Just what is a thyristor?
A thyristor is a high-power semiconductor device, also referred to as a silicon-controlled rectifier. Its structure includes 4 quantities of semiconductor materials, including 3 PN junctions corresponding towards the Anode, Cathode, and control electrode Gate. These 3 poles are definitely 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 operating status. Therefore, thyristors are popular in a variety of electronic circuits, including controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency alteration.
The graphical symbol of any semiconductor device is normally represented from the text symbol “V” or “VT” (in older standards, the letters “SCR”). Furthermore, derivatives of thyristors also have fast thyristors, bidirectional thyristors, reverse conduction thyristors, and light-controlled thyristors. The operating condition from the thyristor is the fact when 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 is used between 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 applied towards the control pole (i.e., K is disconnected), as well as the indicator light fails to 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, as well as a forward voltage is applied towards the control electrode (called a trigger, as well as the applied voltage is referred to as trigger voltage), the indicator light switches on. Which means that 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 taken off (that is, K is excited again), the indicator light still glows. This implies that the thyristor can continue to conduct. At this time, in order to cut off the conductive thyristor, the power supply Ea has to be cut off or reversed.
- Reverse blocking
As shown in Figure d above, although a forward voltage is applied towards the control electrode, a reverse voltage is applied between the anode and cathode, as well as the indicator light fails to glow at this time. This implies that the thyristor is not conducting and may reverse blocking.
- In conclusion
1) If the thyristor is subjected to a reverse anode voltage, the thyristor is at a reverse blocking state whatever voltage the gate is subjected to.
2) If 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 within the forward conduction state, the thyristor characteristic, that is, the controllable characteristic.
3) If the thyristor is excited, as long as there is a specific forward anode voltage, the thyristor will always be excited regardless of the gate voltage. Which is, following the thyristor is excited, the gate will lose its function. The gate only functions as a trigger.
4) If the thyristor is on, as well as 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 needs to be applied between the anode as well as the cathode, and an appropriate forward voltage ought to be applied between the gate as well as the cathode. To turn off a conducting thyristor, the forward voltage between the anode and cathode has to be cut off, or perhaps the voltage has to be reversed.
Working principle of thyristor
A thyristor is actually a unique triode composed of three PN junctions. It may be equivalently thought to be consisting of a PNP transistor (BG2) and an NPN transistor (BG1).
- In case a forward voltage is applied between the anode and cathode from the thyristor without applying a forward voltage towards 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. In case a forward voltage is applied towards the control electrode at this time, BG1 is triggered to generate basics 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 large current appears within the emitters of the two transistors, that is, the anode and cathode from the thyristor (how big the current is really based on how big the load and how big Ea), therefore the thyristor is completely excited. This conduction process is done in an exceedingly short time.
- Following the thyristor is excited, its conductive state will be maintained from the positive feedback effect from the tube itself. Even when the forward voltage from the control electrode disappears, it is still within the conductive state. Therefore, the function of the control electrode is only to trigger the thyristor to change on. After the thyristor is excited, the control electrode loses its function.
- The best way to switch off the turned-on thyristor is always to lessen the anode current that it is not enough to maintain the positive feedback process. The best way to lessen the anode current is always to cut off the forward power supply Ea or reverse the link of Ea. The minimum anode current required to keep your thyristor within the conducting state is referred to as the holding current from the thyristor. Therefore, strictly speaking, as long as the anode current is under the holding current, the thyristor may be turned off.
Exactly what is the difference between a transistor as well as a thyristor?
Transistors usually include 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 task of any 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 popular in amplification, switches, oscillators, and other elements of electronic circuits.
Thyristors are mostly found in electronic circuits including 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 attain current amplification.
The thyristor is excited or off by manipulating the trigger voltage from 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 may be used in similar applications in some instances, because of their different structures and operating principles, they have got 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 could 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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