Q3 Mosfet Fried to a Crisp

I just got a 2 12v rail psu and connected 12v1 to the hot end and motors while 12v2 was connected to my heated bed, grounded through D8 -. The bed seemed to get hot, but it was also uncontrollable. Setting the bed temp to 0 and it climed from 50c to 84c before I pulled the plug. After a few restarts I finally got a hot end around 210 and heated bed around 55. About 3 hours later I go to print again and my USB tty dies whenever I turn on the 12v supply. Tracking down the dmesg leads me to see that it's something about too much power on the USB and a protection circuit on the host PC. So... finally, after looking at the RAMPS I see Q3 fried open.

I have 4 2 ohm resistors in series and parallel (2 in series, two in parallel, seriesed together) to give 1.25 ohms of resistance (1/2 + 1/2 + 1/4). That should only be about 82 watts and 7 amps. Can RAMPS (1.2) not handle that? Is there anyway to heat a bed with it?

How exactly did you connect the power to D8? Mine has the ground wire of the bed connected to D8 ground, and the positive of the bed connected directly to the PSU, not D8. I did it like this from reading this forum topic: Heated Bed and RAMPS 1.2

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Germanicus

yes, that's exactly how I connected the wires. The positive was directly from the psu 12v2 4 pin molex and ground was only on d8 negative side.

The MOSFETs should have a small resistor (e.g. 100R) in series with the gate close to the pin. Without that you can get oscillations in the GHz range that can cause them to fry. I am not saying that is the problem here as other people have it working, but it is a possibility and an example of poor circuit design.

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