Specifically what is a thyristor?
A thyristor is really a high-power semiconductor device, also known as a silicon-controlled rectifier. Its structure consists of four 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 in 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 popular in various electronic circuits, such as controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency alteration.
The graphical symbol of a Thyristor is normally represented from the text symbol “V” or “VT” (in older standards, the letters “SCR”). Furthermore, derivatives of thyristors also include fast thyristors, bidirectional thyristors, reverse conduction thyristors, and lightweight-controlled thyristors. The working condition in the thyristor is that when a forward voltage is used, the gate should have a trigger current.
Characteristics of thyristor
- Forward blocking
As shown in Figure a above, when an ahead voltage is used in between the anode and cathode (the anode is connected to the favorable pole in the power supply, and the cathode is connected 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 does not illuminate. This demonstrates that the thyristor is not 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 towards the control electrode (referred to as a trigger, and the applied voltage is known as trigger voltage), the indicator light switches on. Which means that the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, right after the thyristor is turned on, even if the voltage around the control electrode is taken off (that is certainly, K is turned on again), the indicator light still glows. This demonstrates that the thyristor can continue to conduct. At the moment, so that you can shut down the conductive thyristor, the power supply Ea must be shut down 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 in between the anode and cathode, and the indicator light does not illuminate at this time. This demonstrates that the thyristor is not conducting and can reverse blocking.
- In conclusion
1) When the thyristor is exposed to a reverse anode voltage, the thyristor is within a reverse blocking state regardless of what voltage the gate is exposed to.
2) When the thyristor is exposed to a forward anode voltage, the thyristor will only conduct if the gate is exposed to a forward voltage. At the moment, the thyristor is incorporated in the forward conduction state, which is the thyristor characteristic, that is certainly, the controllable characteristic.
3) When the thyristor is turned on, as long as you will find a specific forward anode voltage, the thyristor will always be turned on regardless of the gate voltage. Which is, right after the thyristor is turned on, the gate will lose its function. The gate only serves as a trigger.
4) When the thyristor is on, and the primary circuit voltage (or current) decreases to close to zero, the thyristor turns off.
5) The problem for your thyristor to conduct is that a forward voltage ought to be applied in between the anode and the cathode, as well as an appropriate forward voltage ought to be applied in between the gate and the cathode. To transform off a conducting thyristor, the forward voltage in between the anode and cathode must be shut down, or perhaps the voltage must be reversed.
Working principle of thyristor
A thyristor is actually a distinctive triode made up of three PN junctions. It can be equivalently viewed as comprising a PNP transistor (BG2) as well as an NPN transistor (BG1).
- When a forward voltage is used in 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 basics current Ig. BG1 amplifies this current, along with 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 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 enter a saturated conduction state quickly. A big current appears in the emitters of the two transistors, that is certainly, the anode and cathode in the thyristor (the size of the current is in fact dependant on the size of the burden and the size of Ea), and so the thyristor is completely turned on. This conduction process is done in an exceedingly limited time.
- Right after the thyristor is turned on, its conductive state will be maintained from the positive feedback effect in the tube itself. Even if the forward voltage in the control electrode disappears, it is still in the conductive state. Therefore, the function of the control electrode is only 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 decrease the anode current that it is insufficient to keep up the positive feedback process. The best way to decrease the anode current would be to shut down the forward power supply Ea or reverse the bond of Ea. The minimum anode current necessary to keep your thyristor in the conducting state is known as the holding current in the thyristor. Therefore, as it happens, as long as the anode current is lower than the holding current, the thyristor could be turned off.
Exactly what is the difference between a transistor along with a thyristor?
Transistors usually include a PNP or NPN structure made up of three semiconductor materials.
The thyristor consists of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The task of a transistor depends on electrical signals to control its closing and opening, allowing fast switching operations.
The thyristor demands a forward voltage along with a trigger current in the gate to change on or off.
Transistors are popular in amplification, switches, oscillators, and other facets of electronic circuits.
Thyristors are mainly used in electronic circuits such as 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 turned on or off by controlling the trigger voltage in the control electrode to understand the switching function.
The circuit parameters of thyristors are based on stability and reliability and often have higher turn-off voltage and larger on-current.
To sum up, although transistors and thyristors may be used in similar applications in some cases, because of the different structures and working principles, they may have noticeable variations in performance and use 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 lightweight 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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