What exactly is a thyristor?
A thyristor is a high-power semiconductor device, also called a silicon-controlled rectifier. Its structure contains four quantities of semiconductor materials, including three PN junctions corresponding towards the Anode, Cathode, and control electrode Gate. These three poles are the critical parts in 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 working status. Therefore, thyristors are widely used in a variety of electronic circuits, such as controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency alteration.
The graphical symbol of any semiconductor device is usually represented through the text symbol “V” or “VT” (in older standards, the letters “SCR”). In addition, derivatives of thyristors include fast thyristors, bidirectional thyristors, reverse conduction thyristors, and light-weight-controlled thyristors. The working condition in the thyristor is that when 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 in between the anode and cathode (the anode is connected to the favorable pole in the power supply, as well as the cathode is linked to the negative pole in 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 implies 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 (called a trigger, as well as 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, right after the thyristor is switched on, even when the voltage in the control electrode is taken away (that is certainly, K is switched on again), the indicator light still glows. This implies that the thyristor can continue to conduct. At the moment, so that you can shut down the conductive thyristor, the power supply Ea should be shut down 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 in between the anode and cathode, as well as the indicator light fails to light up currently. This implies that the thyristor is not 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 whatever voltage the gate is exposed to.
2) Once the thyristor is exposed to a forward anode voltage, the thyristor will simply conduct if the gate is exposed to a forward voltage. At the moment, the thyristor is within the forward conduction state, the thyristor characteristic, that is certainly, the controllable characteristic.
3) Once the thyristor is switched on, so long as you will find a specific forward anode voltage, the thyristor will always be switched on whatever the gate voltage. That is, right after the thyristor is switched on, the gate will lose its function. The gate only works as a trigger.
4) Once the thyristor is on, as well as the primary circuit voltage (or current) decreases to seal 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 as well as the cathode, and an appropriate forward voltage should also be applied in between the gate as well as the cathode. To change off a conducting thyristor, the forward voltage in between the anode and cathode should be shut down, or even the voltage should be reversed.
Working principle of thyristor
A thyristor is actually an exclusive triode composed of three PN junctions. It can be equivalently regarded as composed of a PNP transistor (BG2) and an NPN transistor (BG1).
- If a forward voltage is applied 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 continues to be turned off because BG1 has no base current. If a forward voltage is applied towards the control electrode currently, BG1 is triggered to create a base current Ig. BG1 amplifies this current, as well as 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 be introduced the collector of BG2. This current is sent to BG1 for amplification then sent to BG2 for amplification again. Such repeated amplification forms a crucial positive feedback, causing both BG1 and BG2 to enter a saturated conduction state quickly. A sizable current appears within the emitters of the two transistors, that is certainly, the anode and cathode in the thyristor (the size of the current is really based on the size of the stress and the size of Ea), and so the thyristor is completely switched on. This conduction process is completed in an exceedingly short period of time.
- Following the thyristor is switched on, its conductive state will be maintained through the positive feedback effect in the tube itself. Even if the forward voltage in the control electrode disappears, it is actually still within the conductive state. Therefore, the purpose of the control electrode is only to trigger the thyristor to turn on. Once the thyristor is switched on, the control electrode loses its function.
- The only method to shut off the turned-on thyristor is always to decrease the anode current so that it is insufficient to keep up the positive feedback process. The way to decrease the anode current is always to shut down the forward power supply Ea or reverse the connection of Ea. The minimum anode current needed to keep your thyristor within the conducting state is referred to as the holding current in the thyristor. Therefore, strictly speaking, so long as the anode current is less than the holding current, the thyristor may be turned off.
What exactly is the difference between a transistor as well as a thyristor?
Transistors usually contain a PNP or NPN structure composed of three semiconductor materials.
The thyristor is composed of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The job of any transistor relies upon electrical signals to control its opening and closing, allowing fast switching operations.
The thyristor demands a forward voltage as well as a trigger current at the gate to turn on or off.
Transistors are widely used in amplification, switches, oscillators, as well as other facets of electronic circuits.
Thyristors are mainly found in electronic circuits such as controlled rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversions.
Method 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 manipulating the trigger voltage in the control electrode to realize 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 may be used in similar applications sometimes, because of the different structures and working principles, they may have noticeable differences in performance and utilize occasions.
Application scope of thyristor
- In power electronic equipment, thyristors may be used in frequency converters, motor controllers, welding machines, power supplies, etc.
- Within the lighting field, thyristors may be used in dimmers and light-weight control devices.
- In induction cookers and electric water heaters, thyristors could 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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