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Archive for December 30th, 2010

ATX Power Supply vs Heatsinks

Having just installed resistors on those three heatsinks, here’s the results from that ATX power supply.

Loads +5V +12 V -ON -> Gnd PS Tester
0 4.94 11.76 13 17
1 4.81 11.82 70 72
2 4.79 11.92 129 130
3 4.77 12.02 189 192

Notice that the +12 V output increases under load, which turns out to be true because all the outputs share the same transformer: supporting the load on the +5 V output requires more flux, which tends to increase all the other outputs.

The last two columns are the input power from the AC line, with the -ON pin shorted to ground and with a black-box power supply tester that evidently draws a watt or three.

The resistors on each heatsink dissipate a total of 46.6 W:

  • 1 Ω -> 4.82 / 1 = 23.0 W
  • 6 Ω -> 11.92 / 6 = 23.6 W

The box sees 140 W with all three heatsinks powered up.

Three of those fans draw 1.1 A from +12 V, adding another 13.4 W.

Grand total: 153 W. Close enough!

Power supply efficiency at full load is 81% = 153 / 189. Not as good as you’d like, but it’ll suffice for my simple needs.

The drop across a single Molex power connector pin is 11 mV @ 5 A and 4.3 mV @ 2 A: call it 2 mΩ. They’re rated for 11 A with a 30 °C temperature rise, which means I really should use PowerPoles.

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