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’
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
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