Balancing ATX Supplies

A high power ATX supply off a PC is a good way to power your RepRap but requires a bit more care than a purpose built 12V supply. The main issue with some (not all) ATX PSUs is the requirement for a load on the 5V line. To get reliable operation from these PSUs you will need to balance the loads on the 12V vs the 5V lines. This is very often the case with power supplies from the Pentium IV era, as these were designed for heavy current loads on the +5V rail and considerably smaller loads on the +12V rail.

Below you'll see a table of voltage as measured under various loads. The first three show increasing 12V load with the recommended 5V load of a 10 ohm 10W power resistor. The first line represents the load from motors only. The second is with the hot end on and the last is with a heated bed added. You can see that as load is added to the 12V line the 12V line is drawn down and the 5V line floats higher and higher. This is due to the fact that the 12V and 5V lines share a regulator.

Note: Under this configuration my Prusa experienced periodic communication disconnects, and would stop mid print.

The next three lines represent the voltages as more load was added to the 5V line. Each load was a 50W 12V light bulb easily sourced from the nearest hardware store or supermarket as accent lighting. At 5V these lights draw about 2.5 amps each. As you can see the voltages are better regulated as the load on the 5V line increases. I have a clip on ferrite cylinder on the USB cable and still got rare comms disconnect mid print with two lamps connected, so I'll add the third and try again.

12 V Load (Amps)	5 V Load (Amps)	12 V Voltage	5 V Voltage	Total Watts
4	0.5	11.87	5.18	50
6	0.5	11.77	5.2	73
16	0.5	10.98	5.31	178
16	3	11.46	5.23	199
16	5.5	11.77	5.19	216
16	8	11.94	5.15	232

In principle, the latest "Haswell ready" ATX PSUs should never have this problem, as they are designed for heavy current loads on the +12V rail and much smaller loads on the +5V and 3.3V rails.