Specifically what is a thyristor?
A thyristor is really a high-power semiconductor device, also called a silicon-controlled rectifier. Its structure includes 4 levels of semiconductor materials, including three PN junctions corresponding for the Anode, Cathode, and control electrode Gate. These three poles are definitely 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 functioning status. Therefore, thyristors are commonly used in various electronic circuits, such as controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversion.
The graphical symbol of a silicon-controlled rectifier is usually represented from 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-weight-controlled thyristors. The functioning condition from the thyristor is that whenever a forward voltage is applied, 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 between the anode and cathode (the anode is attached to the favorable pole from the power supply, and also the cathode is connected to the negative pole from the power supply). But no forward voltage is applied for the control pole (i.e., K is disconnected), and also the indicator light fails to illuminate. 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, along with a forward voltage is applied for the control electrode (referred to as a trigger, and also the applied voltage is called trigger voltage), the indicator light turns on. Because of this the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, after the thyristor is turned on, even if the voltage in the control electrode is taken off (that is, K is turned on again), the indicator light still glows. This shows that the thyristor can still conduct. Currently, to be able 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 applied for the control electrode, a reverse voltage is applied between the anode and cathode, and also the indicator light fails to illuminate at this time. This shows that the thyristor is not really conducting and may reverse blocking.
- To sum up
1) Once the thyristor is subjected to a reverse anode voltage, the thyristor is in a reverse blocking state whatever voltage the gate is subjected to.
2) Once the thyristor is subjected to a forward anode voltage, the thyristor will only conduct when the gate is subjected to a forward voltage. Currently, the thyristor is incorporated in the forward conduction state, which is the thyristor characteristic, that is, the controllable characteristic.
3) Once the thyristor is turned on, so long as there is a specific forward anode voltage, the thyristor will remain turned on whatever the gate voltage. That is, after the thyristor is turned on, the gate will lose its function. The gate only works as a trigger.
4) Once the thyristor is on, and also the primary circuit voltage (or current) decreases to close to zero, the thyristor turns off.
5) The disorder for that thyristor to conduct is that a forward voltage needs to be applied between the anode and also the cathode, as well as an appropriate forward voltage should also be applied between the gate and also the cathode. To turn off a conducting thyristor, the forward voltage between the anode and cathode must be shut down, or even the voltage must be reversed.
Working principle of thyristor
A thyristor is basically a unique triode composed of three PN junctions. It may be equivalently viewed as consisting of a PNP transistor (BG2) as well as an NPN transistor (BG1).
- If a forward voltage is applied between the anode and cathode from the thyristor without applying a forward voltage for the control electrode, although both BG1 and BG2 have forward voltage applied, the thyristor remains turned off because BG1 has no base current. If a forward voltage is applied for 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 the collector. This current is precisely the base current of BG2. After amplification by BG2, a ß1ß2Ig current will likely be introduced the collector of BG2. This current is brought to BG1 for amplification and after that brought 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 large current appears inside the emitters of the two transistors, that is, the anode and cathode from the thyristor (the size of the current is in fact based on the size of the burden and the size of Ea), therefore the thyristor is totally turned on. This conduction process is done in a really short period of time.
- Right after the thyristor is turned on, its conductive state will likely 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 purpose of the control electrode is simply to trigger the thyristor to turn on. Once the thyristor is turned on, the control electrode loses its function.
- The only way to switch off the turned-on thyristor is to reduce the anode current so that it is insufficient to keep up the positive feedback process. The best way to reduce the anode current is to shut down the forward power supply Ea or reverse the connection of Ea. The minimum anode current necessary to keep the thyristor inside the conducting state is called the holding current from the thyristor. Therefore, strictly speaking, so long as the anode current is less than the holding current, the thyristor can be turned off.
What exactly is the difference between a transistor along with a thyristor?
Transistors usually consist of a PNP or NPN structure composed 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 a transistor relies upon electrical signals to control its closing and opening, allowing fast switching operations.
The thyristor demands a forward voltage along with a trigger current on the gate to turn on or off.
Transistors are commonly used in amplification, switches, oscillators, along with other facets of electronic circuits.
Thyristors are mostly 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 accomplish current amplification.
The thyristor is turned on or off by controlling the trigger voltage from the control electrode to realize 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 may be used in similar applications in some cases, because of their different structures and functioning principles, they may have noticeable variations in performance and make use of occasions.
Application scope of thyristor
- In power electronic equipment, thyristors may be used in frequency converters, motor controllers, welding machines, power supplies, etc.
- Inside the lighting field, thyristors may 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 for the heating element.
- In electric vehicles, transistors may be used in motor controllers.
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