Just what is a thyristor?
A thyristor is really a high-power semiconductor device, also known as a silicon-controlled rectifier. Its structure includes four levels of semiconductor components, including three PN junctions corresponding to the Anode, Cathode, and control electrode Gate. These three poles are definitely 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 popular 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 silicon-controlled rectifier is usually 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 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 utilized involving the anode and cathode (the anode is linked to the favorable pole of the power supply, and also the cathode is connected to the negative pole of the power supply). But no forward voltage is applied to the control pole (i.e., K is disconnected), and also the indicator light fails to illuminate. This demonstrates that the thyristor is not 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, and also the applied voltage is referred to as trigger voltage), the indicator light switches on. This means that the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, right after the thyristor is excited, even if the voltage around the control electrode is taken off (that is, K is excited again), the indicator light still glows. This demonstrates that the thyristor can carry on and 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 to the control electrode, a reverse voltage is applied involving the anode and cathode, and also the indicator light fails to illuminate at this time. This demonstrates that the thyristor is not conducting and may reverse blocking.
- To sum up
1) When the thyristor is exposed to a reverse anode voltage, the thyristor is in a reverse blocking state whatever voltage the gate is exposed to.
2) When the thyristor is exposed to a forward anode voltage, the thyristor will simply conduct when the gate is exposed to a forward voltage. At the moment, the thyristor is within the forward conduction state, which is the thyristor characteristic, that is, the controllable characteristic.
3) When the thyristor is excited, provided that there exists a specific forward anode voltage, the thyristor will always be excited no matter the gate voltage. That is certainly, right after the thyristor is excited, the gate will lose its function. The gate only functions as a trigger.
4) When the thyristor is on, and also the primary circuit voltage (or current) decreases to close to zero, the thyristor turns off.
5) The condition for your thyristor to conduct is the fact that a forward voltage needs to be applied involving the anode and also the cathode, as well as an appropriate forward voltage ought to be applied involving the gate and also the cathode. To turn off a conducting thyristor, the forward voltage involving the anode and cathode has to be cut off, or perhaps the voltage has to be reversed.
Working principle of thyristor
A thyristor is actually a unique triode made up of three PN junctions. It could be equivalently viewed as consisting of a PNP transistor (BG2) as well as an NPN transistor (BG1).
- In case a forward voltage is applied involving the anode and cathode of the thyristor without applying a forward voltage to the control electrode, although both BG1 and BG2 have forward voltage applied, the thyristor is still turned off because BG1 has no base current. In case a forward voltage is applied to 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 the 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 then sent to BG2 for amplification again. Such repeated amplification forms a vital positive feedback, causing both BG1 and BG2 to enter a saturated conduction state quickly. A sizable current appears inside the emitters of these two transistors, that is, the anode and cathode of the thyristor (the size of the current is in fact based on the size of the load and the size of Ea), and so the thyristor is completely excited. This conduction process is finished in a very 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 still inside the conductive state. Therefore, the purpose of the control electrode is only to trigger the thyristor to turn on. Once the thyristor is excited, the control electrode loses its function.
- The best way to turn off the turned-on thyristor is always to lessen the anode current that it is not enough to keep up the positive feedback process. The best way to lessen the anode current is always to cut off the forward power supply Ea or reverse the connection of Ea. The minimum anode current required to maintain the thyristor inside the conducting state is referred to as the holding current of the thyristor. Therefore, as it happens, provided that the anode current is less than the holding current, the thyristor could be turned off.
What is the distinction between a transistor along with a thyristor?
Transistors usually consist of 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 job of any transistor relies upon electrical signals to control its opening and closing, allowing fast switching operations.
The thyristor demands a forward voltage along with a trigger current at the gate to turn on or off.
Transistors are popular in amplification, switches, oscillators, and other aspects of electronic circuits.
Thyristors are mostly utilized in electronic circuits such as 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 attain current amplification.
The thyristor is excited or off by controlling the trigger voltage of the control electrode to understand the switching function.
The circuit parameters of thyristors are based on stability and reliability and usually have higher turn-off voltage and larger on-current.
To sum up, although transistors and thyristors can be used in similar applications in some instances, because of the different structures and operating principles, they have got 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.
- In the lighting field, thyristors can be used in dimmers and light control devices.
- In induction cookers and electric water heaters, thyristors could 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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