Circuit monitors ac-power loss

Dennis Eichenberg, Parma Heights, OH -- EDN, 11/24/99

The circuit in Figure 1 provides a simple, nonvolatile means of monitoring critical ac-power failures. Monitoring the power is important in such systems as heating and refrigeration, in which damage can occur if the ac power goes down for an extended period. The circuit in Figure 1 requires little power from the ac-power system. The quiescent battery current of approximately 5 µA provides long battery life in this application. The optocoupler consists of a neon lamp and a photocell. When the ac-power system is active, the neon lamp lights via current-limiting resistor R₁. A neon lamp is ideal for this application because of its low power drain. The resistance of the photocell in the optocoupler is low when the cell is illuminated and high when it's unilluminated. R₂ serves as a current-limiting resistor for the photocell.

IC_1A of the quad two-input NAND-gate Schmitt trigger serves as a buffer for the optocoupler. The output of IC_1A is normally high when the ac power is present and goes low when a power failure occurs. IC_1B is configured as a negative-edge-triggered, half-monostable multivibrator. It produces a positive pulse with a width of approximately 0.8R₃C₁, or approximately 0.8 sec, whenever the ac power goes down. The output pulse drives transistor Q₁ via the current-limiting resistor, R₄, to activate solenoid S₁. D₁ is a flyback diode to protect Q₁ from the inductive spike S₁ generates when it becomes deactivated. You can use S₁ to trigger a flag or to provide a similar visual alarm. The flag remains in position until you manually reset it to prepare the circuit for the next power failure. (DI #2446).