Just what is a thyristor?
A thyristor is actually a high-power semiconductor device, also called a silicon-controlled rectifier. Its structure includes four levels of semiconductor components, including three PN junctions corresponding to the Anode, Cathode, and control electrode Gate. These three poles would be 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 working status. Therefore, thyristors are popular in various electronic circuits, such as controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversion.
The graphical symbol of a silicon-controlled rectifier is generally represented through 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 light-weight-controlled thyristors. The working condition from 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 can be used between the anode and cathode (the anode is connected to the favorable pole from the power supply, and also the cathode is connected to the negative pole from the power supply). But no forward voltage is applied to the control pole (i.e., K is disconnected), and also the indicator light fails to light up. This shows that the thyristor is not really conducting and contains forward blocking capability.
- Controllable conduction
As shown in Figure b above, when K is closed, and a forward voltage is applied to the control electrode (known as a trigger, and also the applied voltage is referred to 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 when the voltage in the control electrode is taken away (which is, K is switched on again), the indicator light still glows. This shows that the thyristor can continue to conduct. At the moment, so that you can 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 applied to the control electrode, a reverse voltage is applied between the anode and cathode, and also the indicator light fails to light up currently. This shows that the thyristor is not really conducting and may reverse blocking.
- In summary
1) Once the thyristor is put through a reverse anode voltage, the thyristor is at a reverse blocking state regardless of what voltage the gate is put through.
2) Once the thyristor is put through a forward anode voltage, the thyristor will simply conduct once the gate is put through a forward voltage. At the moment, the thyristor is within the forward conduction state, which is the thyristor characteristic, which is, the controllable characteristic.
3) Once the thyristor is switched on, so long as there is a specific forward anode voltage, the thyristor will remain switched on whatever the gate voltage. That is certainly, 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, and also the primary circuit voltage (or current) decreases to close to zero, the thyristor turns off.
5) The problem for the thyristor to conduct is the fact that a forward voltage needs to be applied between the anode and also the cathode, as well as an appropriate forward voltage should also be applied between the gate and also the cathode. To transform 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 basically an exclusive triode made from three PN junctions. It can be equivalently viewed as comprising a PNP transistor (BG2) as well as an NPN transistor (BG1).
- If a forward voltage is applied between the anode and cathode from the thyristor without applying a forward voltage to 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. If a forward voltage is applied to the control electrode currently, BG1 is triggered to produce basics 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 is going to 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 get into a saturated conduction state quickly. A big current appears in the emitters of these two transistors, which is, the anode and cathode from the thyristor (the dimensions of the current is really determined by the dimensions of the load and the dimensions of Ea), and so the thyristor is totally switched on. This conduction process is done in a very short period of time.
- After the thyristor is switched on, its conductive state is going to be maintained through the positive feedback effect from the tube itself. Whether or not the forward voltage from the control electrode disappears, it is still in the conductive state. Therefore, the function of the control electrode is simply to trigger the thyristor to change on. After the thyristor is switched on, the control electrode loses its function.
- The only method to shut off the turned-on thyristor is always to reduce the anode current that it is inadequate to keep up the positive feedback process. How you can reduce the anode current is always to cut off the forward power supply Ea or reverse the connection of Ea. The minimum anode current needed to maintain the thyristor in the conducting state is referred to as the holding current from the thyristor. Therefore, strictly speaking, so long as the anode current is lower than the holding current, the thyristor could be turned off.
What is the distinction between a transistor and 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 work 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 in the gate to change on or off.
Transistors are popular in amplification, switches, oscillators, as well as 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.
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 from 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 working principles, they have got noticeable variations 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 light-weight control devices.
- In induction cookers and electric water heaters, thyristors can be used to control the current flow to the heating element.
- In electric vehicles, transistors can be utilized in motor controllers.
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