Specifically what is a thyristor?
A thyristor is actually a high-power semiconductor device, also referred to as a silicon-controlled rectifier. Its structure contains four quantities of semiconductor materials, including three PN junctions corresponding to the Anode, Cathode, and control electrode Gate. These three poles are the critical parts of 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 widely used in various electronic circuits, including controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency alteration.
The graphical symbol of the silicon-controlled rectifier is generally represented by 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-controlled thyristors. The working condition of the thyristor is the fact each time 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 can be used in between the anode and cathode (the anode is attached to the favorable pole of the power supply, and the cathode is attached to the negative pole of the power supply). But no forward voltage is used to the control pole (i.e., K is disconnected), and the indicator light fails to light up. This demonstrates that the thyristor will not be 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 to the control electrode (known as a trigger, and the applied voltage is called trigger voltage), the indicator light turns on. Which 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 in the control electrode is taken away (that is certainly, K is excited again), the indicator light still glows. This demonstrates that the thyristor can carry on and conduct. Currently, in order to 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 to the control electrode, a reverse voltage is used in between the anode and cathode, and the indicator light fails to light up at this time. This demonstrates that the thyristor will not be conducting and can reverse blocking.
- In conclusion
1) Once the thyristor is put through a reverse anode voltage, the thyristor is at a reverse blocking state no matter what voltage the gate is put through.
2) Once the thyristor is put through a forward anode voltage, the thyristor is only going to conduct if the gate is put through a forward voltage. Currently, the thyristor is incorporated in the forward conduction state, the thyristor characteristic, that is certainly, the controllable characteristic.
3) Once the thyristor is excited, as long as there exists a specific forward anode voltage, the thyristor will remain excited regardless of the gate voltage. That is, right after the thyristor is excited, the gate will lose its function. The gate only works as a trigger.
4) Once the thyristor is on, and the primary circuit voltage (or current) decreases to seal 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 and the cathode, as well as an appropriate forward voltage also need 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 even the voltage must be reversed.
Working principle of thyristor
A thyristor is actually a unique triode made from three PN junctions. It may be equivalently thought to be comprising a PNP transistor (BG2) as well as an NPN transistor (BG1).
- When a forward voltage is used in between 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 continues to be turned off because BG1 has no base current. When a forward voltage is used to the control electrode at this time, BG1 is triggered to create 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 likely be brought in the collector of BG2. This current is delivered to BG1 for amplification then 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 these two transistors, that is certainly, the anode and cathode of the thyristor (how big the current is really dependant on how big the stress and how big Ea), so the thyristor is totally excited. This conduction process is finished in a very short period of time.
- After the thyristor is excited, its conductive state will likely be maintained by the positive feedback effect of the tube itself. Even when the forward voltage of the control electrode disappears, it is actually still in the conductive state. Therefore, the purpose of the control electrode is simply to trigger the thyristor to change on. Once the thyristor is excited, the control electrode loses its function.
- The only method to switch off the turned-on thyristor is to lessen the anode current that it is inadequate to keep up the positive feedback process. The best way to lessen the anode current is to shut down the forward power supply Ea or reverse the connection of Ea. The minimum anode current necessary to maintain the thyristor in the conducting state is called the holding current of the thyristor. Therefore, as it happens, as long as the anode current is under the holding current, the thyristor may be turned off.
Exactly what is the difference between a transistor along with a thyristor?
Transistors usually consist of a PNP or NPN structure made from three semiconductor materials.
The thyristor consists of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The job of the transistor relies on electrical signals to control its closing and opening, allowing fast switching operations.
The thyristor needs a forward voltage along with a trigger current in the gate to change on or off.
Transistors are widely used in amplification, switches, oscillators, along with other facets of electronic circuits.
Thyristors are mostly found in electronic circuits including 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 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 in most cases have higher turn-off voltage and larger on-current.
To sum up, although transistors and thyristors can be utilized in similar applications sometimes, because of the different structures and working principles, they have noticeable variations in performance and make use of occasions.
Application scope of thyristor
- In power electronic equipment, thyristors can be utilized in frequency converters, motor controllers, welding machines, power supplies, etc.
- Within the lighting field, thyristors can be utilized in dimmers and light control devices.
- In induction cookers and electric water heaters, thyristors may be used to control the current flow to the heating element.
- In electric vehicles, transistors can be utilized in motor controllers.
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