Abstract: An inherent-security signal comparator (COMP) is driven by two or more input signals (V1, *V2) coming from a control system (.mu.P1, .mu.P2) and generates an alternate output signal (C) of a predetermined frequency whenever both its input signals are identical and terminates its output signal when its input signals wander off from identity, or any of the circuit components of the comparator break down, or its d.c. supply is interrupted. An amplifier (AMP) driven by the comparator drives an isolating transformer (TR) coupled to a voltage rectifier/regulator device (REG) for the supply to the control system. Preferably an oscillator (OSC), whose output is connected to the input of the amplifier (AMP) in common with the output of the comparator, is supplied by a voltage pulse generated by a monostable circuit (T3-R7-R8-R9-C5-C6-D1-D2) which is momentarily enabled at the start-up of the supply circuit.