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Protection of equipment from increased mains voltage

Measuring and household appliances can be easily disabled if the supply voltage is too high or too low. The voltage-sensitive circuit on the 555 integral timer disconnects the equipment from the mains when the voltage in it is out of the specified range.

Such a circuit provides better protection of devices than a voltage regulator, which is effective for detecting and compensating for short-term voltage changes. As shown in the figure, the mains voltage is reduced to approximately 15 V using a step-down transformer whose conversion ratio depends on the voltage on the primary winding. The secondary voltage is rectified, filtered by a capacitor and fed to a 12-volt regulator, which forms the offset of the timer and the transistor KT315 used as a sensing element. Unstabilized voltage, which varies in proportion to the mains voltage, goes to the potentiometers R1 and R3, which set the upper and lower limits.

The 555 timer operates in bistable mode, and its state is directly dependent on the voltages at the plant inputs and the reset (terminals 2 and 4, respectively).

In the normal mode, when the supply voltage is within the specified limits, the unstabilized voltage at the middle terminal of the resistor R1 opens the silicon zener diode and enters the transistor in the saturation state. The voltage at terminal 4 of the timer rises sharply, tending to 12 V and, when it reaches 2/3 of the 12 V bias voltage, that is, 8 V, the potential at pin 3 rises and turns on the relay.

If the AC mains voltage is less than the lower limit, the voltage at the middle terminal of the resistor R1 is too small to turn on the Zener diode KC166, and therefore the relay is de-energized. When the mains voltage exceeds the upper set limit, the DC voltage at output 2 exceeds 1/3 of the 12V supply voltage, resulting in a reduced output potential of 3, and the relay shuts down.

The upper and lower limits can be set with an accuracy of ± 5 V when using precision potentiometers. Due to the avalanche-like nature of the breakdown in a silicon zener diode, the circuit has no hysteresis. Voltage emissions occurring in the network are effectively suppressed by a large filter capacitor.

Date of publication 10.05.2003gg.

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