Missing phase detection circuit for use with three-phase power sources

Apparatus for monitoring a poly-phase power supply of the type providing a predetermined series of phase voltages of a common frequency, each phase voltage of the series comprising a bipolar alternating voltage, said apparatus comprising:

• a. a corresponding series of pulse forming means operatively responsive to said series of phase voltages, respectively, to provide pulsed signals during the excursion of the respective phase voltage waveform in a predetermined one of the power supply polarities;
• b. gating means having a corresponding series of inputs for receiving the outputs of said series of pulse forming means, said gating means being operative to produce a first logical output state except during the receipt of the pulsed signal at any of its inputs;
• and
• c. a retriggerable monostable multivibrator having its trigger signal input connected to the output of said gating means, said multivibrator being operative in response to the application of a first logical signal state to its trigger signal input to produce a first logical signal state at its output for a predetermined duration of time greater than the full period of alternation of the individual phase signals to detect the absence of said phase voltages, said multivibrator being retriggerable to renew said predetermined duration of said first logical signal state in response to successive applications of a first logical signal state to its trigger input prior to termination of the first state.