@▷ AC Power Monitor | Diagram for Schematic

AC Power Monitor

This AC Power Monitor continuously watches the AC power line voltage for both under-voltage and missing cycles. When it detects a total of 5 or 6 consecutive missing half-cycles (50mS, 50/60HZ), it drops a relay and starts a timer. A power contactor is slaved to the relay contacts. Its purpose is to protect power-loss sensitive industrial equipment from brief power glitches by preventing immediate restart.

After the specified time delay (2 to 10S), the equipment may automatically restart. Although this may seem like a simple task, note that the condition sensed is the absence of a periodic signal and that the timer may or may not have external power available. electroschematics.com is not really into industrial controls, but this may be the beginning—this is where I spent most of my life.

This function could also be performed by a PLC (programmable logic controller) with a UPS (unterruptable power source), but such would be expensive and consume additional panel space.

Power Monitor Schematic

AC Power Monitor Circuit Schematic

Two key components (transformer and relay) are documented on the power monitor schematic — there is no bill of materials.

Power supply

Key to this functioning properly is its power-loss ride-through capability. Note that this circuit continues to time out even after the power is interrupted. To run the circuitry for the entire time period, energy is stored in a large filter capacitor (10,000uf). That way if the power resumes during the timeout period, the timer remains alive and functioning so that the sensitive equipment may not restart.
The raw capacitor voltage ranges from about 14 to 24V, and is series regulated to 12V via U2, an LM7812 voltage regulator.

The power transformer (T1) provides low voltage power and power line isolation. Its primary is reconnectable for 115 /230VAC.

Capacitor Bleeder

At the end of the timeout period, the bleeder circuit kicks in to dump the remaining charge. A 4000 series CMOS latch consisting of U3B & U3C is set by the positive transition of U1-3. Should the power resume while the capacitor is bleeding down, the latch is reset via the signal at the collector of Q3 so that the bleeder driver Darlington (Q1 & Q2) turns off. When all of the charge is dissipated by the bleeder resistor (R3), the circuit is free to immediately restart and again monitor voltage glitches.

The MC14093 is a good choice for the power monitor application. It has Schmitt trigger inputs for handling slow input signal transitions, and may be powered by the 12V Vcc. It is identical in pin-out to the more common CD4011. Note that the CD4011 should also function acceptably, but I did not try it in the circuit.


The timer is the good old 555 (actually, it is the TLC555 due to its low quiescent current). It is configured as a monostable multivibrator that is triggered via a positive voltage at U1-6. To assure that the internal latch of the 555 is set in the proper state during power-up, C7 works against Vcc (rather than common) and U1-2 is held low for 0.5 sec via U3A, C6 and R10.

The timeout period may be increased substantially by increasing the size of the timing capacitor C7. Note that C1 must be adjusted likewise to provide additional ride-through. 60 seconds is not unreasonable.

AC voltage threshold detector

Note that the cathodes of D1 & D2 are unfiltered—this is necessary for rapid sensing of power loss—a filter capacitor would tend to hold up the voltage thus interfering with the voltage loss timer. The filter capacitor (C1) is isolated via D3. The Vbe of Q3 (0.65V) is the voltage threshold comparator. This voltage is multiplied by the ratio of (R4, R5 & R6) to R6.

The function of the voltage threshold comparator is to keep C5 discharged. At each line voltage zero-crossing C5 charges slightly, but requires a full 50mS to reach the threshold of U1-6. Should the voltage remain below the threshold of Q3, C5 starts to charge significantly—if 5 or 6 half-cycles are lost in succession, the voltage across C5 charges to the 8V threshold of U1-6 and triggers the timer.

Proper adjustment of R4 is required. To do so, connect a Variac to the power terminals and reduce the voltage to the desired AC threshold voltage (e.g. 170VAC). Then increase the value of R16 until the LEDs change state.

Power Monitor Oscillographs

Power Monitor Photos

Glossary of undocumented words and idioms (for our ESL friends)

e.g. common abbreviation – example given