Specifically what is a thyristor?
A thyristor is a high-power semiconductor device, also known as a silicon-controlled rectifier. Its structure includes 4 quantities of semiconductor elements, including three PN junctions corresponding towards the Anode, Cathode, and control electrode Gate. These three 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, like controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency alteration.
The graphical symbol of any Thyristor is usually represented through the text symbol “V” or “VT” (in older standards, the letters “SCR”). Additionally, derivatives of thyristors also have fast thyristors, bidirectional thyristors, reverse conduction thyristors, and lightweight-controlled thyristors. The working condition in the thyristor is the fact that each time a forward voltage is used, the gate needs to have a trigger current.
Characteristics of thyristor
- Forward blocking
As shown in Figure a above, when an ahead voltage can be used between the anode and cathode (the anode is attached 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 used towards the control pole (i.e., K is disconnected), and the indicator light fails to illuminate. This implies that the thyristor is not really 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 used towards the control electrode (known as a trigger, and 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 switched on, even when the voltage on the control electrode is taken off (that is certainly, K is switched on again), the indicator light still glows. This implies that the thyristor can still conduct. Currently, to be able to stop the conductive thyristor, the power supply Ea should 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 between the anode and cathode, and the indicator light fails to illuminate at this time. This implies that the thyristor is not really conducting and may reverse blocking.
- To sum up
1) Once the thyristor is exposed to a reverse anode voltage, the thyristor is at a reverse blocking state whatever voltage the gate is exposed to.
2) Once the thyristor is exposed to a forward anode voltage, the thyristor will only conduct once the gate is exposed to a forward voltage. Currently, the thyristor is in the forward conduction state, which is the thyristor characteristic, that is certainly, the controllable characteristic.
3) Once the thyristor is switched on, provided that there exists a specific forward anode voltage, the thyristor will remain switched on regardless of the gate voltage. Which is, following the thyristor is switched on, the gate will lose its function. The gate only functions as a trigger.
4) Once the thyristor is on, and the primary circuit voltage (or current) decreases to close to zero, the thyristor turns off.
5) The problem for the thyristor to conduct is the fact that a forward voltage should be applied between the anode and the cathode, and an appropriate forward voltage should also be applied between the gate and the cathode. To turn off a conducting thyristor, the forward voltage between the anode and cathode should be stop, or the voltage should be reversed.
Working principle of thyristor
A thyristor is basically a distinctive triode made up of three PN junctions. It could be equivalently thought to be comprising a PNP transistor (BG2) and an NPN transistor (BG1).
- When a forward voltage is used 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 is still turned off because BG1 has no base current. When a forward voltage is used towards the control electrode at this time, BG1 is triggered to generate a base 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 is going to be brought in the collector of BG2. This current is sent to BG1 for amplification and after that 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 big current appears in the emitters of these 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 stress and the dimensions of Ea), and so the thyristor is entirely switched on. This conduction process is completed in an exceedingly limited time.
- Following the thyristor is switched on, its conductive state is going to be maintained through the positive feedback effect in the tube itself. Even when the forward voltage in the control electrode disappears, it is actually still in the conductive state. Therefore, the purpose of the control electrode is just to trigger the thyristor to turn on. When the thyristor is switched on, the control electrode loses its function.
- The best way to switch off the turned-on thyristor is always to reduce the anode current that it is inadequate to maintain the positive feedback process. The best way to reduce the anode current is always to stop the forward power supply Ea or reverse the link of Ea. The minimum anode current required to keep the thyristor in the conducting state is referred to as the holding current in the thyristor. Therefore, as it happens, provided that the anode current is lower than the holding current, the thyristor can be turned off.
Exactly what is the difference 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 made up of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The task of any transistor relies on electrical signals to control its opening and closing, allowing fast switching operations.
The thyristor requires a forward voltage as well as a trigger current in the gate to turn on or off.
Transistors are commonly used in amplification, switches, oscillators, as well as other facets of electronic circuits.
Thyristors are mostly used in electronic circuits like 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 switched on or off by controlling the trigger voltage in the control electrode to comprehend the switching function.
The circuit parameters of thyristors are based on stability and reliability and usually have higher turn-off voltage and larger on-current.
To sum up, although transistors and thyristors can be utilized in similar applications in some cases, due to their different structures and working principles, they have noticeable variations 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.
- Inside 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 towards the heating element.
- In electric vehicles, transistors can be utilized in motor controllers.
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