Balancing ATX Supplies

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A 400~500W ATX PSU is a good way to power your RepRap but requires a bit more care than a purpose built 12V PSU. The main issue with some (not all) ATX PSUs is the requirement for a load on the 5V rail. To get reliable operation from these PSUs you will need to balance the loads on the 12V vs the 5V rails. 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. Nowadays (October 2014) the reverse is true: most PCs draw considerably more current from the 12V rail than from the 5V and 3.3V rails.

Below you'll see a table of voltage as measured under various loads for an old ATX PSU that has issues with unbalanced loads. The first three show increasing 12V load with the recommended "dummy" 5V load of a 10 ohm 10W power resistor (i.e. 500ma). The first line represents the 12V 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 the load is increased on the 12V rail, the voltage decreases and the 5V rail voltage floats higher and higher. This is due to the fact that the 12V and 5V lines share a regulator. Another symptom that often occurs with PSUs that cannot handle unbalanced loads is that the transformer coils will emit a clearly audible high-pitch whine, that increases in loudness as the load on the 12V rail increases.

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 rail. Each load was a 50W 12V light bulb easily sourced from the nearest hardware store or supermarket as accent lighting. At 5V these light bulbs draw about 2.5 amps each. As you can see the voltages are better regulated as the load on the 5V rail increases. I have a clip on ferrite cylinder on the USB cable and still got rare comms disconnect mid print with two light bulbs 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

Generally speaking, the latest "Haswell ready" or "Haswell compatible" ATX PSUs - commonly available since the beginning of 2014 - should not 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.