Just what is a thyristor?
A thyristor is a high-power semiconductor device, also referred to as a silicon-controlled rectifier. Its structure contains 4 levels of semiconductor elements, including 3 PN junctions corresponding for the Anode, Cathode, and control electrode Gate. These 3 poles would be 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 popular in various electronic circuits, like controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency alteration.
The graphical symbol of a 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 light-controlled thyristors. The operating condition in the thyristor is the fact that whenever a forward voltage is applied, the gate should 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 linked to the favorable pole in the power supply, as well as the cathode is attached to the negative pole in the power supply). But no forward voltage is applied for the control pole (i.e., K is disconnected), as well as the indicator light fails to illuminate. This demonstrates 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 applied for the control electrode (known as a trigger, as well as 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 turned on, even when the voltage around the control electrode is taken off (that is certainly, K is turned on again), the indicator light still glows. This demonstrates that the thyristor can carry on and conduct. Currently, in order to shut down the conductive thyristor, the power supply Ea must be shut down or reversed.
- Reverse blocking
As shown in Figure d above, although a forward voltage is applied for the control electrode, a reverse voltage is applied between the anode and cathode, as well as the indicator light fails to illuminate at the moment. This demonstrates that the thyristor will not be conducting and may reverse blocking.
- To sum up
1) When the thyristor is put through a reverse anode voltage, the thyristor is at a reverse blocking state whatever voltage the gate is put through.
2) When the thyristor is put through a forward anode voltage, the thyristor is only going to conduct once the gate is put through a forward voltage. Currently, the thyristor is incorporated in the forward conduction state, the thyristor characteristic, that is certainly, the controllable characteristic.
3) When the thyristor is turned on, provided that you will find a specific forward anode voltage, the thyristor will always be turned on whatever the gate voltage. That is certainly, right after the thyristor is turned on, the gate will lose its function. The gate only works as a trigger.
4) When the thyristor is on, as well as the primary circuit voltage (or current) decreases to close to zero, the thyristor turns off.
5) The disorder for that thyristor to conduct is the fact that a forward voltage ought to be applied between the anode as well as the cathode, and an appropriate forward voltage should also be applied between the gate as well as the cathode. To turn off a conducting thyristor, the forward voltage between the anode and cathode must be shut down, or the voltage must be reversed.
Working principle of thyristor
A thyristor is essentially a unique triode composed of three PN junctions. It could be equivalently thought to be composed of a PNP transistor (BG2) and an NPN transistor (BG1).
- When a forward voltage is applied 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 is still switched off because BG1 has no base current. When a forward voltage is applied for the control electrode at the moment, BG1 is triggered to create a base current Ig. BG1 amplifies this current, along with 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 introduced the collector of BG2. This current is sent to BG1 for amplification then sent to BG2 for amplification again. Such repeated amplification forms a crucial positive feedback, causing both BG1 and BG2 to get in a saturated conduction state quickly. A large current appears inside the emitters of the two transistors, that is certainly, the anode and cathode in the thyristor (how big the current is in fact dependant on how big the load and how big Ea), so the thyristor is entirely turned on. This conduction process is finished in an exceedingly short period of time.
- After the thyristor is turned on, its conductive state is going to be maintained by the positive feedback effect in the tube itself. Whether or not the forward voltage in the control electrode disappears, it really is still inside the conductive state. Therefore, the function of the control electrode is simply to trigger the thyristor to change on. Once the thyristor is turned on, the control electrode loses its function.
- The only method to turn off the turned-on thyristor is always to lessen the anode current so that it is not enough to maintain the positive feedback process. How you can lessen the anode current is always to shut down the forward power supply Ea or reverse the bond of Ea. The minimum anode current needed to maintain the thyristor inside the conducting state is called the holding current in the thyristor. Therefore, as it happens, provided that the anode current is less than the holding current, the thyristor can be switched off.
What exactly is the difference between a transistor along with a thyristor?
Transistors usually consist of a PNP or NPN structure composed of three semiconductor materials.
The thyristor is made up of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The job of a transistor relies upon electrical signals to control its opening and closing, allowing fast switching operations.
The thyristor demands a forward voltage along with a trigger current on the gate to change on or off.
Transistors are popular in amplification, switches, oscillators, as well as other elements of electronic circuits.
Thyristors are mostly found 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 turned on or off by manipulating the trigger voltage in 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 summarize, although transistors and thyristors can be used in similar applications in some instances, because of the different structures and operating principles, they have noticeable variations in performance and make use of occasions.
Application scope of thyristor
- In power electronic equipment, thyristors can be used in frequency converters, motor controllers, welding machines, power supplies, etc.
- Within the lighting field, thyristors can be used 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 used in motor controllers.
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