what is the SCR | operation of SCR | SCR working principle |


SCR fill name silicon controlled rectifier. A silicon controlled rectifier is a semiconductor device that alts as a true electronic switch. It can be change alternating current and at the same time can control the amount of power fed to the load. SCR combines the features of a rectifier and a transistor.


As show in figure it is a four layer three terminal device, layer being alternately p- type and N- type silicon junction are marked J1, J2, J3 where as terminals are anode , cathode and gate.

Principle of operation of SCR

Three modes of operation;

1. forward blocking region.
2. forward conduction region.
3. reverse blocking region.

1. Forward blocking region;

In this mode of operation, the anode (+) is given a positive voltage while the cathode (−) is given a negative voltage, keeping the gate at zero (0) potential i.e. disconnected. In this case junction J1 and J3 are forward-biased, while J2 is reverse-biased, due to which only a small leakage current exists from the anode to the cathode until the applied voltage reaches its break-over value, at which J2 undergoes avalanche breakdown, and at this break-over voltage it starts conducting, but below break-over voltage it offers very high resistance to the current and is said to be in the off state.

2. Forward conduction region;

An SCR can be brought from blocking mode to conduction mode in two ways: Either by increasing the voltage between anode and cathode beyond the break-over voltage, or by applying a positive pulse at the gate. Once the SCR starts conducting, no more gate voltage is required to maintain it in the ON state.

3. reverse conduction region ;

When a negative voltage is applied to the anode and a positive voltage to the cathode, the SCR is in reverse blocking mode, making J1 and J3 reverse biased and J2 forward biased. The device behaves as two reverse-biassed diodes connected in series. A small leakage current flows. This is the reverse blocking mode. If the reverse voltage is increased, then at critical breakdown level, called the reverse breakdown voltage (VBR), an avalanche occurs at J1 and J3 and the reverse current increases rapidly. SCR are available with reverse blocking capability, which adds to the forward voltage drop because of the need to have a long, low-doped P1 region. (If one cannot determine which region is P1, a labeled diagram of layers and junctions can help.) Usually, the reverse blocking voltage rating and forward blocking voltage rating are the same. The typical application for a reverse blocking SCR is in current-source inverters.


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