Given that every PC has a powerful, well-regulated PSU that supplies, among other things, a 12 V rail, why not make use of it to produce a PSU variable from 1.25 to 10 V? Well, that’s just what we’re proposing to do here. This power supply can also be used as an addition to a conventional bench supply in order to simulate an analog voltage if the bench supply has only one output.
The conversion part is entrusted to the cheap and very popular MC34063 DC-DC converter, arranged as a step-down. Using a switching solution makes it possible to limit losses due to the Joule effect. The MC34063 is associated with a microcontroller, aided by an LCD display (1×16 characters) which lets you display the output voltage along with the current being supplied by the PSU (connect K3 to pins 4 and 5 of K4). Under ideal conditions, 700 mA may be drawn, but don’t worry, the IC includes a current limiter which will come into action as soon as you go over the limit.
Program the microcontroller using the software available from [1] and adjust P2 to make the displayed output voltage correspond to the actual value. Note that certain single-line 16-character displays behave like a display with two lines of eight characters. The download contains two HEX files for dealing with both eventualities.
Once the PSU has been assembled, you will be able to house it in one of your computer’s spare slots for a 5¼-inch floppy disk drive.
One last little detail: to allow a more accurate setting of the output voltage, you can include a second 1 kΩ potentiometer in series with P1.
[1] www.elektor.com/090863
