as long as there exists a specific forward anode voltage, the thyristor will remain excited no matter the gate voltage. That is, after the thyristor is excited, the gate will lose its function. The gate only works as a trigger.
4) Once the thyristor is on, as well as the primary circuit voltage (or current) decreases to seal to zero, the thyristor turns off.
5) The disorder for that thyristor to conduct is the fact a forward voltage should be applied involving the anode as well as the cathode, plus an appropriate forward voltage also need to be applied involving the gate as well as the cathode. To transform off a conducting thyristor, the forward voltage involving the anode and cathode must be stop, or the voltage must be reversed.
Working principle of thyristor
A thyristor is actually an exclusive triode made up of three PN junctions. It may be equivalently thought to be composed of a PNP transistor (BG2) plus an NPN transistor (BG1).
- If a forward voltage is used involving the anode and cathode of the thyristor without applying a forward voltage towards 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 used towards the control electrode at the moment, BG1 is triggered to create a base 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 will likely 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 in a saturated conduction state quickly. A big current appears in the emitters of the two transistors, which is, the anode and cathode of the thyristor (the dimensions of the current is in fact dependant on the dimensions of the burden and the dimensions of Ea), therefore the thyristor is totally excited. This conduction process is finished in an exceedingly short time.
- Following the thyristor is excited, its conductive state will likely be maintained from the positive feedback effect of the tube itself. Even if the forward voltage of the control electrode disappears, it is still in the conductive state. Therefore, the function of the control electrode is only to trigger the thyristor to change on. When the thyristor is excited, the control electrode loses its function.
- The only way to switch off the turned-on thyristor is to reduce the anode current that it is inadequate to keep up the positive feedback process. How you can reduce the anode current is to stop the forward power supply Ea or reverse the connection of Ea. The minimum anode current necessary to keep the thyristor in the conducting state is referred to as the holding current of the thyristor. Therefore, strictly speaking, as long as the anode current is under the holding current, the thyristor may be turned off.
What is the distinction between a transistor and a thyristor?
Transistors usually contain a PNP or NPN structure made up of three semiconductor materials.
The thyristor is made up of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The work of the transistor depends on electrical signals to control its closing and opening, allowing fast switching operations.
The thyristor needs a forward voltage and a trigger current in the gate to change on or off.
Transistors are commonly used in amplification, switches, oscillators, as well as other elements of electronic circuits.
Thyristors are mostly used in electronic circuits like 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 excited or off by controlling the trigger voltage of the control electrode to realize 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 sometimes, due to their different structures and working principles, they 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 lightweight control devices.
- In induction cookers and electric water heaters, thyristors may be used to control the current flow towards the heating element.
- In electric vehicles, transistors can be utilized in motor controllers.
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