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Heatbed not heating.

Posted by Arquit3D 
Heatbed not heating.
April 22, 2017 09:59AM
Hi, I have a problem that I've not been able to solve with the general indications found around the web, I thought that maybe someone could help in here.

I have a Prusa i3 running with Ramps 1.4.

When everything started, I was printing and suddenly the printbed stopped heating. Then, never turned on again.
I have checked voltages on PSU, Ramps terminals and D8 terminal. Everywhere around 12V or slightly higher (12.3 on the PSU). Thermistor works fine.
When I check voltage in the heatbed terminals, I get 0.0V when not heating, but when I turn on heating in Repetier, the voltage increases to 30-40 mV (millivolts, yes). Of course, there is no increase in the temperature.
I thought about a Mosfet or the polyfuse, but I completely replaced the Ramps board and the problem still persist, so I don't think that a brand new Ramps board is giving me the same problems, even though it could be possible.
Resistante between terminals in the heatbed is 1.3 Ohm (I read that it should be around 0.8) but even if it is a little bit over the theory, it should still slowly heat, right?
I have not seen any damage in the heatbed, so considering it was working properly before, I should assume it is still working fine as it has no secrets.
Last option for me is the PSU, is giving 12V but maybe it's not providing the right power? A new one is on its way, but again, if the power is not enough, I should get some lectures on the temperature sensor increasing temperature slowly right?
I don't know about the ATMega/Ramps communication, but could be possible that the ATMega controls the activation of the heatbed or this is just not possible? Movement on axis and heating in the extruder is working without noticeable problems,
Can firmware be corrupted somehow? I have not tried yet to reupload it but I'm running out of possible options.
Re: Heatbed not heating.
April 22, 2017 04:19PM
When you try to heat the bed, does LED2 on the ramps board light up ?
If it does not light up then either the mosfet is o/c or there is no signal from the mega to turn the mosfet on.
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