Design Ideas: March 1, 1996

You can use the MC34161P universal voltage monitor to build a simple ac power-line monitor (a voltage-sensing relay). In this application (Figure 1), the mode select pin (pin 7) connects to the internal 2.55V reference (pin 1), and the IC functions as a positive-voltage window detector.

A small signal transformer provides total isolation from the power lines and performs a unique role in the circuit. During the positive half cycles of the line voltage, the transformer functions as a conventional power transformer feeding a 15V power supply through a half-wave rectifier. When the line voltage is negative, the transformer becomes a potential transformer accurately sensing the amplitude of the power line's voltage. During negative half cycles the transformer is essentially unloaded, thus eliminating all errors due to changes in loading (such as relay energizing or de-energizing).

The voltage-sensing circuit consists of R₁ through R₅, C₁, and D₁. The circuit responds to the peak value of the (assumed) sinusoidal line voltage. The response times to increasing and decreasing voltages are different; C₁ charges very quickly to the peak of the transformer's secondary voltage but discharges slowly through the voltage-divider network. This feature enables the voltage monitor to ignore momentary dips in line voltage and to quickly disconnect the load in case of overvoltage. Three LEDs provide visual indications of low, normal (in-band), or high line voltage. The relay de-energizes when the voltage is either too low or too high, which in turn disconnects the load from the line. (DI #1831)