What exactly is a thyristor?
A thyristor is a high-power semiconductor device, also called a silicon-controlled rectifier. Its structure contains 4 levels of semiconductor materials, including three PN junctions corresponding for the Anode, Cathode, and control electrode Gate. These three poles are the critical parts of the thyristor, allowing it to 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 a variety of electronic circuits, like controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversion.
The graphical symbol of any Thyristor is usually represented through 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 lightweight-controlled thyristors. The functioning condition of the thyristor is the fact 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 can be used in between the anode and cathode (the anode is connected 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 implies that the thyristor is not 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 for the control electrode (referred to as a trigger, and the applied voltage is referred to as trigger voltage), the indicator light turns on. Because of this the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, following the thyristor is excited, even if the voltage in the control electrode is taken away (which is, K is excited again), the indicator light still glows. This implies that the thyristor can carry on and conduct. At this time, to be able 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 for the control electrode, a reverse voltage is applied in between the anode and cathode, and the indicator light fails to glow currently. This implies that the thyristor is not conducting and will reverse blocking.
- To sum up
1) If the thyristor is exposed to a reverse anode voltage, the thyristor is at a reverse blocking state whatever voltage the gate is exposed to.
2) If the thyristor is exposed to a forward anode voltage, the thyristor will simply conduct once the gate is exposed to a forward voltage. At this time, the thyristor is in the forward conduction state, the thyristor characteristic, which is, the controllable characteristic.
3) If the thyristor is excited, provided that there is a specific forward anode voltage, the thyristor will remain excited no matter the gate voltage. Which is, following the thyristor is excited, the gate will lose its function. The gate only serves as a trigger.
4) If the thyristor is on, and the primary circuit voltage (or current) decreases to close to zero, the thyristor turns off.
5) The disorder for your thyristor to conduct is the fact that a forward voltage needs to be applied in between the anode and the cathode, as well as 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 cut off, or even the voltage has to be reversed.
Working principle of thyristor
A thyristor is basically a unique triode composed of three PN junctions. It can be equivalently viewed as consisting of a PNP transistor (BG2) as well as an NPN transistor (BG1).
- In case a forward voltage is applied in 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 continues to be turned off because BG1 has no base current. In case a forward voltage is applied for the control electrode currently, BG1 is triggered to create basics 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 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 get into a saturated conduction state quickly. A large current appears in the emitters of these two transistors, which is, the anode and cathode of the thyristor (the size of the current is actually dependant on the size of the burden and the size of Ea), so the thyristor is totally excited. This conduction process is done in a really short period of time.
- After the thyristor is excited, its conductive state will likely be maintained through the positive feedback effect of the tube itself. Whether or not the forward voltage of the control electrode disappears, it is actually still in the conductive state. Therefore, the function of the control electrode is simply to trigger the thyristor to transform on. Once the thyristor is excited, the control electrode loses its function.
- The only way to turn off the turned-on thyristor would be to reduce the anode current that it is not enough to keep the positive feedback process. The best way to reduce the anode current would be to cut off the forward power supply Ea or reverse the connection of Ea. The minimum anode current necessary to keep the thyristor in the conducting state is referred to as the holding current of the thyristor. Therefore, as it happens, provided that the anode current is under the holding current, the thyristor may be turned off.
What is the difference between a transistor as well as a thyristor?
Transistors usually contain 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 job of any transistor depends on electrical signals to control its closing and opening, allowing fast switching operations.
The thyristor needs a forward voltage as well as a trigger current on the gate to transform on or off.
Transistors are widely used in amplification, switches, oscillators, as well as other facets of electronic circuits.
Thyristors are mainly found in electronic circuits like 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 excited or off by controlling the trigger voltage of the control electrode to comprehend the switching function.
The circuit parameters of thyristors are based on stability and reliability and often have higher turn-off voltage and larger on-current.
To summarize, although transistors and thyristors may be used in similar applications in some cases, because of the different structures and functioning principles, they have got noticeable variations in performance and use occasions.
Application scope of thyristor
- In power electronic equipment, thyristors may be used in frequency converters, motor controllers, welding machines, power supplies, etc.
- Within the lighting field, thyristors may be used in dimmers and lightweight control devices.
- In induction cookers and electric water heaters, thyristors may 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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