So what is a thyristor?
A thyristor is actually a high-power semiconductor device, also known as a silicon-controlled rectifier. Its structure includes 4 quantities of semiconductor elements, including 3 PN junctions corresponding to the Anode, Cathode, and control electrode Gate. These 3 poles are the critical parts from 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 a variety of electronic circuits, including controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversion.
The graphical symbol of any Thyristor is usually represented from the text symbol “V” or “VT” (in older standards, the letters “SCR”). In addition, derivatives of thyristors also include fast thyristors, bidirectional thyristors, reverse conduction thyristors, and lightweight-controlled thyristors. The working condition from 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 can be used involving the anode and cathode (the anode is connected to the favorable pole from the power supply, as well as the cathode is attached to the negative pole from the power supply). But no forward voltage is used to the control pole (i.e., K is disconnected), as well as the indicator light does not illuminate. This demonstrates that the thyristor is not really conducting and has forward blocking capability.
- Controllable conduction
As shown in Figure b above, when K is closed, and a forward voltage is used to the control electrode (referred to as a trigger, as well as the applied voltage is referred to as trigger voltage), the indicator light turns on. Which means that the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, following the thyristor is switched on, whether or not the voltage in the control electrode is taken away (which is, K is switched on again), the indicator light still glows. This demonstrates that the thyristor can carry on and conduct. At this time, to be able 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 used to the control electrode, a reverse voltage is used involving the anode and cathode, as well as the indicator light does not illuminate currently. This demonstrates that the thyristor is not really conducting and may reverse blocking.
- In summary
1) Once the thyristor is subjected to a reverse anode voltage, the thyristor is at a reverse blocking state regardless of what voltage the gate is subjected to.
2) Once the thyristor is subjected to a forward anode voltage, the thyristor is only going to conduct once the gate is subjected to a forward voltage. At this time, the thyristor is incorporated in the forward conduction state, the thyristor characteristic, which is, the controllable characteristic.
3) Once the thyristor is switched on, as long as there exists a specific forward anode voltage, the thyristor will stay switched on regardless of the gate voltage. That is certainly, following the thyristor is switched on, the gate will lose its function. The gate only functions as a trigger.
4) Once the thyristor is on, as well as the primary circuit voltage (or current) decreases to seal to zero, the thyristor turns off.
5) The problem for the thyristor to conduct is the fact that a forward voltage needs to be applied involving the anode as well as the cathode, and an appropriate forward voltage also need to be applied involving the gate as well as the cathode. To transform off a conducting thyristor, the forward voltage involving the anode and cathode must be stop, or perhaps the voltage must be reversed.
Working principle of thyristor
A thyristor is essentially a distinctive triode made up of three PN junctions. It may be equivalently viewed as composed of a PNP transistor (BG2) and an NPN transistor (BG1).
- In case a forward voltage is used involving the anode and cathode from the thyristor without applying a forward voltage to the control electrode, although both BG1 and BG2 have forward voltage applied, the thyristor is still turned off because BG1 has no base current. In case a forward voltage is used to the control electrode currently, BG1 is triggered to generate basics current Ig. BG1 amplifies this current, and a ß1Ig current is obtained in their 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 brought to BG1 for amplification and then brought to BG2 for amplification again. Such repeated amplification forms a vital positive feedback, causing both BG1 and BG2 to enter a saturated conduction state quickly. A sizable current appears in the emitters of these two transistors, which is, the anode and cathode from the thyristor (how big the current is actually dependant on how big the load and how big Ea), and so the thyristor is entirely switched on. This conduction process is done in a very limited time.
- Right after the thyristor is switched on, its conductive state is going to be maintained from the positive feedback effect from the tube itself. Even if the forward voltage from the control electrode disappears, it is actually still in the conductive state. Therefore, the function of the control electrode is only to trigger the thyristor to transform on. When the thyristor is switched on, the control electrode loses its function.
- The only method to shut off the turned-on thyristor is always to lessen the anode current so that it is insufficient 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 in the conducting state is referred to as the holding current from the thyristor. Therefore, as it happens, as long as the anode current is lower than the holding current, the thyristor could be turned off.
Exactly what is the distinction between a transistor and a thyristor?
Transistors usually include 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 any transistor depends on electrical signals to control its closing and opening, allowing fast switching operations.
The thyristor needs a forward voltage and a trigger current in the gate to transform on or off.
Transistors are popular in amplification, switches, oscillators, as well as other aspects of electronic circuits.
Thyristors are mostly utilized 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 accomplish current amplification.
The thyristor is switched on or off by manipulating the trigger voltage from the control electrode to understand the switching function.
The circuit parameters of thyristors are related to stability and reliability and often have higher turn-off voltage and larger on-current.
To sum up, although transistors and thyristors can be used in similar applications in some instances, because of their different structures and working 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 could be used to control the current flow to the heating element.
- In electric vehicles, transistors can be used in motor controllers.
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