So what is a thyristor?
A thyristor is really a high-power semiconductor device, also known as a silicon-controlled rectifier. Its structure contains four quantities of semiconductor materials, including 3 PN junctions corresponding for the Anode, Cathode, and control electrode Gate. These 3 poles are definitely 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 widely used in a variety of electronic circuits, like controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency alteration.
The graphical symbol of the semiconductor device is usually 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 in the thyristor is the fact that whenever a forward voltage is applied, the gate needs to have a trigger current.
Characteristics of thyristor
- Forward blocking
As shown in Figure a above, when an ahead voltage is utilized in between the anode and cathode (the anode is linked to the favorable pole in the power supply, as well as the cathode is linked 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 light up. 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 applied for the control electrode (called a trigger, as well as the applied voltage is known as trigger voltage), the indicator light turns on. This means that the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, following the thyristor is switched on, even if the voltage in the control electrode is taken away (that is, K is switched on again), the indicator light still glows. This implies that the thyristor can still conduct. Currently, in order to stop the conductive thyristor, the power supply Ea has to be stop 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, as well as the indicator light fails to light up at this time. This implies that the thyristor will not be conducting and will reverse blocking.
- In conclusion
1) Once the thyristor is exposed to a reverse anode voltage, the thyristor is at 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 when the gate is exposed to a forward voltage. Currently, the thyristor is in the forward conduction state, the thyristor characteristic, that 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 whatever the gate voltage. Which is, 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 close to zero, the thyristor turns off.
5) The condition for your thyristor to conduct is the fact that a forward voltage ought to be applied in between the anode as well as the cathode, as well as an appropriate forward voltage also need to be applied in between the gate as well as the cathode. To transform off a conducting thyristor, the forward voltage in between the anode and cathode has to be stop, or perhaps the voltage has to be reversed.
Working principle of thyristor
A thyristor is actually an exclusive triode made from three PN junctions. It can be equivalently regarded as consisting of a PNP transistor (BG2) as well as an NPN transistor (BG1).
- When a forward voltage is applied in 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 this time, BG1 is triggered to produce a base current Ig. BG1 amplifies this current, as well as 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 be introduced the collector of BG2. This current is delivered to BG1 for amplification then delivered 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 within the emitters of the two transistors, that is, the anode and cathode in the thyristor (how big the current is actually dependant on how big the burden and how big Ea), so the thyristor is completely switched on. This conduction process is completed in a really short period of time.
- After the thyristor is switched on, its conductive state will be maintained from the positive feedback effect in the tube itself. Whether or not the forward voltage in the control electrode disappears, it is still within the conductive state. Therefore, the purpose of the control electrode is just to trigger the thyristor to change on. When the thyristor is switched on, the control electrode loses its function.
- The best way to shut off the turned-on thyristor is to lessen the anode current so that it is not enough to keep the positive feedback process. How you can lessen the anode current is to stop the forward power supply Ea or reverse the connection of Ea. The minimum anode current needed to maintain the thyristor within the conducting state is known as the holding current in the thyristor. Therefore, as it happens, as long as the anode current is lower than the holding current, the thyristor can be switched off.
What is the distinction between a transistor as well as a thyristor?
Transistors usually include a PNP or NPN structure made from three semiconductor materials.
The thyristor consists of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The task of the transistor relies on electrical signals to control its closing and opening, allowing fast switching operations.
The thyristor requires a forward voltage as well as a trigger current in 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 like controlled rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversions.
Method of working
The transistor controls the collector current by holding the base current to attain current amplification.
The thyristor is switched on or off by controlling the trigger voltage in the control electrode to understand 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 summarize, although transistors and thyristors can be utilized in similar applications in some instances, due to their different structures and operating principles, they have noticeable differences in performance and use 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 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 utilized in motor controllers.
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