So what is a thyristor?
A thyristor is a high-power semiconductor device, also known as a silicon-controlled rectifier. Its structure includes four quantities of semiconductor materials, including three PN junctions corresponding for the Anode, Cathode, and control electrode Gate. These three poles are the critical parts in the thyristor, allowing it to 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 commonly used in a variety of electronic circuits, including controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversion.
The graphical symbol of a silicon-controlled rectifier is usually represented by the text symbol “V” or “VT” (in older standards, the letters “SCR”). In addition, derivatives of thyristors also have fast thyristors, bidirectional thyristors, reverse conduction thyristors, and lightweight-controlled thyristors. The functioning condition in the thyristor is 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 can be used between the anode and cathode (the anode is attached 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 used for the control pole (i.e., K is disconnected), and the indicator light does not light up. 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, along with a forward voltage is used for the control electrode (known as a trigger, and the applied voltage is referred to as 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, whether or not the voltage on the control electrode is removed (which is, K is excited again), the indicator light still glows. This shows that the thyristor can still conduct. Currently, in order to stop the conductive thyristor, the power supply Ea should 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 between the anode and cathode, and the indicator light does not light up at this time. This shows that the thyristor will not be conducting and will reverse blocking.
- To sum up
1) When the thyristor is exposed to a reverse anode voltage, the thyristor is at a reverse blocking state regardless of what voltage the gate is exposed to.
2) When the thyristor is exposed to a forward anode voltage, the thyristor will simply conduct if the gate is exposed to a forward voltage. Currently, the thyristor is incorporated in the forward conduction state, the thyristor characteristic, which is, the controllable characteristic.
3) When the thyristor is excited, provided that there exists a specific forward anode voltage, the thyristor will stay excited whatever the gate voltage. That is certainly, following the thyristor is excited, the gate will lose its function. The gate only functions as a trigger.
4) When the thyristor is on, and the primary circuit voltage (or current) decreases to seal 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 the cathode, plus an appropriate forward voltage ought to be applied between the gate and the cathode. To change off a conducting thyristor, the forward voltage between the anode and cathode should be stop, or the voltage should be reversed.
Working principle of thyristor
A thyristor is essentially a distinctive triode made up of three PN junctions. It could be equivalently thought to be consisting of a PNP transistor (BG2) plus an NPN transistor (BG1).
- When a forward voltage is used 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 continues to be switched off because BG1 has no base current. When a forward voltage is used for the control electrode at this time, 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 will be introduced 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 enter a saturated conduction state quickly. A large current appears inside the emitters of these 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), and so the thyristor is completely excited. This conduction process is finished in an exceedingly short time.
- Right after the thyristor is excited, 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 really is still inside the conductive state. Therefore, the function of the control electrode is just to trigger the thyristor to turn on. Once the thyristor is excited, the control electrode loses its function.
- The only way to shut off the turned-on thyristor would be to lessen the anode current that it is inadequate to maintain the positive feedback process. The best way to 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 the thyristor inside the conducting state is referred to as the holding current in the thyristor. Therefore, strictly speaking, provided that the anode current is under the holding current, the thyristor may be switched off.
What exactly is the difference between a transistor along with 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 work of a transistor depends on electrical signals to control its closing and opening, allowing fast switching operations.
The thyristor demands a forward voltage along with a trigger current on the gate to turn on or off.
Transistors are commonly used in amplification, switches, oscillators, and other facets of electronic circuits.
Thyristors are mainly utilized 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 accomplish 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 can 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 can be used in frequency converters, motor controllers, welding machines, power supplies, etc.
- Inside the lighting field, thyristors can be used in dimmers and lightweight 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 used in motor controllers.
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