Specifically what is a thyristor?
A thyristor is really a high-power semiconductor device, also referred to as a silicon-controlled rectifier. Its structure includes 4 quantities of semiconductor elements, including 3 PN junctions corresponding towards the Anode, Cathode, and control electrode Gate. These 3 poles are the critical parts of 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 functioning status. Therefore, thyristors are commonly used in different electronic circuits, like controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency alteration.
The graphical symbol of the Thyristor is usually represented through the text symbol “V” or “VT” (in older standards, the letters “SCR”). Additionally, derivatives of thyristors include fast thyristors, bidirectional thyristors, reverse conduction thyristors, and light-controlled thyristors. The functioning condition of the thyristor is the fact each time a forward voltage is used, 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 connected to the favorable pole of the power supply, and also the cathode is connected to the negative pole of the power supply). But no forward voltage is used towards the control pole (i.e., K is disconnected), and also the indicator light will not illuminate. This implies 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, as well as a forward voltage is used towards the control electrode (referred to as a trigger, and also the applied voltage is referred to as trigger voltage), the indicator light switches on. This 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 in the control electrode is taken off (that is, K is turned on again), the indicator light still glows. This implies that the thyristor can still conduct. At this time, to be able to cut off the conductive thyristor, the power supply Ea must be cut off or reversed.
- Reverse blocking
As shown in Figure d above, although a forward voltage is used towards the control electrode, a reverse voltage is used between the anode and cathode, and also the indicator light will not illuminate at this time. This implies that the thyristor will not be conducting and can reverse blocking.
- In conclusion
1) Once the thyristor is exposed to a reverse anode voltage, the thyristor is within a reverse blocking state no matter what voltage the gate is exposed to.
2) Once the thyristor is exposed to a forward anode voltage, the thyristor is only going to conduct if the gate is exposed to a forward voltage. At this time, the thyristor is in the forward conduction state, the thyristor characteristic, that is, the controllable characteristic.
3) Once the thyristor is turned on, provided that there is a specific forward anode voltage, the thyristor will remain 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) Once the thyristor is on, and also the primary circuit voltage (or current) decreases to close to zero, the thyristor turns off.
5) The disorder for your thyristor to conduct is the fact a forward voltage should be applied between the anode and also the cathode, plus an appropriate forward voltage should also be applied between the gate and also the cathode. To turn off a conducting thyristor, the forward voltage between the anode and cathode must be cut off, or even the voltage must be reversed.
Working principle of thyristor
A thyristor is actually a unique triode made up of three PN junctions. It may be equivalently thought to be comprising a PNP transistor (BG2) plus an NPN transistor (BG1).
- In case a forward voltage is used between the anode and cathode of the thyristor without applying a forward voltage towards the control electrode, although both BG1 and BG2 have forward voltage applied, the thyristor remains turned off because BG1 has no base current. In case a forward voltage is used towards the control electrode at this time, BG1 is triggered to generate a base current Ig. BG1 amplifies this current, as well as 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 after that delivered 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 sizable current appears within the emitters of the two transistors, that is, the anode and cathode of the thyristor (the dimensions of the current is actually determined by the dimensions of the stress and the dimensions of Ea), therefore the thyristor is completely turned on. This conduction process is completed in a very limited time.
- After the thyristor is turned on, its conductive state will be maintained through the positive feedback effect of the tube itself. Even when the forward voltage of the control electrode disappears, it really is still within the conductive state. Therefore, the purpose of the control electrode is only to trigger the thyristor to change on. When the thyristor is turned on, the control electrode loses its function.
- The only method to switch off the turned-on thyristor is to reduce the anode current that it is insufficient to maintain the positive feedback process. The best way to reduce the anode current is to cut off the forward power supply Ea or reverse the bond of Ea. The minimum anode current required to keep your thyristor within the conducting state is referred to as the holding current of the thyristor. Therefore, as it happens, provided that the anode current is under the holding current, the thyristor can be turned off.
Exactly what is the difference between a transistor as well as a thyristor?
Transistors usually contain 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 the transistor relies upon electrical signals to control its opening and closing, allowing fast switching operations.
The thyristor requires a forward voltage as well as a trigger current on the gate to change on or off.
Transistors are commonly used in amplification, switches, oscillators, as well as other elements of electronic circuits.
Thyristors are mostly used in electronic circuits like 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 turned on or off by manipulating the trigger voltage of 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 sum up, although transistors and thyristors may be used in similar applications sometimes, because of the different structures and functioning principles, they have noticeable differences in performance and utilize occasions.
Application scope of thyristor
- In power electronic equipment, thyristors may be used in frequency converters, motor controllers, welding machines, power supplies, etc.
- In the lighting field, thyristors may be used in dimmers and light control devices.
- In induction cookers and electric water heaters, thyristors can be used to control the current flow towards the heating element.
- In electric vehicles, transistors may be used in motor controllers.
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