This circuit was
designed to protect a dual rail power supply from shorts across the two
rails. It uses an optocoupler to monitor each supply rail, with the
internal LEDs powered from ZD2 and ZD3 and the associated resistors. While the LEDs
are on, the optocoupler’s internal transistors are both turned on which
ensures that transistor Q1 is on and relay RLY1 is energised. If either
rail is short-circuited, the associated optocoupler is turned off,
robbing Q1 of base current and the relay then drops out to disconnect
the supply rails. Operation is restored by pressing the reset button.
The value of ZD1 and the associated resistor should be chosen to suit
the supply and relay coil voltages.