SCR Triggering using UJT

How to trigger an SCR using UJT ?

One common application of the uni junction transistor is the triggering of the other devices such as the SCR, triac etc. The basic elements of such a triggering circuit are shown in figure. The resistor R_E is chosen so that the load line determined by R_E passes through the device characteristic in the negative resistance region, that is, to the right of the peak point but to the left of the valley point, as shown in figure. If the load line does not pass to the right of the peak point P, the device cannot turn on.

For ensuring turn-on of UJT

R_E <    V_BB – V_p­ / I_P

This can be established as below

Consider the peak point at which I_RE = I_p and V_E = V_P

(the equality I_RE = I_P is valid because the charging current of capacitor, at this instant is zero, that is, the capacitor, at this particular instant, is changing from a charging state to

a discharging state). Then V_E = V_BB – I_RE R_E

So, R_E(MAX) =  V_BB – V_E­ / I_RE = V_BB – V_p­ / I_P at the peak point.

At the valley point, V

I_E = I_V and V_E = V_V so that

V_E = V_BB – I_RE R_E

So R_E(MIN) = V_BB – V_E / I_RE = V_BB – V_V / I_V or for ensuring turn-off.

R_E > = V_BB – V_V / I_V

So, the range of resistor R_E is given as

V_BB – V_P / I_P >R_E > V_BB – V_V / I_V

The resistor R is chosen small enough so as to ensure that SCR is not turned on by voltage V_R when emitter terminal E is open or I_E = 0

The voltage V_R = RV_BB/R + R_BB for open-emitter terminal.

The capacitor C determines the time interval between triggering pulses and the time duration of each pulse. By varying R_E, we can change the time constant R_E C and alter the point at which the UJT fires. This allows us to control the conduction angle of the SCR, which means the control of load current.