So what is a thyristor?
A thyristor is actually a high-power semiconductor device, also known as a silicon-controlled rectifier. Its structure includes four quantities of semiconductor components, including three PN junctions corresponding to the Anode, Cathode, and control electrode Gate. These three poles are 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 operating status. Therefore, thyristors are commonly used in various electronic circuits, such as controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversion.
The graphical symbol of a semiconductor device is generally represented through 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 light-controlled thyristors. The operating condition in the thyristor is the fact when a forward voltage is used, 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 involving the anode and cathode (the anode is connected to the favorable pole in the power supply, and the cathode is connected to the negative pole in the power supply). But no forward voltage is used to the control pole (i.e., K is disconnected), and the indicator light does not glow. This implies that the thyristor is not 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 to the control electrode (referred to as a trigger, and the applied voltage is called trigger voltage), the indicator light turns on. Because of this the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, after the thyristor is excited, even if the voltage around the control electrode is taken away (that is, K is excited again), the indicator light still glows. This implies that the thyristor can continue to conduct. Currently, to be able 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 to the control electrode, a reverse voltage is used involving the anode and cathode, and the indicator light does not glow at this time. This implies that the thyristor is not conducting and can reverse blocking.
- In summary
1) When the thyristor is exposed to a reverse anode voltage, the thyristor is in a reverse blocking state no matter what voltage the gate is exposed to.
2) When the thyristor is exposed to a forward anode voltage, the thyristor will only conduct when the gate is exposed to a forward voltage. Currently, the thyristor is incorporated in the forward conduction state, the thyristor characteristic, that is, the controllable characteristic.
3) When the thyristor is excited, as long as you will find a specific forward anode voltage, the thyristor will always be excited regardless of the gate voltage. That is, after 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 close to zero, the thyristor turns off.
5) The problem for your thyristor to conduct is the fact a forward voltage should be applied involving the anode and the cathode, plus an appropriate forward voltage also need to be applied involving the gate and the cathode. To change off a conducting thyristor, the forward voltage involving the anode and cathode should be stop, or the voltage should be reversed.
Working principle of thyristor
A thyristor is basically a distinctive triode made up of three PN junctions. It may be equivalently thought to be comprising a PNP transistor (BG2) plus an NPN transistor (BG1).
- If a forward voltage is used involving 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 continues to be turned off because BG1 has no base current. If a forward voltage is used to the control electrode at this time, BG1 is triggered to produce 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 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 crucial positive feedback, causing both BG1 and BG2 to enter a saturated conduction state quickly. A sizable current appears within the emitters of these two transistors, that is, the anode and cathode in the thyristor (the size of the current is in fact determined by the size of the stress and the size of Ea), so the thyristor is entirely excited. This conduction process is finished in an exceedingly limited time.
- After the thyristor is excited, its conductive state will be maintained through the positive feedback effect in the tube itself. Even when the forward voltage in the control electrode disappears, it is actually still within the conductive state. Therefore, the purpose of the control electrode is only to trigger the thyristor to transform on. When the thyristor is excited, the control electrode loses its function.
- The best way to turn off the turned-on thyristor is always to lessen the anode current that it is not enough to keep the positive feedback process. How you can lessen the anode current is always to stop the forward power supply Ea or reverse the bond of Ea. The minimum anode current required to maintain the thyristor within the conducting state is called the holding current in the thyristor. Therefore, as it happens, as long as the anode current is under the holding current, the thyristor may be turned off.
What 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 made up 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 opening and closing, allowing fast switching operations.
The thyristor needs a forward voltage along with a trigger current in the gate to transform on or off.
Transistors are commonly used in amplification, switches, oscillators, along with other elements of electronic circuits.
Thyristors are mainly utilized in electronic circuits such as 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 controlling the trigger voltage in 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 utilized in similar applications in some instances, due to their different structures and operating principles, they may have noticeable variations in performance and use 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 could 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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