Specifically what is a thyristor?
A thyristor is actually a high-power semiconductor device, also known as a silicon-controlled rectifier. Its structure contains four levels of semiconductor elements, including three PN junctions corresponding to the Anode, Cathode, and control electrode Gate. These three poles would be the critical parts from 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 working status. Therefore, thyristors are widely used in various electronic circuits, such as controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency alteration.
The graphical symbol of the silicon-controlled rectifier is usually represented through the text symbol “V” or “VT” (in older standards, the letters “SCR”). Furthermore, derivatives of thyristors include fast thyristors, bidirectional thyristors, reverse conduction thyristors, and light-controlled thyristors. The working condition from the thyristor is that when 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 is utilized between the anode and cathode (the anode is linked to the favorable pole from the power supply, and also the cathode is linked to the negative pole from the power supply). But no forward voltage is applied to the control pole (i.e., K is disconnected), and also the indicator light does not illuminate. This implies that the thyristor will not be conducting and has forward blocking capability.
- Controllable conduction
As shown in Figure b above, when K is closed, and a forward voltage is applied to the control electrode (referred to as a trigger, and also the applied voltage is called trigger voltage), the indicator light turns on. This means that 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 removed (which is, K is excited again), the indicator light still glows. This implies that the thyristor can carry on and conduct. At this time, so that you can 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 to the control electrode, a reverse voltage is applied between the anode and cathode, and also the indicator light does not illuminate at this time. This implies that the thyristor will not be conducting and may reverse blocking.
- In conclusion
1) When the thyristor is subjected to a reverse anode voltage, the thyristor is at a reverse blocking state whatever voltage the gate is subjected to.
2) When the thyristor is subjected to a forward anode voltage, the thyristor is only going to conduct when 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 excited, provided that you will find a specific forward anode voltage, the thyristor will always be excited no matter the gate voltage. That is, following the thyristor is excited, the gate will lose its function. The gate only serves as a trigger.
4) When the thyristor is on, and also the primary circuit voltage (or current) decreases to close to zero, the thyristor turns off.
5) The problem for your thyristor to conduct is that a forward voltage should be applied between the anode and also the cathode, and an appropriate forward voltage ought to be applied between the gate and also the cathode. To turn off a conducting thyristor, the forward voltage between the anode and cathode has to be cut off, or the voltage has to be reversed.
Working principle of thyristor
A thyristor is actually an exclusive triode made up of three PN junctions. It could be equivalently regarded as composed of a PNP transistor (BG2) and an NPN transistor (BG1).
- When a forward voltage is applied between the anode and cathode from the thyristor without applying a forward voltage to the control electrode, although both BG1 and BG2 have forward voltage applied, the thyristor remains turned off because BG1 has no base current. When a forward voltage is applied to the control electrode at this time, BG1 is triggered to create basics current Ig. BG1 amplifies this current, and 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 sent to BG1 for amplification and then sent to BG2 for amplification again. Such repeated amplification forms a crucial 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, which is, the anode and cathode from the thyristor (the size of the current is really based on the size of the burden and the size of Ea), and so the thyristor is entirely excited. This conduction process is completed in a very limited time.
- After the thyristor is excited, its conductive state is going to be maintained through the positive feedback effect from the tube itself. Even when the forward voltage from the control electrode disappears, it really is still inside the conductive state. Therefore, the function of the control electrode is simply to trigger the thyristor to transform on. When the thyristor is excited, the control electrode loses its function.
- The best way to shut off the turned-on thyristor is always to reduce the anode current that it is not enough to keep up the positive feedback process. The best way to reduce the anode current is always to cut off the forward power supply Ea or reverse the link of Ea. The minimum anode current needed to keep your thyristor inside the conducting state is called the holding current from the thyristor. Therefore, as it happens, provided that the anode current is less than the holding current, the thyristor can be turned off.
Exactly what is the difference between a transistor and a thyristor?
Transistors usually contain a PNP or NPN structure made up of three semiconductor materials.
The thyristor is composed 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 opening and closing, allowing fast switching operations.
The thyristor demands a forward voltage and a trigger current on the gate to transform on or off.
Transistors are widely used in amplification, switches, oscillators, and other facets of electronic circuits.
Thyristors are mostly utilized 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 excited or off by manipulating the trigger voltage from 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 sum up, although transistors and thyristors can be utilized in similar applications in some instances, due to their different structures and working principles, they have got noticeable differences in performance and utilize occasions.
Application scope of thyristor
- In power electronic equipment, thyristors can be utilized in frequency converters, motor controllers, welding machines, power supplies, etc.
- Inside the lighting field, thyristors can be utilized in dimmers and light control devices.
- In induction cookers and electric water heaters, thyristors could be used to control the current flow to the heating element.
- In electric vehicles, transistors can be utilized in motor controllers.
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