What exactly is a thyristor?
A thyristor is actually a high-power semiconductor device, also referred to as a silicon-controlled rectifier. Its structure consists of 4 levels of semiconductor materials, including 3 PN junctions corresponding towards the Anode, Cathode, and control electrode Gate. These 3 poles are 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, like controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversion.
The graphical symbol of the Thyristor is normally represented through 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 lightweight-controlled thyristors. The functioning condition from the thyristor is that each time a forward voltage is applied, the gate should have a trigger current.
Characteristics of thyristor
- Forward blocking
As shown in Figure a above, when an ahead voltage is used involving the anode and cathode (the anode is connected to the favorable pole from the power supply, as well as the cathode is connected to the negative pole from the power supply). But no forward voltage is applied towards the control pole (i.e., K is disconnected), as well as the indicator light does not illuminate. This implies 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 called trigger voltage), the indicator light switches on. Because of this the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, following the thyristor is excited, even when the voltage in the control electrode is taken away (that is certainly, K is excited again), the indicator light still glows. This implies that the thyristor can continue to conduct. Currently, in order to 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 involving the anode and cathode, as well as the indicator light does not illuminate at this time. This implies that the thyristor is not really conducting and can reverse blocking.
- In conclusion
1) If the thyristor is put through a reverse anode voltage, the thyristor is at a reverse blocking state whatever 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, that is certainly, the controllable characteristic.
3) If the thyristor is excited, provided that there is a specific forward anode voltage, the thyristor will always be excited regardless of the gate voltage. That is certainly, following the thyristor is excited, the gate will lose its function. The gate only functions as a trigger.
4) If the thyristor is on, as well as the primary circuit voltage (or current) decreases to seal to zero, the thyristor turns off.
5) The disorder for that thyristor to conduct is that a forward voltage should be applied involving the anode as well as the cathode, plus an appropriate forward voltage ought to be applied involving the gate as well as the cathode. To transform 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 basically a unique triode composed of three PN junctions. It may be equivalently regarded as consisting of a PNP transistor (BG2) plus an NPN transistor (BG1).
- In case a forward voltage is applied involving 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 applied towards the control electrode at this time, BG1 is triggered to produce a base 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 brought in the collector of BG2. This current is delivered to BG1 for amplification and 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 large current appears in the emitters of the two transistors, that is certainly, the anode and cathode from the thyristor (how big the current is really dependant on how big the burden and how big Ea), therefore the thyristor is entirely excited. This conduction process is finished in a very short period of time.
- Following the thyristor is excited, its conductive state will likely be maintained through the positive feedback effect from the tube itself. Whether or not the forward voltage from the control electrode disappears, it is still in the conductive state. Therefore, the function of the control electrode is only to trigger the thyristor to change on. Once the thyristor is excited, the control electrode loses its function.
- The only way to shut off the turned-on thyristor is to reduce the anode current that it is insufficient to keep up the positive feedback process. How you can reduce the anode current is to cut off the forward power supply Ea or reverse the link of Ea. The minimum anode current required to keep your thyristor in the conducting state is called the holding current from the thyristor. Therefore, strictly speaking, provided that the anode current is under 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 composed of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The task of the transistor relies on electrical signals to control its closing and opening, allowing fast switching operations.
The thyristor requires a forward voltage along with a trigger current at the gate to change on or off.
Transistors are commonly used in amplification, switches, oscillators, along with other elements of electronic circuits.
Thyristors are mostly utilized in electronic circuits like 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 accomplish current amplification.
The thyristor is excited or off by manipulating the trigger voltage from the control electrode to realize 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 summarize, although transistors and thyristors can be used in similar applications in some cases, because of their different structures and functioning principles, they have noticeable variations in performance and use 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 lightweight control devices.
- In induction cookers and electric water heaters, thyristors may 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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