What is a thyristor?
A thyristor is a high-power semiconductor device, also known as a silicon-controlled rectifier. Its structure contains 4 quantities of semiconductor components, including 3 PN junctions corresponding towards the Anode, Cathode, and control electrode Gate. These 3 poles would be 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 commonly used in various electronic circuits, including controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversion.
The graphical symbol of a silicon-controlled rectifier is normally represented through the text symbol “V” or “VT” (in older standards, the letters “SCR”). In addition, derivatives of thyristors also have fast thyristors, bidirectional thyristors, reverse conduction thyristors, and light-controlled thyristors. The working condition in the thyristor is the fact that each time a forward voltage is applied, the gate needs to 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 linked to the favorable pole in the power supply, and the cathode is linked to the negative pole in the power supply). But no forward voltage is applied towards the control pole (i.e., K is disconnected), and the indicator light does not light up. This demonstrates that the thyristor is not conducting and has forward blocking capability.
- Controllable conduction
As shown in Figure b above, when K is closed, and a forward voltage is applied towards the control electrode (referred to as a trigger, and 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, whether or not the voltage in the control electrode is removed (that is certainly, K is excited again), the indicator light still glows. This demonstrates that the thyristor can still conduct. At the moment, so that you can cut off the conductive thyristor, the power supply Ea must be cut off or reversed.
- Reverse blocking
As shown in Figure d above, although a forward voltage is applied towards the control electrode, a reverse voltage is applied between the anode and cathode, and the indicator light does not light up at the moment. This demonstrates that the thyristor is not conducting and can reverse blocking.
- In conclusion
1) Once the thyristor is put through a reverse anode voltage, the thyristor is in a reverse blocking state whatever voltage the gate is put through.
2) Once the thyristor is put through a forward anode voltage, the thyristor will simply conduct if the gate is put through a forward voltage. At the moment, the thyristor is incorporated in the forward conduction state, the thyristor characteristic, that is certainly, the controllable characteristic.
3) Once the thyristor is excited, provided that you will find a specific forward anode voltage, the thyristor will always be excited whatever the gate voltage. Which is, after the thyristor is excited, the gate will lose its function. The gate only serves as a trigger.
4) Once the thyristor is on, and the primary circuit voltage (or current) decreases to seal to zero, the thyristor turns off.
5) The disorder for the thyristor to conduct is the fact that a forward voltage needs to be applied between the anode and the cathode, and an appropriate forward voltage ought to be applied between the gate and the cathode. To transform off a conducting thyristor, the forward voltage between the anode and cathode must be cut off, or perhaps the voltage must be reversed.
Working principle of thyristor
A thyristor is essentially a unique triode made up of three PN junctions. It can be equivalently regarded as comprising a PNP transistor (BG2) and an NPN transistor (BG1).
- If a forward voltage is applied between the anode and cathode in 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 applied towards the control electrode at the moment, BG1 is triggered to generate a base 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 likely be brought in the collector of BG2. This current is delivered to BG1 for amplification then 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 certainly, the anode and cathode in the thyristor (the dimensions of the current is really based on the dimensions of the burden and the dimensions of Ea), and so the thyristor is entirely excited. This conduction process is completed in an exceedingly limited time.
- After the thyristor is excited, its conductive state will likely be maintained through the positive feedback effect in the tube itself. Whether or not the forward voltage in the control electrode disappears, it really is still inside the conductive state. Therefore, the purpose of the control electrode is only to trigger the thyristor to transform on. When the thyristor is excited, the control electrode loses its function.
- The only way to switch off the turned-on thyristor would be to decrease the anode current so that it is not enough to keep the positive feedback process. How you can decrease 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, strictly speaking, provided that the anode current is less than the holding current, the thyristor can be turned off.
What exactly 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 job of a 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 on the gate to transform on or off.
Transistors are commonly used in amplification, switches, oscillators, as well as other aspects of electronic circuits.
Thyristors are mostly used 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 accomplish current amplification.
The thyristor is excited or off by controlling 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 can be utilized in similar applications sometimes, due to their 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.
- Within the lighting field, thyristors can be utilized in dimmers and light control devices.
- In induction cookers and electric water heaters, thyristors could 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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