Just what is a thyristor?
A thyristor is actually a high-power semiconductor device, also called a silicon-controlled rectifier. Its structure includes 4 quantities of semiconductor elements, including 3 PN junctions corresponding to the Anode, Cathode, and control electrode Gate. These 3 poles would be 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 different electronic circuits, including 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 have fast thyristors, bidirectional thyristors, reverse conduction thyristors, and light-weight-controlled thyristors. The functioning condition from the thyristor is the fact that each time a forward voltage is applied, the gate will need to have a trigger current.
Characteristics of thyristor
- Forward blocking
As shown in Figure a above, when an ahead voltage is used involving the anode and cathode (the anode is connected to the favorable pole from the power supply, as well as the cathode is attached to the negative pole from the power supply). But no forward voltage is applied to the control pole (i.e., K is disconnected), as well as the indicator light fails to glow. This shows that the thyristor is not really conducting and has forward blocking capability.
- Controllable conduction
As shown in Figure b above, when K is closed, along with a forward voltage is applied to the control electrode (referred to as a trigger, as well as the applied voltage is known as trigger voltage), the indicator light turns on. This means that the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, after the thyristor is switched on, whether or not the voltage around the control electrode is taken away (which is, K is switched on again), the indicator light still glows. This shows that the thyristor can still conduct. Currently, in order to cut off the conductive thyristor, the power supply Ea must be cut off or reversed.
- Reverse blocking
As shown in Figure d above, although a forward voltage is applied to the control electrode, a reverse voltage is applied involving the anode and cathode, as well as the indicator light fails to glow currently. This shows that the thyristor is not really conducting and will reverse blocking.
- In conclusion
1) Once the thyristor is exposed to a reverse anode voltage, the thyristor is within a reverse blocking state no matter what voltage the gate is exposed to.
2) Once the thyristor is exposed to a forward anode voltage, the thyristor is only going to conduct if the gate is exposed to a forward voltage. Currently, the thyristor is within the forward conduction state, which is the thyristor characteristic, which is, the controllable characteristic.
3) Once the thyristor is switched on, provided that there is a specific forward anode voltage, the thyristor will remain switched on regardless of the gate voltage. That is certainly, after the thyristor is switched on, the gate will lose its function. The gate only serves as a trigger.
4) Once the thyristor is on, as well as the primary circuit voltage (or current) decreases to close 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 involving the anode as well as the cathode, and an appropriate forward voltage ought to be applied involving the gate as well as the cathode. To turn off a conducting thyristor, the forward voltage involving the anode and cathode must be cut off, or the voltage must be reversed.
Working principle of thyristor
A thyristor is essentially a unique triode made from three PN junctions. It can be equivalently thought to be comprising a PNP transistor (BG2) and an NPN transistor (BG1).
- In case a forward voltage is applied involving the anode and cathode from the thyristor without applying a forward voltage to the control electrode, although both BG1 and BG2 have forward voltage applied, the thyristor continues to be switched off because BG1 has no base current. In case a forward voltage is applied to the control electrode currently, BG1 is triggered to generate a base current Ig. BG1 amplifies this current, along with a ß1Ig current is obtained in their 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 delivered to BG1 for amplification and after that delivered to BG2 for amplification again. Such repeated amplification forms a vital positive feedback, causing both BG1 and BG2 to get into a saturated conduction state quickly. A big current appears inside the emitters of these two transistors, which is, the anode and cathode from the thyristor (the dimensions of the current is actually determined by the dimensions of the burden and the dimensions of Ea), and so the thyristor is entirely switched on. This conduction process is done in a really short time.
- After 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 actually still inside the conductive state. Therefore, the function of the control electrode is only to trigger the thyristor to transform on. After the thyristor is switched on, the control electrode loses its function.
- The only way to shut off the turned-on thyristor would be to decrease the anode current so that it is inadequate to maintain the positive feedback process. The way to decrease the anode current would be to cut off the forward power supply Ea or reverse the link of Ea. The minimum anode current needed to keep your thyristor inside the conducting state is known as the holding current from the thyristor. Therefore, as it happens, provided that the anode current is less than the holding current, the thyristor may be switched off.
Exactly what is the difference between a transistor along with a thyristor?
Transistors usually include a PNP or NPN structure made from three semiconductor materials.
The thyristor is made up of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The work of any transistor relies upon electrical signals to control its opening and closing, allowing fast switching operations.
The thyristor requires a forward voltage along with a trigger current on the gate to transform on or off.
Transistors are widely used in amplification, switches, oscillators, along with other elements of electronic circuits.
Thyristors are mostly found in electronic circuits including 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 accomplish 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 related to stability and reliability and in most cases have higher turn-off voltage and larger on-current.
To sum up, although transistors and thyristors can be used in similar applications sometimes, because of their different structures and functioning principles, they 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.
- Inside the lighting field, thyristors can be used in dimmers and light-weight control devices.
- In induction cookers and electric water heaters, thyristors can be used to control the current flow to the heating element.
- In electric vehicles, transistors can be used in motor controllers.
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