What is Thyristor(SCR)?
Thyristor is also known as SCR. It is a three terminal, four layer semiconductor device. Many layers of silicon is used. Gate terminal controls the device.
Once the device is ON it acts like a rectifier and so it is called as SCR. It has common properties of diodes, resistors and transistors.
Thyristor is a unidirectional device and conducts current in only one direction. It can be used only for switching and cannot be used for amplification.
Symbol of SCR:
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| Symbol of SCR |
The symbol of SCR looks like the diode symbol and additionally we have the Gate terminal entering at the junction. It has three terminals Anode, Cathode and Gate.
Construction of SCR:
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| Construction of SCR |
Thyristor or SCR consists of four P-N-P-N layer and have three PN junctions J1,J2, J3 in series. It has three terminals Anode, Cathode and Gate. Gate terminal is attached to the P type layer near the Cathode terminal.
The two transistor model shows that SCR is a combination of one PNP transistor and one NPN transistor.
The emitter of PNP transistor is taken as the Anode terminal, emitter of NPN transistor is taken as Cathode and base of PNP is taken as the Gate terminal. The base of PNP is connected to the collector of NPN and collector of PNP is connected to the NPN transistor.
Working of SCR:
Reverse blocking mode of SCR:
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| Reverse blocking mode of SCR |
The thyristor is switched off if no voltage is applied at the Gate terminal. When SCR is reverse biased the anode is connected to the negative end and Cathode is connected to the positive end, the junctions J1 and J3 are reverse biased and J2 is forward biased.
It acts like the conventional diode in reverse biased condition. Only reverse saturation current flows. Reverse breakdown happens like the diode, when it exceeds the safe voltage.
Forward blocking mode of SCR:
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| Forward blocking mode of SCR |
When SCR is forward biased the anode is connected to the positive end and Cathode is connected to the negative end, the junctions J1 and J3 are forward biased and J2 is reverse biased.
It acts like the conventional diode in forward biased condition. Here also no current flows, only small saturation current flows.
Forward conduction mode of SCR:
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| Forward conduction mode of SCR |
When the thyristor is forward biased and small voltage is applied at the gate terminal, the device is ON.
In the two transistors model when the gate voltage is applied the lower transistor switches ON which in turn switches ON the upper transistor.
This continues till the power supply applied to gate and anode terminals is stopped. The current flow continues even when the applied voltage at the gate terminal is stopped.
Characteristics of SCR:
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| Characteristics of SCR |
When the gate voltage is not applied and when the anode and cathode terminals are forward or reverse biased, it blocks the current flow and only saturation current flows.
When the voltage is applied at the gate terminal the current flows in the lower transistor which turns ON the upper transistor and this continues like a latch.
Once the device is ON, gate terminal has no effect in controlling the current. To stop the device all the power supplies should be switched OFF.
Advantages:
- It is easy to turn ON
- SCR or thyristor can handle large voltage, current and power
- It is possible to control AC power
- It can be protected with the help of fuse
- It is of low cost
Disadvantages:
- It conducts in only one direction, so it controls during only half cycle.
- It cannot be used at high frequencies
- Gate current should be positive
- It is very difficult to turn off SCR
Applications of SCR:
- Controlled Rectifiers
- Inverters
- Pulse circuits
- Timing circuits
- Latching relays
- Temperature control systems
- Remote switching units
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