What is a thyristor?
A thyristor is actually a high-power semiconductor device, also called a silicon-controlled rectifier. Its structure consists of 4 quantities of semiconductor elements, including three PN junctions corresponding for the Anode, Cathode, and control electrode Gate. These three poles are definitely 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 working status. Therefore, thyristors are commonly used in a variety of electronic circuits, including controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversion.
The graphical symbol of any semiconductor device is generally represented from the text symbol “V” or “VT” (in older standards, the letters “SCR”). In addition, derivatives of thyristors also include fast thyristors, bidirectional thyristors, reverse conduction thyristors, and lightweight-controlled thyristors. The working condition of the thyristor is the fact 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 can be used between the anode and cathode (the anode is attached to the favorable pole of the power supply, and the cathode is connected to the negative pole of the power supply). But no forward voltage is applied for the control pole (i.e., K is disconnected), and the indicator light fails to glow. This shows that the thyristor is not conducting and has 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 (called a trigger, and the applied voltage is known as trigger voltage), the indicator light switches on. Because of this the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, right after the thyristor is excited, even when the voltage around the control electrode is taken away (that is certainly, K is excited again), the indicator light still glows. This shows that the thyristor can still conduct. At the moment, so that you can shut down the conductive thyristor, the power supply Ea should 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 the indicator light fails to glow at the moment. This shows that the thyristor is not conducting and will reverse blocking.
- To sum up
1) Once the thyristor is subjected to a reverse anode voltage, the thyristor is at a reverse blocking state no matter what voltage the gate is subjected to.
2) Once the thyristor is subjected to a forward anode voltage, the thyristor will simply conduct if the gate is subjected to a forward voltage. At the moment, the thyristor is in the forward conduction state, the thyristor characteristic, that is certainly, the controllable characteristic.
3) Once the thyristor is excited, provided that you will find a specific forward anode voltage, the thyristor will always be excited regardless of the gate voltage. That is certainly, 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 problem for that thyristor to conduct is the fact a forward voltage needs to be applied between the anode and the cathode, and an appropriate forward voltage ought to be applied between the gate and the cathode. To turn off a conducting thyristor, the forward voltage between the anode and cathode should be shut down, or perhaps the voltage should be reversed.
Working principle of thyristor
A thyristor is actually an exclusive triode composed of three PN junctions. It may be equivalently viewed as consisting of a PNP transistor (BG2) and an NPN transistor (BG1).
- When a forward voltage is applied between the anode and cathode of the thyristor without applying a forward voltage for the control electrode, although both BG1 and BG2 have forward voltage applied, the thyristor is still turned off because BG1 has no base current. When a forward voltage is applied for the control electrode at the moment, BG1 is triggered to generate a base 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 is going to be brought in the collector of BG2. This current is brought to BG1 for amplification then brought to BG2 for amplification again. Such repeated amplification forms a vital positive feedback, causing both BG1 and BG2 to get into a saturated conduction state quickly. A big current appears in the emitters of the two transistors, that is certainly, the anode and cathode of the thyristor (the dimensions of the current is really dependant on the dimensions of the stress and the dimensions of Ea), so the thyristor is completely excited. This conduction process is finished in a very short time.
- Right after the thyristor is excited, its conductive state is going to be maintained from the positive feedback effect of the tube itself. Even if the forward voltage of the control electrode disappears, it is still in the conductive state. Therefore, the purpose of the control electrode is only to trigger the thyristor to transform on. When the thyristor is excited, the control electrode loses its function.
- The only way to turn off the turned-on thyristor is always to reduce the anode current so that it is insufficient to keep the positive feedback process. The best way to reduce the anode current is always to shut down the forward power supply Ea or reverse the link of Ea. The minimum anode current needed to keep your thyristor in the conducting state is known as the holding current of the thyristor. Therefore, strictly speaking, provided that the anode current is lower than 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 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 any transistor relies upon electrical signals to control its opening and closing, allowing fast switching operations.
The thyristor requires a forward voltage along with a trigger current on the gate to transform on or off.
Transistors are commonly used in amplification, switches, oscillators, as well as other facets 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 attain current amplification.
The thyristor is excited or off by manipulating the trigger voltage of the control electrode to comprehend the switching function.
The circuit parameters of thyristors are related to stability and reliability and often have higher turn-off voltage and larger on-current.
To summarize, although transistors and thyristors can be used in similar applications in some instances, because of the different structures and working principles, they have got noticeable differences 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 could be used to control the current flow for the heating element.
- In electric vehicles, transistors can be used in motor controllers.
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