Protection of equipment from increased mains voltage using an integrated timer. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Protection of equipment from emergency operation of the network
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Measuring and electrical appliances can be easily damaged by excessive increase or decrease in supply voltage. Voltage-sensitive circuitry on the Type 555 integral timer disconnects the equipment from the mains when the voltage in it goes outside the specified limits.
This arrangement provides better instrument protection than a voltage regulator, which is effective in detecting and compensating for momentary voltage fluctuations. As shown in the figure, the mains voltage is reduced to approximately 15 V by means of a step-down transformer, the transformation ratio of which depends on the voltage on the primary winding. The secondary voltage is rectified, filtered by a capacitor, and fed to a 12V regulator that biases the timer and the KT315 transistor used as a sensing element. An unstabilized voltage, which varies in proportion to the mains voltage, is supplied to the potentiometers R1 and R3, which set the upper and lower limits.
The Type 555 timer operates in bistable mode, and its state directly depends on the voltages at the set and reset inputs (pins 2 and 4, respectively).
(click to enlarge)
In normal mode, when the supply voltage is within the specified limits, the unregulated voltage at the middle terminal of the resistor R1 opens the silicon zener diode and enters the transistor into saturation. The voltage at pin 4 of the timer rises sharply, tending to 12 V, and when it reaches 2/3 of the timer's 12 V bias voltage, i.e. 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 resistor R1 is too low to turn on the KC166 silicon zener diode, and therefore the relay is de-energized. When the mains voltage exceeds the upper limit, the DC voltage at output 2 exceeds 1/3 of the 12V supply voltage, causing the potential of output 3 to drop and the relay turns off.
The upper and lower limits can be set with an accuracy of ±5 V using precision potentiometers. Due to the avalanche nature of the breakdown in the silicon zener diode, the circuit has no hysteresis. Voltage surges occurring in the network are effectively suppressed by a large filter capacitor.
Author: RJ Patel; Publication: N. Bolshakov, rf.atnn.ru
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