Specifically what is a thyristor?
A thyristor is actually a high-power semiconductor device, also known as a silicon-controlled rectifier. Its structure contains 4 quantities of semiconductor components, including 3 PN junctions corresponding to the Anode, Cathode, and control electrode Gate. These 3 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 popular in different electronic circuits, like controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversion.
The graphical symbol of any silicon-controlled rectifier is normally represented by the text symbol “V” or “VT” (in older standards, the letters “SCR”). Furthermore, derivatives of thyristors also include fast thyristors, bidirectional thyristors, reverse conduction thyristors, and lightweight-controlled thyristors. The working condition of 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 can be used in between the anode and cathode (the anode is attached 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 to 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, along with a forward voltage is applied to the control electrode (known as a trigger, and 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, right after the thyristor is excited, even if the voltage around the control electrode is removed (that is, K is excited again), the indicator light still glows. This implies that the thyristor can carry on and conduct. At the moment, in order to stop the conductive thyristor, the power supply Ea must be stop 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 in between the anode and cathode, and the indicator light fails to glow at the moment. This implies that the thyristor is not conducting and may reverse blocking.
- To sum up
1) If the thyristor is subjected to a reverse anode voltage, the thyristor is at a reverse blocking state no matter what voltage the gate is subjected to.
2) If the thyristor is subjected to a forward anode voltage, the thyristor is only going to conduct if the gate is subjected to a forward voltage. At the moment, the thyristor is within the forward conduction state, which is the thyristor characteristic, that is, the controllable characteristic.
3) If the thyristor is excited, so long as you will find a specific forward anode voltage, the thyristor will remain excited no matter the gate voltage. Which is, right after the thyristor is excited, the gate will lose its function. The gate only functions 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 condition for that thyristor to conduct is the fact that a forward voltage ought to be applied in between the anode and the cathode, as well as 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 stop, or even the voltage must be reversed.
Working principle of thyristor
A thyristor is essentially a unique triode made from three PN junctions. It can be equivalently thought to be 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 to the control electrode, although both BG1 and BG2 have forward voltage applied, the thyristor remains switched off because BG1 has no base current. In case a forward voltage is applied to the control electrode at the moment, BG1 is triggered to generate basics 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 sent to BG1 for amplification then sent 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 sizable current appears in the emitters of the two transistors, that is, the anode and cathode of the thyristor (how big the current is in fact determined by how big the stress and how big Ea), and so the thyristor is totally excited. This conduction process is finished in a really limited time.
- Right after the thyristor is excited, its conductive state is going to be maintained by the positive feedback effect of the tube itself. Even when the forward voltage of the control electrode disappears, it is still in the conductive state. Therefore, the purpose of the control electrode is just to trigger the thyristor to turn on. After the thyristor is excited, the control electrode loses its function.
- The best way to shut off the turned-on thyristor is to lessen the anode current that it is insufficient to keep the positive feedback process. The way to lessen the anode current is to stop the forward power supply Ea or reverse the connection of Ea. The minimum anode current necessary to maintain the thyristor in the conducting state is called the holding current of the thyristor. Therefore, strictly speaking, so long as the anode current is lower than the holding current, the thyristor may be switched off.
What is the difference between a transistor along with a thyristor?
Transistors usually contain a PNP or NPN structure made from three semiconductor materials.
The thyristor consists 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 opening and closing, allowing fast switching operations.
The thyristor demands a forward voltage along with a trigger current at the gate to turn on or off.
Transistors are popular in amplification, switches, oscillators, and 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.
Way 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 realize the switching function.
The circuit parameters of thyristors are based on stability and reliability and usually 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 make use of occasions.
Application scope of thyristor
- In power electronic equipment, thyristors can be utilized in frequency converters, motor controllers, welding machines, power supplies, etc.
- In the lighting field, thyristors can be utilized in dimmers and lightweight control devices.
- In induction cookers and electric water heaters, thyristors may 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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