A variable power supply is a basic piece of
equipment in any electronics workshop, repair
shop, and in the tool kit of every electronics
engineer. The presented power supply, in spite of
its simple design, boasts good parameters. It
makes possible the stepless adjustment of output
voltage in the range from 0V to 30V as well as the
stepless adjustment of current up to 1A.
The entire unit is based on an LM324 quad
operational amplifier. The D amplifier provides
a source of reference voltage. A power supply
error amplifier is built on circuit C. The B circuit
serves to measure output current, while the A
circuit works as a comparator controlling the LED signaling the power supply's switching to
c u r r e n t r e g u l a t i o n . T h e P 1 p o t e n t i o m e t e r s e r v e s
to adjust the output voltage. The P2 potentiometer adjusts the threshold limiting the supply
current. The PR1 trimmer potentiometer should be set to the upper range of current
adjustments as follows: The P2 potentiometer should be set at maximum. A load should be
applied to the power supply output e.g. a barretter with a resistance of over a dozen W. An
amp meter should have a serial connection with the resistor. In adjusting the output voltage,
the current flowing through the resistor should be set to 1A. The PR1 trimmer potentiometer
should be rotated until the LED goes on. The entire power supply is assembled on a single
printed circuit board. The T2 output transistor should be screwed to a radiator with a minimum
surface area of 1dm2. The supply transformer should provide a maximum voltage of 25V so
that the voltage at the C1 capacitor does not exceed 33V (the allowable value for the LM324
circuit). The transformer load current should be at least equal to the maximum power supply
current load. When assembled in accordance with the diagram, the device operates correctly
immediately upon powering up. The power supply output should be locked out by
a noninductive capacitor, e.g. a ceramic model with a capacity of 100μF/50V as close to the
output terminals as possible. The power supply is much more convenient to use if it is provided
with a volt and current meter. The J-25 set (digital millivoltmeter) as manufactured by our
company is a perfect solution. Current measurements may be conducted by attaching the
milivoltmeter to the terminals of the R14 resistor. Voltage measurements are taken by
attaching the milivoltmeter to the power supply output through a 1:100 divider.