Just what is a thyristor?
A thyristor is actually a high-power semiconductor device, also referred to as a silicon-controlled rectifier. Its structure contains 4 levels of semiconductor components, including three PN junctions corresponding for the Anode, Cathode, and control electrode Gate. These three poles would be the critical parts of 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 working status. Therefore, thyristors are commonly used in different electronic circuits, such as controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversion.
The graphical symbol of any Thyristor is usually represented from the text symbol “V” or “VT” (in older standards, the letters “SCR”). Additionally, derivatives of thyristors also include fast thyristors, bidirectional thyristors, reverse conduction thyristors, and light-controlled thyristors. The working condition of the thyristor is that when 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 in between the anode and cathode (the anode is connected to the favorable pole of the power supply, and the cathode is connected to the negative pole of the power supply). But no forward voltage is used for the control pole (i.e., K is disconnected), and the indicator light does not glow. This implies 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 used for the control electrode (referred to as a trigger, and the applied voltage is called 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 turned on, even when the voltage in the control electrode is taken away (that is, K is turned on again), the indicator light still glows. This implies that the thyristor can carry on and conduct. Currently, in order to 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 for the control electrode, a reverse voltage is used in between the anode and cathode, and the indicator light does not glow currently. This implies that the thyristor will not be conducting and will reverse blocking.
- In conclusion
1) If 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) If the thyristor is put through a forward anode voltage, the thyristor will only conduct if the gate is put through a forward voltage. Currently, the thyristor is incorporated in the forward conduction state, the thyristor characteristic, that is, the controllable characteristic.
3) If the thyristor is turned on, provided that you will find a specific forward anode voltage, the thyristor will stay turned on whatever the gate voltage. That is certainly, following the thyristor is turned on, the gate will lose its function. The gate only functions as a trigger.
4) If the thyristor is on, and the primary circuit voltage (or current) decreases to seal to zero, the thyristor turns off.
5) The condition for that thyristor to conduct is that a forward voltage ought to be applied in between the anode and the cathode, plus an appropriate forward voltage ought to be applied in between the gate and the cathode. To turn off a conducting thyristor, the forward voltage in between the anode and cathode must be stop, or perhaps the voltage must be reversed.
Working principle of thyristor
A thyristor is actually a distinctive triode made from three PN junctions. It may be equivalently viewed as comprising a PNP transistor (BG2) plus an NPN transistor (BG1).
- When a forward voltage is used in between the anode and cathode of the thyristor without applying a forward voltage for the control electrode, although both BG1 and BG2 have forward voltage applied, the thyristor remains turned off because BG1 has no base current. When a forward voltage is used for the control electrode currently, BG1 is triggered to produce basics current Ig. BG1 amplifies this current, as well as 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 be introduced the collector of BG2. This current is delivered to BG1 for amplification and after that delivered to BG2 for amplification again. Such repeated amplification forms an essential 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, the anode and cathode of the thyristor (how big the current is in fact dependant on how big the stress and how big Ea), so the thyristor is completely turned on. This conduction process is finished in an exceedingly limited time.
- Right after the thyristor is turned on, its conductive state will be maintained from the positive feedback effect of the tube itself. Whether or not the forward voltage of the control electrode disappears, it is actually still inside the conductive state. Therefore, the function of the control electrode is simply to trigger the thyristor to change on. After the thyristor is turned on, the control electrode loses its function.
- The only method to turn off the turned-on thyristor would be to lessen the anode current that it is insufficient to keep up the positive feedback process. The best way to lessen the anode current would be to stop the forward power supply Ea or reverse the bond of Ea. The minimum anode current necessary to keep your thyristor inside the conducting state is called the holding current of the thyristor. Therefore, as it happens, provided that the anode current is less than the holding current, the thyristor can 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 from three semiconductor materials.
The thyristor consists of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The task of any transistor depends on electrical signals to control its opening and closing, allowing fast switching operations.
The thyristor needs a forward voltage as well as a trigger current on the gate to change on or off.
Transistors are commonly used in amplification, switches, oscillators, along with other elements of electronic circuits.
Thyristors are mainly utilized in electronic circuits such as 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 attain current amplification.
The thyristor is turned on or off by controlling the trigger voltage of the control electrode to comprehend 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 used in similar applications in some instances, because of their different structures and working principles, they may have noticeable variations in performance and make use of occasions.
Application scope of thyristor
- In power electronic equipment, thyristors can be used in frequency converters, motor controllers, welding machines, power supplies, etc.
- Within the lighting field, thyristors can be used in dimmers and light control devices.
- In induction cookers and electric water heaters, thyristors may be used to control the current flow for the heating element.
- In electric vehicles, transistors can be used in motor controllers.
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