Too low voltage between bed heater pins, bed won't warm up (Sangui1.3a,ATX)

The problem: When turning on the extruder heater, and measuring voltages directly at the out-PINs of my sanguinololu, the extruder heater voltage reads 12.08V, but the bed heater reads a puny 6 to 9 millivolts and oscillates between those 2 values. Consequence: Extruder heats fine, bed doesn't.

I'm using a ATX1 260W PSU without dummy load yet.
I was thinking if it were the dummy load problem, the extruder heater should have a faulty voltage as well, but since it's working fine, I'm wondering what it could be.

I just installed my first heatbed, so it is also kinda unlikely that I have already blown the MOSFET....

Also: Resistance on both extruder heater and bed heater "+ to ground" lines (measured at the boards pins) are roughly the same at ~1kOhm, so I guess I also haven't shorted the pins while soldering.

Any help on that matter would be greatly appreciated

Edited 1 time(s). Last edit at 06/30/2012 12:13PM by Gara.

First off, a 260W ATX PSU will not provide enough current on the 12V line to power the heated bed. I would expect the extruder voltage to drop when the bed is turned on if everything else is working correctly though.

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The PSU is rated for continuous output of 17A on 12V, which subtracting the 5A for the steppers+extruder, should give 12A for the bed, which is close but should suffice. Also, this problem should only arise if all of those are powered simultaneously. When I tested though, it was one at a time, ie extruder/steppers off when measuring heater voltage.