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 4 levels of semiconductor components, including 3 PN junctions corresponding towards 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 working status. Therefore, thyristors are popular in various electronic circuits, including controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency alteration.
The graphical symbol of any Thyristor is usually represented from the text symbol “V” or “VT” (in older standards, the letters “SCR”). Furthermore, derivatives of thyristors also include fast thyristors, bidirectional thyristors, reverse conduction thyristors, and light-weight-controlled thyristors. The working condition from the thyristor is the fact each time a forward voltage is used, the gate needs to 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 linked to the favorable pole from the power supply, and the cathode is attached to the negative pole from the power supply). But no forward voltage is used towards the control pole (i.e., K is disconnected), and the indicator light will not glow. This shows that the thyristor is not really 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 used towards the control electrode (known as a trigger, and 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, right after the thyristor is excited, even when the voltage on the control electrode is taken off (which is, K is excited again), the indicator light still glows. This shows that the thyristor can still conduct. Currently, in order to shut down the conductive thyristor, the power supply Ea has to be shut down or reversed.
- Reverse blocking
As shown in Figure d above, although a forward voltage is used towards the control electrode, a reverse voltage is used in between the anode and cathode, and the indicator light will not glow at the moment. This shows that the thyristor is not really conducting and will reverse blocking.
- In summary
1) If the thyristor is put through a reverse anode voltage, the thyristor is within a reverse blocking state no matter what voltage the gate is put through.
2) If the thyristor is put through a forward anode voltage, the thyristor will only conduct once the gate is put through a forward voltage. Currently, the thyristor is in the forward conduction state, the thyristor characteristic, which is, the controllable characteristic.
3) If the thyristor is excited, provided that you will find a specific forward anode voltage, the thyristor will stay excited regardless of the gate voltage. That is, right after the thyristor is excited, the gate will lose its function. The gate only functions as a trigger.
4) If 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 needs to be applied in between the anode and the cathode, and an appropriate forward voltage should also be applied in between the gate and the cathode. To transform off a conducting thyristor, the forward voltage in between the anode and cathode has to be shut down, or perhaps the voltage has to be reversed.
Working principle of thyristor
A thyristor is actually an exclusive triode made up of three PN junctions. It can be equivalently regarded as comprising a PNP transistor (BG2) and an NPN transistor (BG1).
- In case a forward voltage is used in between the anode and cathode from 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. In case a forward voltage is used towards the control electrode at the moment, BG1 is triggered to generate a base current Ig. BG1 amplifies this current, as well as 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 delivered to BG1 for amplification then delivered 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 large current appears inside the emitters of the two transistors, which is, the anode and cathode from the thyristor (how big the current is in fact determined by how big the load and how big Ea), so the thyristor is completely excited. This conduction process is done in an exceedingly limited time.
- Right after the thyristor is excited, its conductive state will be maintained from the positive feedback effect from the tube itself. Whether or not the forward voltage from the control electrode disappears, it is still inside the conductive state. Therefore, the function of the control electrode is only to trigger the thyristor to transform on. Once the thyristor is excited, the control electrode loses its function.
- The only method to switch off the turned-on thyristor is always to lessen the anode current that it is not enough to maintain the positive feedback process. The way to lessen the anode current is always to shut down the forward power supply Ea or reverse the connection of Ea. The minimum anode current required to keep the thyristor inside the conducting state is known as the holding current from the thyristor. Therefore, as it happens, provided that the anode current is lower than the holding current, the thyristor could be turned off.
What exactly is the difference between a transistor as well as a thyristor?
Transistors usually consist of a PNP or NPN structure made up of three semiconductor materials.
The thyristor is made up of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The job of any transistor relies on electrical signals to control its closing and opening, allowing fast switching operations.
The thyristor requires a forward voltage as well as a trigger current at the gate to transform on or off.
Transistors are popular in amplification, switches, oscillators, and other elements of electronic circuits.
Thyristors are mostly utilized in electronic circuits including 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 achieve current amplification.
The thyristor is excited or off by managing 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 summarize, although transistors and thyristors can be used in similar applications in some instances, due to their different structures and working principles, they 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.
- Within 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 towards the heating element.
- In electric vehicles, transistors can be used in motor controllers.
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