So what is a thyristor?
A thyristor is really a high-power semiconductor device, also called a silicon-controlled rectifier. Its structure contains four levels of semiconductor elements, including three PN junctions corresponding for the Anode, Cathode, and control electrode Gate. These three poles are definitely 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 working status. Therefore, thyristors are widely used in various electronic circuits, including 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 have fast thyristors, bidirectional thyristors, reverse conduction thyristors, and light-controlled thyristors. The working condition in the thyristor is the fact that each time 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 used involving the anode and cathode (the anode is linked to the favorable pole in the power supply, and also the cathode is linked to the negative pole in the power supply). But no forward voltage is used for the control pole (i.e., K is disconnected), and also the indicator light does not light up. This shows 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 used for the control electrode (known as a trigger, and also the applied voltage is known as trigger voltage), the indicator light switches on. Which means that the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, right after the thyristor is turned on, whether or not the voltage around the control electrode is removed (which is, K is turned on again), the indicator light still glows. This shows that the thyristor can still conduct. Currently, so that you can 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 used for the control electrode, a reverse voltage is used involving the anode and cathode, and also the indicator light does not light up at the moment. This shows that the thyristor is not conducting and will reverse blocking.
- To sum up
1) When the thyristor is put through a reverse anode voltage, the thyristor is at a reverse blocking state no matter what voltage the gate is put through.
2) When the thyristor is put through a forward anode voltage, the thyristor is only going to conduct if the gate is put through a forward voltage. Currently, the thyristor is within the forward conduction state, which is the thyristor characteristic, which is, the controllable characteristic.
3) When the thyristor is turned on, provided that there exists a specific forward anode voltage, the thyristor will stay turned on no matter the gate voltage. That is certainly, right after the thyristor is turned on, the gate will lose its function. The gate only functions 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 condition for that thyristor to conduct is the fact that a forward voltage ought to be applied involving the anode and also the cathode, as well as an appropriate forward voltage should also 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 stop, or the voltage must be reversed.
Working principle of thyristor
A thyristor is actually an exclusive triode made from three PN junctions. It may be equivalently thought to be comprising a PNP transistor (BG2) as well as an NPN transistor (BG1).
- If a forward voltage is used involving 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 is still turned off because BG1 has no base current. If a forward voltage is used for the control electrode at the moment, BG1 is triggered to create basics 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 be brought in the collector of BG2. This current is delivered to BG1 for amplification and then delivered to BG2 for amplification again. Such repeated amplification forms a crucial positive feedback, causing both BG1 and BG2 to get into a saturated conduction state quickly. A sizable current appears inside the emitters of the two transistors, which is, the anode and cathode in the thyristor (the size of the current is in fact dependant on the size of the stress and the size of Ea), and so the thyristor is totally turned on. This conduction process is completed in a really short period of time.
- Following the thyristor is turned on, its conductive state will be maintained by the positive feedback effect in the tube itself. Even if the forward voltage in the control electrode disappears, it is still inside the conductive state. Therefore, the purpose of the control electrode is only to trigger the thyristor to transform on. After the thyristor is turned on, the control electrode loses its function.
- The only way to shut off the turned-on thyristor would be to lessen the anode current so that it is inadequate to keep the positive feedback process. How you can lessen the anode current would be to stop 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 known as the holding current in the thyristor. Therefore, strictly speaking, provided that the anode current is under the holding current, the thyristor may be turned off.
What exactly is the difference between a transistor as well as a thyristor?
Transistors usually contain 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 job of any transistor relies upon electrical signals to control its closing and opening, allowing fast switching operations.
The thyristor needs a forward voltage as well as a trigger current at the gate to transform on or off.
Transistors are widely used in amplification, switches, oscillators, as well as other facets of electronic circuits.
Thyristors are mainly used in electronic circuits including 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 turned on or off by controlling the trigger voltage in the control electrode to understand the switching function.
The circuit parameters of thyristors are based on stability and reliability and often have higher turn-off voltage and larger on-current.
To summarize, although transistors and thyristors can be utilized in similar applications in some cases, because of their different structures and working principles, they have got noticeable variations 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 light control devices.
- In induction cookers and electric water heaters, thyristors can be used to control the current flow for the heating element.
- In electric vehicles, transistors can be utilized in motor controllers.
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