Just what is a thyristor?
A thyristor is actually a high-power semiconductor device, also referred to as a silicon-controlled rectifier. Its structure includes 4 levels of semiconductor elements, including 3 PN junctions corresponding towards the Anode, Cathode, and control electrode Gate. These 3 poles are definitely the critical parts from 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 operating status. Therefore, thyristors are widely used in a variety of electronic circuits, like controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency alteration.
The graphical symbol of a Thyristor is usually represented from the text symbol “V” or “VT” (in older standards, the letters “SCR”). Furthermore, derivatives of thyristors include fast thyristors, bidirectional thyristors, reverse conduction thyristors, and light-weight-controlled thyristors. The operating condition from the thyristor is the fact that each time a forward voltage is applied, the gate should have a trigger current.
Characteristics of thyristor
- Forward blocking
As shown in Figure a above, when an ahead voltage is used between the anode and cathode (the anode is attached to the favorable pole from the power supply, as well as the cathode is linked to the negative pole from the power supply). But no forward voltage is applied towards the control pole (i.e., K is disconnected), as well as the indicator light fails to light up. This shows that the thyristor is not conducting and contains forward blocking capability.
- Controllable conduction
As shown in Figure b above, when K is closed, as well as a forward voltage is applied towards the control electrode (known as a trigger, as well as the applied voltage is known as trigger voltage), the indicator light turns on. Which means that the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, after the thyristor is switched on, even if the voltage in the control electrode is removed (which is, K is switched on again), the indicator light still glows. This shows that the thyristor can still conduct. Currently, 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 applied towards the control electrode, a reverse voltage is applied between the anode and cathode, as well as the indicator light fails to light up at this time. This shows that the thyristor is not conducting and will reverse blocking.
- In summary
1) If the thyristor is exposed to a reverse anode voltage, the thyristor is in a reverse blocking state regardless of what voltage the gate is exposed to.
2) If the thyristor is exposed to a forward anode voltage, the thyristor is only going to conduct when the gate is exposed to a forward voltage. Currently, the thyristor is within the forward conduction state, the thyristor characteristic, which is, the controllable characteristic.
3) If the thyristor is switched on, so long as there is a specific forward anode voltage, the thyristor will stay switched on regardless of the gate voltage. Which is, after the thyristor is switched on, the gate will lose its function. The gate only works as a trigger.
4) If 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 that a forward voltage needs to be applied between the anode as well as the cathode, and an appropriate forward voltage ought to be applied between the gate as well as the cathode. To change off a conducting thyristor, the forward voltage between the anode and cathode should be cut off, or perhaps the voltage should be reversed.
Working principle of thyristor
A thyristor is basically a distinctive triode composed of three PN junctions. It can be equivalently viewed as consisting of a PNP transistor (BG2) and an NPN transistor (BG1).
- If a forward voltage is applied between the anode and cathode from the thyristor without applying a forward voltage towards the control electrode, although both BG1 and BG2 have forward voltage applied, the thyristor remains turned off because BG1 has no base current. If a forward voltage is applied towards the control electrode at this time, BG1 is triggered to generate basics current Ig. BG1 amplifies this current, as well as 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 brought to BG1 for amplification and then brought to BG2 for amplification again. Such repeated amplification forms a vital positive feedback, causing both BG1 and BG2 to get in a saturated conduction state quickly. A sizable current appears within the emitters of these two transistors, which is, the anode and cathode from the thyristor (the size of the current is really dependant on the size of the load and the size of Ea), therefore the thyristor is totally switched on. This conduction process is done in a really limited time.
- Following the thyristor is switched on, its conductive state will likely be maintained from the positive feedback effect from the tube itself. Even if the forward voltage from the control electrode disappears, it is still within the conductive state. Therefore, the function of the control electrode is simply to trigger the thyristor to change on. When the thyristor is switched on, the control electrode loses its function.
- The only method to turn off the turned-on thyristor is always to lessen the anode current so that it is inadequate to maintain the positive feedback process. How you can lessen the anode current is always to cut off the forward power supply Ea or reverse the bond of Ea. The minimum anode current necessary to keep your thyristor within the conducting state is known as the holding current from the thyristor. Therefore, strictly speaking, so long as the anode current is lower than the holding current, the thyristor may be turned off.
Exactly what is the difference between a transistor as well as a thyristor?
Transistors usually include a PNP or NPN structure composed of three semiconductor materials.
The thyristor consists of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The work of a transistor relies on electrical signals to control its closing and opening, allowing fast switching operations.
The thyristor needs a forward voltage as well as a trigger current at the gate to change on or off.
Transistors are widely used in amplification, switches, oscillators, as well as other aspects of electronic circuits.
Thyristors are mainly 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 achieve current amplification.
The thyristor is switched on or off by manipulating the trigger voltage from the control electrode to comprehend 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 sum up, although transistors and thyristors may be used in similar applications sometimes, because of their different structures and operating principles, they have got noticeable variations in performance and make use of occasions.
Application scope of thyristor
- In power electronic equipment, thyristors may be used in frequency converters, motor controllers, welding machines, power supplies, etc.
- In the lighting field, thyristors may be used in dimmers and light-weight 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 may be used in motor controllers.
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