Specifically what 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 3 PN junctions corresponding for the Anode, Cathode, and control electrode Gate. These 3 poles are the critical parts in 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 operating status. Therefore, thyristors are popular in various electronic circuits, such as controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency alteration.
The graphical symbol of any silicon-controlled rectifier is normally represented from the text symbol “V” or “VT” (in older standards, the letters “SCR”). Additionally, derivatives of thyristors include fast thyristors, bidirectional thyristors, reverse conduction thyristors, and light-weight-controlled thyristors. The operating condition in the thyristor is the fact that each time 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 in between the anode and cathode (the anode is connected to the favorable pole in the power supply, and the cathode is attached to the negative pole in the power supply). But no forward voltage is applied for the control pole (i.e., K is disconnected), and the indicator light will not glow. This shows that the thyristor will not be conducting and it has forward blocking capability.
- Controllable conduction
As shown in Figure b above, when K is closed, and a forward voltage is applied for the control electrode (known as a trigger, and the applied voltage is called trigger voltage), the indicator light turns on. Which means that the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, following the thyristor is switched on, whether or not the voltage in the control electrode is taken off (which is, K is switched on again), the indicator light still glows. This shows that the thyristor can carry on and conduct. Currently, to be able to cut off the conductive thyristor, the power supply Ea must 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 will not glow currently. This shows that the thyristor will not be conducting and can reverse blocking.
- In conclusion
1) If the thyristor is put through a reverse anode voltage, the thyristor is within 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 when the gate is put through a forward voltage. Currently, the thyristor is incorporated in the forward conduction state, which is the thyristor characteristic, which is, the controllable characteristic.
3) If the thyristor is switched on, as long as you will find a specific forward anode voltage, the thyristor will stay switched on whatever the gate voltage. Which is, following the thyristor is switched on, the gate will lose its function. The gate only works as a trigger.
4) If the thyristor is on, and the primary circuit voltage (or current) decreases to seal to zero, the thyristor turns off.
5) The condition for that thyristor to conduct is the fact that a forward voltage needs to be applied in between the anode and the cathode, and an appropriate forward voltage also need to be applied in between the gate and the cathode. To change off a conducting thyristor, the forward voltage in between the anode and cathode must be cut off, or even the voltage must be reversed.
Working principle of thyristor
A thyristor is essentially a unique triode composed of three PN junctions. It could be equivalently thought to be consisting of a PNP transistor (BG2) and an NPN transistor (BG1).
- When a forward voltage is applied in between the anode and cathode in the thyristor without applying a forward voltage for 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 for the control electrode currently, BG1 is triggered to generate a base 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 delivered to BG1 for amplification then delivered 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 sizable current appears in the emitters of the two transistors, which is, the anode and cathode in the thyristor (the dimensions of the current is actually based on the dimensions of the load and the dimensions of Ea), so the thyristor is entirely switched on. This conduction process is completed in an exceedingly short time.
- Following the thyristor is switched on, its conductive state is going to be maintained from the positive feedback effect in the tube itself. Whether or not the forward voltage in the control electrode disappears, it really is still in the conductive state. Therefore, the function of the control electrode is simply to trigger the thyristor to change on. After the thyristor is switched on, the control electrode loses its function.
- The only method to shut off the turned-on thyristor is to lessen the anode current so that it is inadequate to keep the positive feedback process. The best way to lessen the anode current is to cut off 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 called the holding current in the thyristor. Therefore, as it happens, as long as the anode current is under the holding current, the thyristor can be turned off.
Exactly what is the distinction between a transistor and 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 work of any transistor relies upon electrical signals to control its closing and opening, allowing fast switching operations.
The thyristor demands a forward voltage and a trigger current at the gate to change on or off.
Transistors are popular in amplification, switches, oscillators, along with other aspects of electronic circuits.
Thyristors are mostly utilized in electronic circuits such as 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 achieve current amplification.
The thyristor is switched on or off by controlling the trigger voltage in 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 sum up, although transistors and thyristors can be used in similar applications in some instances, because of their different structures and operating principles, they have got noticeable variations in performance and utilize occasions.
Application scope of thyristor
- In power electronic equipment, thyristors can be used in frequency converters, motor controllers, welding machines, power supplies, etc.
- Inside the lighting field, thyristors can be used in dimmers and light-weight 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 can be used in motor controllers.
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