Just what is a thyristor?
A thyristor is a high-power semiconductor device, also called a silicon-controlled rectifier. Its structure consists of 4 levels of semiconductor elements, including 3 PN junctions corresponding towards the Anode, Cathode, and control electrode Gate. These 3 poles are the critical parts of 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 operating status. Therefore, thyristors are commonly 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 generally represented through the text symbol “V” or “VT” (in older standards, the letters “SCR”). In addition, derivatives of thyristors also have fast thyristors, bidirectional thyristors, reverse conduction thyristors, and light-controlled thyristors. The operating condition of the thyristor is the fact 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 utilized in between the anode and cathode (the anode is attached to the favorable pole of the power supply, as well as the cathode is attached to the negative pole of 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 demonstrates that the thyristor is not really 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 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, following the thyristor is excited, even when the voltage on the control electrode is removed (which is, K is excited again), the indicator light still glows. This demonstrates that the thyristor can continue to conduct. At the moment, so that you can cut off the conductive thyristor, the power supply Ea has to 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 in between the anode and cathode, as well as the indicator light fails to light up at the moment. This demonstrates that the thyristor is not really conducting and may reverse blocking.
- In conclusion
1) When the thyristor is subjected to a reverse anode voltage, the thyristor is within a reverse blocking state no matter what voltage the gate is subjected to.
2) When the thyristor is subjected to a forward anode voltage, the thyristor is only going to conduct once the gate is subjected to a forward voltage. At the moment, the thyristor is in the forward conduction state, the thyristor characteristic, which is, the controllable characteristic.
3) When the thyristor is excited, so long as there is a specific forward anode voltage, the thyristor will remain excited whatever the gate voltage. That is certainly, following the thyristor is excited, the gate will lose its function. The gate only works as a trigger.
4) When 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 the thyristor to conduct is the fact a forward voltage should be applied in between the anode as well as the cathode, as well as an appropriate forward voltage also need to 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 has to be cut off, or perhaps the voltage has to be reversed.
Working principle of thyristor
A thyristor is essentially an exclusive triode composed of three PN junctions. It can be equivalently regarded as consisting of a PNP transistor (BG2) as well as an NPN transistor (BG1).
- If a forward voltage is applied in between the anode and cathode of 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. If a forward voltage is applied towards the control electrode at the moment, BG1 is triggered to produce a base 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 brought in the collector of BG2. This current is sent to BG1 for amplification and after that sent 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 big current appears in the emitters of the two transistors, which is, the anode and cathode of the thyristor (the size of the current is actually determined by the size of the burden and the size of Ea), so the thyristor is totally excited. This conduction process is done in a really limited time.
- After the thyristor is excited, its conductive state will be maintained through the positive feedback effect of the tube itself. Whether or not the forward voltage of the control electrode disappears, it is actually still in the conductive state. Therefore, the purpose of the control electrode is only to trigger the thyristor to transform on. After the thyristor is excited, the control electrode loses its function.
- The best way to turn off the turned-on thyristor is to lessen the anode current that it is not enough to maintain the positive feedback process. The best way to lessen the anode current is to cut off the forward power supply Ea or reverse the bond of Ea. The minimum anode current required to maintain the thyristor in the conducting state is referred to as the holding current of the thyristor. Therefore, strictly speaking, so long as the anode current is lower than the holding current, the thyristor could be turned off.
Exactly what is the distinction between a transistor along with a thyristor?
Transistors usually consist of 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 closing and opening, 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 commonly used in amplification, switches, oscillators, and other facets of electronic circuits.
Thyristors are mainly utilized 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 accomplish current amplification.
The thyristor is excited or off by managing the trigger voltage of the control electrode to realize 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 summarize, although transistors and thyristors can be used in similar applications sometimes, due to their different structures and operating principles, they may have noticeable differences 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 control devices.
- In induction cookers and electric water heaters, thyristors can be used to control the current flow towards the heating element.
- In electric vehicles, transistors can be used in motor controllers.
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