Bench Power Supply
This unit delivers 0 to 20 volts at up to 4 amps in 0.1 volt increments. The entire device runs on a PIC16F870 (about $3 in small quantities). This is basically a switching power supply with the voltage regulation done in software. The PIC used here has analog inputs (used to measure voltage and current) and hardware PWM (pulse width modulation) output used to control the power.
Only two controls are used on the front
panel - an 'on/off'' push button and a rotary encoder. The on/off
button is a 'soft' control; the unit actually stays powered up all of
the time. In the 'off'' mode, the display is blanked except for
the far right decimal which acts as a standby indicator, and the voltage
output is set to zero. Also, while in this mode, the rotary
encoder is not active so that the previously selected voltage will be
maintained when the unit is powered back on.
This design has a 'current cutout' which
will turn off power completely. To access the cutout limit, turn
the unit on, then hold down the button for about 2 seconds. The
left display shows 'ALI' and the right displays shows the current.
To go back to normal operation, hold button for 2 seconds again.
The displays are standard red 7-segment LED's. They are multiplexed in software to simplify the circuit design. A set of 6 small pnp transistors activates each one in turn at about a 100 hz scan rate.
Because of the nature of switching supplies, it is actually possible to get more amperes out of the unit than the transformer is rated for. I got over 4 amps out at 5 volts. As you increase the voltage, less current is available.
The software listing
is included here allong with the Object File,as
is the Schematic.Like other projects shown on this site,
you are welcome to use whatever information you want but this is not
a step-by-step guide to making your own. Some of the parts used
here were scrounged from the junk box and the values are unknown - they
just seem to work.