Balancing ATX Supplies

A modern 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 these supplies is the requirement for a load on the 5V line. To get reliable opperation you will need to balance the loads on the 12V vs the 5V lines.

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