Mains voltage deviation indicator. 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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When powering some radio equipment from an alternating current network, it is necessary to monitor the stability of its voltage and, if the voltage deviates above the permissible norm, either turn off the equipment or change the voltage on it, for example, using an autotransformer.
To signal mains voltage deviations, you can use the proposed indicator (Fig. 7.12), made on three LEDs and two dinistors.
A feature of the device is the inclusion of indicators (LEDs) at each positive half-cycle of the mains voltage, but only at a certain amplitude equal to the threshold, and turning off when the instantaneous voltage value drops to zero. This eliminates hysteresis and increases the accuracy of the display. The indicator can be connected both to a mains socket and to an autotransformer socket. At the input of the indicator there is a voltage limiter from the VD1 diode and the VD2 zener diode, and after it three indication chains connected in parallel follow. The first of them, consisting of a resistor R1 and an LED, is designed to indicate the presence of mains voltage. The remaining chains, consisting of voltage dividers, threshold devices on dinistors and LEDs connected in series with them, are intended directly for indicating voltage deviations.
The variable resistor R3 sets the lower threshold when the mains voltage drops, say, by 5%, and R5 sets the upper threshold when the voltage rises by the same amount. If the mains voltage is normal, the HL1 and HL2 LEDs are lit. When the voltage drops, the HL2 LED goes out, and HL1 continues to glow. When the voltage rises, all LEDs light up. It is most convenient to set up the indicator with an autotransformer, at the output of which you can set the desired voltage, and with variable resistors to adjust the threshold for turning on the corresponding LEDs.
If the HL2 LED does not go out at low voltage, you will have to increase the resistance of the resistor R2. Having strengthened the variable resistors on the front panel of the structure and provided them with a graduated scale for the thresholds of operation of a particular LED, the indicator can be used for a wide variety of purposes.
Author: Semyan A.P.
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