Huxley heatbed won't heat up

Hi RepRappers,

I've got my Huxley built and wired and I'm following through the commissioning stage of the instructions on the wiki. All the stepper motors are working fine, the hot end heats up correctly, but the heatbed doesn't respond to commands to heat up. I've tried rewiring it and I've checked the connections with a multimeter and there don't seem to be any issues. Pronterface sends the signal to heat up and doesn't report any errors but the temp never changes (thermistor reading stays at room temp). Anyone got any suggestions for what to try next?

Thanks in advance!

My guess is the wires aren't connected to the right terminals. What controller are you using?

From your post, I'm working on the assumption that your thermistor is working correctly. EDIT: I also presume that the firmware is doing its job properly, so add that to the checklist if the other tests don't result in a problem source.

wiring, bad FET, bad I/O, bad heated bed

wiring: visual check, continuity checks. Make sure to check for both open and short-circuit conditions

bad FET: use ammeter and dummy load in place of heated bed assembly, test for current on manual operation. use
R_L ~= 10 R_heated-bed
to make it easy (A 5W 10ohm resistor is easy to come by and a useful item to have on hand if you don't already)
Note: this test isn't 100% accurate, since it's not pushing the FET up into the 10-15A range, but it should do for a primary validation of the FET.

bad I/O: manually operate heated bed using pronterface and check for logic hi/lo condition on changed status (ie, On/Off) at the gate input to the FET

bad heated bed: Check resistance and/or connect ammeter inline while powering directly from supply

Edited 2 time(s). Last edit at 03/31/2013 04:11PM by xiando.

Make sure the temp is actually reading. When I first built mine, I thought the reading was at room temperature, but found the thermisitor was shorted and the temp line should be right in between 0 and 50, not sitting on the bottom.

Like xiando mentioned, check resistance of the bed with a multimeter with power off. I'd then power back on and check to see if voltage is being sent from your board to the hot bed. Check all of these right at the board terminals. You should be able to tell if all of the wiring is good from this and that the heat bed is right.

If everything still looks good, attach the heat bed directly to your power supply and see if it starts to heat up.