Circuit knowledge

This is a Simple Squib Firing Circuit Diagram . Capacitor Cl is charged to +28 V through Rl and stores energy for firing the squib. A positive pulse of 1 mA applied to the gate of SCR1 will cause it to conduct, discharging Cl into the squib load XI. 

With the load in the cathode schema, the cathode rises immediately to + 28 V as soon as the SCR is triggered on. DiodeD1 decouples the gate from the gate trigger source, allowing the gate to rise in potential along with the cathode so that the negative gate-to-cathode voltage rating is not exceeded. 

This schema will reset itself after test firing, since the available current through Rl is less than the holding current of the SCR. After Cl has been discharged, the SCR automatically turns off—allowing Cl to recharge.