So what is a thyristor?
A thyristor is really a high-power semiconductor device, also known as a silicon-controlled rectifier. Its structure consists of 4 quantities of semiconductor components, including 3 PN junctions corresponding for the Anode, Cathode, and control electrode Gate. These 3 poles are definitely 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 working status. Therefore, thyristors are widely used in a variety of electronic circuits, including controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversion.
The graphical symbol of a silicon-controlled rectifier is usually represented from the text symbol “V” or “VT” (in older standards, the letters “SCR”). Additionally, derivatives of thyristors also include fast thyristors, bidirectional thyristors, reverse conduction thyristors, and lightweight-controlled thyristors. The working condition of the thyristor is that each time a forward voltage is applied, 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 in between the anode and cathode (the anode is connected to the favorable pole of the power supply, and the cathode is linked to the negative pole of the power supply). But no forward voltage is applied for the control pole (i.e., K is disconnected), and the indicator light fails to illuminate. This implies that the thyristor will not be conducting and contains forward blocking capability.
- Controllable conduction
As shown in Figure b above, when K is closed, and a forward voltage is applied for the control electrode (known as a trigger, and the applied voltage is called trigger voltage), the indicator light switches on. Which 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 removed (that is, K is turned on again), the indicator light still glows. This implies that the thyristor can continue to conduct. At the moment, 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 in between the anode and cathode, and the indicator light fails to illuminate currently. This implies that the thyristor will not be conducting and may reverse blocking.
- In conclusion
1) When the thyristor is exposed to a reverse anode voltage, the thyristor is within a reverse blocking state regardless of what voltage the gate is exposed to.
2) When the thyristor is exposed to a forward anode voltage, the thyristor will simply conduct if the gate is exposed to a forward voltage. At the moment, the thyristor is in the forward conduction state, which is the thyristor characteristic, that is, the controllable characteristic.
3) When the thyristor is turned on, as long as there exists a specific forward anode voltage, the thyristor will always be turned on no matter the gate voltage. That is, right after the thyristor is turned on, 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 seal to zero, the thyristor turns off.
5) The condition for the thyristor to conduct is that a forward voltage ought to be applied in between the anode and the cathode, plus an appropriate forward voltage should also be applied in between the gate and the cathode. To turn off a conducting thyristor, the forward voltage in between the anode and cathode must be shut down, or even the voltage must be reversed.
Working principle of thyristor
A thyristor is basically an exclusive triode made up of three PN junctions. It may be equivalently thought to be consisting of a PNP transistor (BG2) plus an NPN transistor (BG1).
- If a forward voltage is applied in between the anode and cathode of the thyristor without applying a forward voltage for the control electrode, although both BG1 and BG2 have forward voltage applied, the thyristor remains turned off because BG1 has no base current. If a forward voltage is applied for the control electrode currently, BG1 is triggered to create a base current Ig. BG1 amplifies this current, and a ß1Ig current is obtained in its collector. This current is precisely the base current of BG2. After amplification by BG2, a ß1ß2Ig current will likely 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 get in a saturated conduction state quickly. A sizable current appears inside the emitters of the two transistors, that is, the anode and cathode of the thyristor (how big the current is actually dependant on how big the stress and how big Ea), therefore the thyristor is completely turned on. This conduction process is done in a really short time.
- Right after the thyristor is turned on, its conductive state will likely be maintained from the positive feedback effect of the tube itself. Even if the forward voltage of the control electrode disappears, it really is still inside the conductive state. Therefore, the purpose of the control electrode is just to trigger the thyristor to change on. After the thyristor is turned on, the control electrode loses its function.
- The only way to switch off the turned-on thyristor is always to decrease the anode current so that it is insufficient to maintain the positive feedback process. How you can decrease the anode current is always to shut down the forward power supply Ea or reverse the link of Ea. The minimum anode current needed to keep your thyristor inside the conducting state is called the holding current of the thyristor. Therefore, as it happens, as long as the anode current is lower than the holding current, the thyristor may be turned off.
Exactly what is the difference between a transistor and a thyristor?
Transistors usually consist of a PNP or NPN structure made up of three semiconductor materials.
The thyristor consists of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The job of a transistor relies on electrical signals to control its opening and closing, allowing fast switching operations.
The thyristor needs a forward voltage and a trigger current at the gate to change on or off.
Transistors are widely used in amplification, switches, oscillators, as well as other facets 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 turned on or off by manipulating the trigger voltage of the control electrode to understand 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 can be utilized in similar applications in some cases, because of their different structures and working principles, they may have noticeable differences in performance and make use of occasions.
Application scope of thyristor
- In power electronic equipment, thyristors can be utilized in frequency converters, motor controllers, welding machines, power supplies, etc.
- Within the lighting field, thyristors can be utilized in dimmers and lightweight 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 utilized in motor controllers.
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