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Variable Power Supply

Published on Dec 05, 2016


A variable power supply technique for use with a high efficiency line driver uses a signal peak amplitude to determine a start of a rise time for a power supply to begin supplying operating voltage to the line driver. This ensures that the voltage supplied to the line driver essentially tracks the output signal.

A slew start delay circuit detects when the signal rises above a level that can be supported by a current power supply, and determines a variable delay for switching on a power supply to supply a higher voltage to the line driver. The advantages include reduced power usage, less heat dissipation, and the ability to select a primary power supply that outputs a lower voltage.

The most frequently used device in electronic workshops and laboratories is a universal power supply that provides a variable, fluctuation-free output. Here we present a variable power supply with digital control that is simple and easy to construct. The circuit is built around an adjustable 3-terminal positive-voltage regulator IC LM317, CMOS decade counter IC CD4017, timer IC NE555 and 3-terminal fixed negative-voltage regulator LM7912. The AC mains supply is stepped down by transformer X1 to deliver a secondary output of 12V-0-12V AC, 1A.

The output of the transformer is rectified by a full-wave rectifier comprising diodes D1 through D4. Capacitors C1 through C4 are connected in parallel to rectifier diodes to bypass undesired spikes and provide smooth and fluctuation-free power. Capacitors C5 and C13 are used as filters to eliminate ripple. Here both negative and positive half cycles are used to obtain positive as well as negative DC output. LED1, along with currentlimiting resistor R1, is used for mains ‘on’ indication.

Timer IC NE555 (IC1) is wired as an astable multivibrator. It generates clock pulses when switch S2 is pressed. The output of IC1 is connected, via an RC network, to the clock input of counter IC CD4017 (IC2).