SCR characteristics and Mode of Operation

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Here we are going to a detailed article related to static VI characteristics of SCR. Vi characteristics of SCR can be explained in three modes of operation. Here we will learn static characteristics of SCR theory in detail.


SCR (silicon controlled amplifier) is a solid state device like a transistor and it has similar characteristics as thyratron tube. SCR or thyristor is a semiconductor device consists of P-n-p-n structure. SCr has four layer and three junctions. Scr consists of three terminal: anode, cathode, and gate. Gate terminal is usually kept near to the cathode as per figure.

As per the diagram, outer p region is called anode(A). Outer n region is called cathode ( K) and the gate terminal is also shown near to the cathode.

Static VI characteristics of SCR

Like the diode, SCR blocks the current flow from cathode to the anode. Unlike the diode, SCR only flows the current from anode to cathode when it is triggered into conduction by applying proper gate voltage. For engineering applications of SCR we must know the all static vi characteristics of SCR, dynamic switching characteristics of SCR and gate characteristics of SCR.

VI characteristics of SCR is shown in below figure. A necessary circuit diagram for obtaining scr characteristics is also shown in the figure.

scr characteristics and mode of operation

  • Anode and cathode terminal is connected to the main supply through the load.
  • Gate signal is applied between the gate terminal and cathode terminal.

Vi characteristics of SCR shown in the figure. This characteristics of SCR can be explained in four different modes.

SCR mode of operation

Forward blocking mode (OFF state)

Forward conducting mode (ON state)

Reverse blocking mode

Forward blocking mode

scr forward and reverse bias conditionWhen the anode is positive with respect to cathode and gate terminal is open, the thyristor is said to be in forward bias. As this condition is shown in figure b, Current can pass through junction J1 and J3 but can’t pass through junction J2 because it is in reverse bias. At this condition very small amount of leakage current passes through SCR.

In case of forward voltage increase, reverse biased junction J2 have an avalanche breakdown at a voltage called breakdown voltage. During the voltage, less than breakdown voltage SCR offer a high impedance to the circuit and during this condition scr treated as an open switch condition.

Forward conducting mode

When the anode is positive with respect to the cathode and necessary gate signal is applied to the SCR at a small voltage, current passes through the SCR. A thyristor is brought from forward blocking mode to forward conducting mode by applying required gate voltage.

At this condition, thyristor offers very small impedance and behaves like a closed switch. A thyristor is triggered into ON condition by applying a voltage greater than the breakdown voltage.

As per characteristics, you can see in this mode, the voltage drop is very small across thyristor. The voltage drop across thyristor during this mode depends upon the thyristor. This voltages drop is due to the ohmic drop in thyristor four layers.

Reverse blocking mode

In this condition, a cathode is positive with respect to the anode. Scr is in reverse biased in this mode. Junction J1 and J3 are in reverse biased and J2 is in a forward biased condition. when the voltage applied across the SCR it offers high impedance and very small leakage current passes through SCR.

At the reverse breakdown voltage, an avalanche occurs at junction J1 and J3 and reverse current increase rapidly. Figure a shows reversely biased SCR.


Here we saw Vi characteristics of SCR which are studied as an experiment in many universities. Scr also has dynamic switching characteristics and gate characteristics. Hope you understand all related to SCR characteristics. If you have any question related to this article comment below.

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