What exactly is a thyristor?
A thyristor is really a high-power semiconductor device, also known as a silicon-controlled rectifier. Its structure consists of 4 levels of semiconductor materials, including 3 PN junctions corresponding to the Anode, Cathode, and control electrode Gate. These 3 poles would be 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 functioning status. Therefore, thyristors are widely used in various electronic circuits, like controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversion.
The graphical symbol of a semiconductor device is generally represented from the text symbol “V” or “VT” (in older standards, the letters “SCR”). Additionally, derivatives of thyristors also include fast thyristors, bidirectional thyristors, reverse conduction thyristors, and lightweight-controlled thyristors. The functioning condition in 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 connected to the favorable pole in the power supply, and also the cathode is attached to the negative pole in the power supply). But no forward voltage is applied to the control pole (i.e., K is disconnected), and also the indicator light will not light up. This implies that the thyristor is not conducting and it has forward blocking capability.
- Controllable conduction
As shown in Figure b above, when K is closed, as well as a forward voltage is applied to the control electrode (referred to as a trigger, and also the applied voltage is known as trigger voltage), the indicator light switches on. This means that the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, after the thyristor is excited, even if the voltage on the control electrode is taken off (that is certainly, K is excited again), the indicator light still glows. This implies that the thyristor can continue to 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 to the control electrode, a reverse voltage is applied between the anode and cathode, and also the indicator light will not light up currently. This implies that the thyristor is not conducting and will reverse blocking.
- To sum up
1) Once the thyristor is exposed to a reverse anode voltage, the thyristor is within a reverse blocking state regardless of what voltage the gate is exposed to.
2) Once the thyristor is exposed to a forward anode voltage, the thyristor will only conduct if the gate is exposed to a forward voltage. At the moment, the thyristor is within the forward conduction state, the thyristor characteristic, that is certainly, the controllable characteristic.
3) Once the thyristor is excited, so long as you will find a specific forward anode voltage, the thyristor will always be excited no matter the gate voltage. Which is, 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 also the primary circuit voltage (or current) decreases to close to zero, the thyristor turns off.
5) The problem for that thyristor to conduct is that a forward voltage ought to 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 should be shut down, or perhaps the voltage should be reversed.
Working principle of thyristor
A thyristor is actually a distinctive triode made up of three PN junctions. It could 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 in the thyristor without applying a forward voltage to 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 applied to the control electrode currently, BG1 is triggered to generate a base current Ig. BG1 amplifies this current, as well as a ß1Ig current is obtained in their 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 then delivered to BG2 for amplification again. Such repeated amplification forms an essential positive feedback, causing both BG1 and BG2 to get in a saturated conduction state quickly. A big current appears within the emitters of the two transistors, that is certainly, the anode and cathode in the thyristor (the dimensions of the current is really dependant on the dimensions of the load and the dimensions of Ea), therefore the thyristor is completely excited. This conduction process is finished in a really short time.
- Following the thyristor is excited, its conductive state will likely be maintained from the positive feedback effect in the tube itself. Whether or not the forward voltage in the control electrode disappears, it is still within the conductive state. Therefore, the purpose 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 always to lessen the anode current so that it is inadequate to maintain the positive feedback process. The way to lessen the anode current is always to shut down the forward power supply Ea or reverse the bond of Ea. The minimum anode current needed to keep the thyristor within the conducting state is known as the holding current in the thyristor. Therefore, as it happens, so long as the anode current is lower than the holding current, the thyristor may be switched off.
What exactly is the difference between a transistor as well as a thyristor?
Transistors usually consist of 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 job of a 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 in the gate to change on or off.
Transistors are widely used in amplification, switches, oscillators, along with other elements of electronic circuits.
Thyristors are mostly used in electronic circuits like 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 attain current amplification.
The thyristor is excited or off by managing the trigger voltage in the control electrode to comprehend the switching function.
The circuit parameters of thyristors are related to stability and reliability and in most cases have higher turn-off voltage and larger on-current.
To sum up, although transistors and thyristors may be used in similar applications sometimes, because of the different structures and functioning principles, they have got noticeable differences in performance and utilize occasions.
Application scope of thyristor
- In power electronic equipment, thyristors may be used in frequency converters, motor controllers, welding machines, power supplies, etc.
- Inside the lighting field, thyristors may be used in dimmers and lightweight 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 may be used in motor controllers.
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