Power is applied to the circuit with the initiate switch open. The 25-JtF capacitor charges through the A14, or equivalent, diode and 2.2-KO resistor to full supply voltage. When the initiate switch is closed, the low side of the capacitor is suddenly raised to +12 V. This raises the diode side of the capacitor to approximately +24 V.
The capacitor immediately begins discharging through the seriesconnected 1 and 3.3-MO resistors. Eventually, the C13 gate becomes forward biased, the device turns, and it applies power to the relay. The delay is virtually independent of supply voltage.