Just what is a thyristor?
A thyristor is really a high-power semiconductor device, also referred to as a silicon-controlled rectifier. Its structure contains four levels of semiconductor elements, including three PN junctions corresponding towards the Anode, Cathode, and control electrode Gate. These three poles are 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 functioning status. Therefore, thyristors are widely used in different electronic circuits, like controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversion.
The graphical symbol of the Thyristor is normally 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 lightweight-controlled thyristors. The functioning condition of the thyristor is the fact that whenever 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 used between the anode and cathode (the anode is connected 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 applied towards the control pole (i.e., K is disconnected), and the indicator light will not light up. This implies 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 applied towards the control electrode (called a trigger, and the applied voltage is referred to 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 turned on, even when the voltage in the control electrode is removed (which is, K is turned on again), the indicator light still glows. This implies that the thyristor can carry on and conduct. At this time, in order to cut off the conductive thyristor, the power supply Ea should 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 between the anode and cathode, and the indicator light will not light up at this time. This implies that the thyristor is not really conducting and can reverse blocking.
- In conclusion
1) When the thyristor is subjected to a reverse anode voltage, the thyristor is at a reverse blocking state regardless of what voltage the gate is subjected to.
2) When the thyristor is subjected to a forward anode voltage, the thyristor will only conduct if the gate is subjected to a forward voltage. At this time, the thyristor is incorporated in the forward conduction state, the thyristor characteristic, which is, the controllable characteristic.
3) When the thyristor is turned on, provided that there is a specific forward anode voltage, the thyristor will remain turned on whatever the gate voltage. That is certainly, right after the thyristor is turned on, the gate will lose its function. The gate only serves as a trigger.
4) When the thyristor is on, and the primary circuit voltage (or current) decreases to seal to zero, the thyristor turns off.
5) The disorder for your thyristor to conduct is the fact that a forward voltage ought to be applied between the anode and the cathode, plus an appropriate forward voltage should also be applied between the gate and the cathode. To turn off a conducting thyristor, the forward voltage between the anode and cathode should be cut off, or even the voltage should be reversed.
Working principle of thyristor
A thyristor is basically an exclusive triode made from three PN junctions. It may be equivalently thought to be comprising a PNP transistor (BG2) plus an NPN transistor (BG1).
- If a forward voltage is applied between the anode and cathode of the thyristor without applying a forward voltage towards the control electrode, although both BG1 and BG2 have forward voltage applied, the thyristor is still switched off because BG1 has no base current. If 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, 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 introduced 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 get in a saturated conduction state quickly. A big current appears within the emitters of these two transistors, which is, the anode and cathode of the thyristor (the dimensions of the current is in fact determined by the dimensions of the stress and the dimensions of Ea), so the thyristor is entirely turned on. This conduction process is done in a very limited time.
- Right after the thyristor is turned on, its conductive state will be maintained from the positive feedback effect of the tube itself. Even if the forward voltage of the control electrode disappears, it is still within the conductive state. Therefore, the purpose of the control electrode is just to trigger the thyristor to change on. After the thyristor is turned on, the control electrode loses its function.
- The only method to turn off the turned-on thyristor is to reduce the anode current that it is inadequate to maintain the positive feedback process. The best way to reduce the anode current is to cut off the forward power supply Ea or reverse the link of Ea. The minimum anode current necessary to keep the thyristor within the conducting state is referred to as the holding current of the thyristor. Therefore, strictly speaking, provided that the anode current is less than the holding current, the thyristor can be switched off.
What is the difference between a transistor as well as a thyristor?
Transistors usually include a PNP or NPN structure made from three semiconductor materials.
The thyristor is made up of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The work of the transistor relies on electrical signals to control its closing and opening, allowing fast switching operations.
The thyristor demands a forward voltage as well as a trigger current in the gate to change on or off.
Transistors are widely used in amplification, switches, oscillators, along with other elements of electronic circuits.
Thyristors are mostly found in electronic circuits like 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 achieve current amplification.
The thyristor is turned on or off by manipulating the trigger voltage of the control electrode to comprehend 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 summarize, although transistors and thyristors may be used in similar applications in some cases, due to their different structures and functioning principles, they have got noticeable variations in performance and utilize occasions.
Application scope of thyristor
- In power electronic equipment, thyristors may be used in frequency converters, motor controllers, welding machines, power supplies, etc.
- In the lighting field, thyristors may be used in dimmers and lightweight control devices.
- In induction cookers and electric water heaters, thyristors could be used to control the current flow towards the heating element.
- In electric vehicles, transistors may be used in motor controllers.
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