So what is a thyristor?
A thyristor is really a high-power semiconductor device, also called a silicon-controlled rectifier. Its structure consists of four quantities of semiconductor materials, including three PN junctions corresponding for the Anode, Cathode, and control electrode Gate. These three poles are the critical parts from 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 functioning status. Therefore, thyristors are widely used in various electronic circuits, like controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency alteration.
The graphical symbol of the silicon-controlled rectifier is normally represented through the text symbol “V” or “VT” (in older standards, the letters “SCR”). Furthermore, derivatives of thyristors also have fast thyristors, bidirectional thyristors, reverse conduction thyristors, and light-weight-controlled thyristors. The functioning condition from the thyristor is that when a forward voltage is used, the gate will need to have a trigger current.
Characteristics of thyristor
- Forward blocking
As shown in Figure a above, when an ahead voltage can be used in between 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 used for the control pole (i.e., K is disconnected), and the indicator light will not illuminate. This shows 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, along with a forward voltage is used for the control electrode (called a trigger, and 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, following the thyristor is switched on, even if the voltage on the control electrode is taken off (that is certainly, K is switched on again), the indicator light still glows. This shows that the thyristor can carry on and conduct. At this time, to be able to cut off the conductive thyristor, the power supply Ea should be cut off or reversed.
- Reverse blocking
As shown in Figure d above, although a forward voltage is used for the control electrode, a reverse voltage is used in between the anode and cathode, and the indicator light will not illuminate currently. This shows that the thyristor will not be conducting and may reverse blocking.
- In summary
1) When the thyristor is subjected to a reverse anode voltage, the thyristor is at a reverse blocking state whatever voltage the gate is subjected to.
2) When the thyristor is subjected to a forward anode voltage, the thyristor will simply conduct once the gate is subjected to a forward voltage. At this time, the thyristor is within the forward conduction state, the thyristor characteristic, that is certainly, the controllable characteristic.
3) When the thyristor is switched on, as long as there exists a specific forward anode voltage, the thyristor will remain switched on no matter the gate voltage. That is, following the thyristor is switched on, 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 seal to zero, the thyristor turns off.
5) The condition for the thyristor to conduct is that a forward voltage should be applied in between the anode and the cathode, and an appropriate forward voltage should also be applied in between the gate and the cathode. To change off a conducting thyristor, the forward voltage in between the anode and cathode should be cut off, or even the voltage should be reversed.
Working principle of thyristor
A thyristor is actually an exclusive triode made from three PN junctions. It can be equivalently regarded as comprising a PNP transistor (BG2) and an NPN transistor (BG1).
- If a forward voltage is used in between 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 is still switched off because BG1 has no base current. If a forward voltage is used for the control electrode currently, BG1 is triggered to produce a base current Ig. BG1 amplifies this current, along with 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 introduced 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 sizable current appears within the emitters of the two transistors, that is certainly, the anode and cathode from the thyristor (the size of the current is in fact determined by the size of the stress and the size of Ea), therefore the thyristor is totally switched on. This conduction process is finished in a really short time.
- Following the thyristor is switched on, its conductive state will likely be maintained through the positive feedback effect from the tube itself. Even if the forward voltage from the control electrode disappears, it is actually still within the conductive state. Therefore, the purpose of the control electrode is simply to trigger the thyristor to turn on. Once the thyristor is switched on, the control electrode loses its function.
- The only way to shut off the turned-on thyristor is to lessen the anode current that it is not enough to keep the positive feedback process. The best way to lessen the anode current is to cut off the forward power supply Ea or reverse the bond of Ea. The minimum anode current necessary to keep the thyristor within the conducting state is referred to as the holding current from the thyristor. Therefore, as it happens, as long as the anode current is less than the holding current, the thyristor can be switched off.
Exactly what is the distinction between a transistor along with a thyristor?
Transistors usually contain a PNP or NPN structure made from 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 depends on electrical signals to control its opening and closing, allowing fast switching operations.
The thyristor requires a forward voltage along with a trigger current in the gate to turn on or off.
Transistors are widely used in amplification, switches, oscillators, along with other facets of electronic circuits.
Thyristors are mainly found 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 switched on or off by controlling the trigger voltage from the control electrode to realize the switching function.
The circuit parameters of thyristors are related to stability and reliability and usually have higher turn-off voltage and larger on-current.
To summarize, although transistors and thyristors can be utilized in similar applications in some instances, due to their different structures and functioning principles, they have got noticeable variations 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.
- Within the lighting field, thyristors can be utilized in dimmers and light-weight control devices.
- In induction cookers and electric water heaters, thyristors could 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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