Specifically what is a thyristor?
A thyristor is really a high-power semiconductor device, also called a silicon-controlled rectifier. Its structure contains 4 quantities of semiconductor components, including three PN junctions corresponding towards the Anode, Cathode, and control electrode Gate. These three poles would be 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 widely 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 silicon-controlled rectifier is usually represented from 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-weight-controlled thyristors. The working condition of the thyristor is the fact that whenever a forward voltage is used, 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 involving the anode and cathode (the anode is attached to the favorable pole of the power supply, and also the cathode is linked to the negative pole of the power supply). But no forward voltage is used towards the control pole (i.e., K is disconnected), and also the indicator light does not light up. This demonstrates that the thyristor is not really conducting and has forward blocking capability.
- Controllable conduction
As shown in Figure b above, when K is closed, along with a forward voltage is used towards the control electrode (referred to as a trigger, and also the applied voltage is called trigger voltage), the indicator light switches 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 around the control electrode is removed (that is certainly, K is excited again), the indicator light still glows. This demonstrates that the thyristor can still conduct. At this time, so that you can 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 used towards the control electrode, a reverse voltage is used involving the anode and cathode, and also the indicator light does not light up currently. This demonstrates that the thyristor is not really conducting and may reverse blocking.
- To sum up
1) Once the thyristor is put through a reverse anode voltage, the thyristor is at a reverse blocking state whatever voltage the gate is put through.
2) Once the thyristor is put through a forward anode voltage, the thyristor is only going to conduct when the gate is put through a forward voltage. At this time, the thyristor is incorporated in the forward conduction state, which is the thyristor characteristic, that is certainly, the controllable characteristic.
3) Once the thyristor is excited, as long as there exists a specific forward anode voltage, the thyristor will always be excited whatever the gate voltage. That is, following the thyristor is excited, the gate will lose its function. The gate only works as a trigger.
4) Once the thyristor is on, and also the primary circuit voltage (or current) decreases to close to zero, the thyristor turns off.
5) The condition for your thyristor to conduct is the fact that a forward voltage needs to be applied involving the anode and also the cathode, and an appropriate forward voltage ought to be applied involving the gate and also the cathode. To turn off a conducting thyristor, the forward voltage involving the anode and cathode must be cut off, or the voltage must be reversed.
Working principle of thyristor
A thyristor is essentially a distinctive triode made from three PN junctions. It can be equivalently regarded as comprising a PNP transistor (BG2) and an NPN transistor (BG1).
- When a forward voltage is used involving 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 remains switched off because BG1 has no base current. When a forward voltage is used towards the control electrode currently, BG1 is triggered to generate a base current Ig. BG1 amplifies this current, along with a ß1Ig current is obtained in the 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 vital positive feedback, causing both BG1 and BG2 to get in a saturated conduction state quickly. A big current appears inside the emitters of the two transistors, that is certainly, the anode and cathode of the thyristor (the dimensions of the current is in fact dependant on the dimensions of the burden and the dimensions of Ea), so the thyristor is totally excited. This conduction process is completed in a really short time.
- After the thyristor is excited, its conductive state is going to be maintained from the positive feedback effect of the tube itself. Even when the forward voltage of the control electrode disappears, it really is still inside the conductive state. Therefore, the function of the control electrode is just to trigger the thyristor to change on. After the thyristor is excited, the control electrode loses its function.
- The only method to turn off the turned-on thyristor is to decrease the anode current that it is inadequate to keep up the positive feedback process. The way to decrease the anode current is to cut off the forward power supply Ea or reverse the bond of Ea. The minimum anode current needed to maintain the thyristor inside the conducting state is called the holding current of the thyristor. Therefore, strictly speaking, as long as the anode current is lower than the holding current, the thyristor could be switched off.
Exactly what is the difference between a transistor along with a thyristor?
Transistors usually include a PNP or NPN structure made from three semiconductor materials.
The thyristor is composed of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The task of any transistor depends on electrical signals to control its closing and opening, allowing fast switching operations.
The thyristor needs a forward voltage along with a trigger current in the gate to change on or off.
Transistors are widely used in amplification, switches, oscillators, and other aspects of electronic circuits.
Thyristors are mainly found 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 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 sometimes, because of 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 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 towards the heating element.
- In electric vehicles, transistors can be used in motor controllers.
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