What exactly 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 three PN junctions corresponding for the Anode, Cathode, and control electrode Gate. These three poles are 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 working status. Therefore, thyristors are widely used in different electronic circuits, such as controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversion.
The graphical symbol of any semiconductor device is generally represented from the text symbol “V” or “VT” (in older standards, the letters “SCR”). In addition, derivatives of thyristors include fast thyristors, bidirectional thyristors, reverse conduction thyristors, and lightweight-controlled thyristors. The working condition in the thyristor is the fact 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 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 will not light up. This demonstrates that the thyristor is not 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 (referred to as 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, after the thyristor is switched on, whether or not the voltage in the control electrode is taken away (that is certainly, K is switched on again), the indicator light still glows. This demonstrates that the thyristor can carry on and conduct. At this time, so that you can stop the conductive thyristor, the power supply Ea must 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 will not light up at this time. This demonstrates that the thyristor is not conducting and may reverse blocking.
- In summary
1) If the thyristor is subjected to a reverse anode voltage, the thyristor is within a reverse blocking state no matter what voltage the gate is subjected to.
2) If the thyristor is subjected to a forward anode voltage, the thyristor will only conduct when the gate is subjected to a forward voltage. At this time, the thyristor is incorporated in the forward conduction state, the thyristor characteristic, that is certainly, the controllable characteristic.
3) If the thyristor is switched on, provided that there exists a specific forward anode voltage, the thyristor will stay switched on no matter the gate voltage. Which is, after the thyristor is switched on, 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 ought to be applied in between the anode as well as the cathode, plus 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 must be stop, or the voltage must be reversed.
Working principle of thyristor
A thyristor is basically a distinctive triode composed of three PN junctions. It could be equivalently viewed as consisting of a PNP transistor (BG2) plus 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 remains turned off because BG1 has no base current. When a forward voltage is applied for the control electrode at this time, BG1 is triggered to create a base current Ig. BG1 amplifies this current, along with a ß1Ig current is obtained in its 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 and then sent 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 big current appears in the emitters of these two transistors, that is certainly, the anode and cathode in the thyristor (the size of the current is in fact dependant on the size of the burden and the size of Ea), and so the thyristor is entirely switched on. This conduction process is finished in an exceedingly short time.
- Right after the thyristor is switched on, its conductive state is going to be maintained from the positive feedback effect in the tube itself. Even if the forward voltage in the control electrode disappears, it is still in the conductive state. Therefore, the function of the control electrode is only to trigger the thyristor to transform on. Once the thyristor is switched on, the control electrode loses its function.
- The best way to switch off the turned-on thyristor is always to decrease the anode current so that it is inadequate to maintain the positive feedback process. The way to decrease the anode current is always to stop the forward power supply Ea or reverse the connection of Ea. The minimum anode current necessary to maintain the thyristor in the conducting state is referred to as the holding current in the thyristor. Therefore, strictly speaking, provided that the anode current is under the holding current, the thyristor may be turned off.
Exactly what is the distinction between a transistor along with a thyristor?
Transistors usually include a PNP or NPN structure composed of three semiconductor materials.
The thyristor consists of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The task of any transistor depends on electrical signals to control its opening and closing, allowing fast switching operations.
The thyristor needs a forward voltage along with a trigger current in the gate to transform on or off.
Transistors are widely used in amplification, switches, oscillators, and other elements of electronic circuits.
Thyristors are mainly used in electronic circuits such as 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 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 related to stability and reliability and usually have higher turn-off voltage and larger on-current.
To summarize, although transistors and thyristors can be utilized in similar applications sometimes, because of their different structures and working principles, they 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.
- Inside the lighting field, thyristors can be utilized in dimmers and lightweight control devices.
- In induction cookers and electric water heaters, thyristors may 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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