Bed MOSFET melting adjacent connector

So I recently blew out the Melzi board in my Wanhao i3. I decided to go RAMPS to have a more reliable printer.

Got a RAMPS off eBay and paired it to a Mega clone I already had. Got the motors to move and home without any issues.

Hit the bed with a heat command to make sure everything looks good. Within 10 seconds or so I heard sizzling and saw smoke, so I killed power.

Looks like the heat bed MOSFET got hot and started melting the D8, D9, and D10 connector.

Pulled everything apart and checked all connectors, checked resistance on the bed, everything appeared good.

Put it back together and tried again with same results, MOSFET is getting hot fast.

The MOSFET is a P55NF06. I have read that these are not ideal but they are used successfully.

Is this just a crappy MOSFET or should this work in theory? The heated bed is 200x200 so it should not be drawing a huge amount of amps. I do not have a way currently to measure amp draw on the bed but this should be around 11 amps.

I have a external MOSFET unit on its way from China, but I am concerned that even with that, something is screwed up with my board.

P55 would work so-so, when the 5V supply from the arduino is stable. When it drops to 4.x V ( due to other components on the 5V rail like graphic controllers ) the MOSfet gets really hot, because it won't switch open fully.

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o_lampe
P55 would work so-so, when the 5V supply from the arduino is stable. When it drops to 4.x V ( due to other components on the 5V rail like graphic controllers ) the MOSfet gets really hot, because it won't switch open fully.

Thanks, this sounds like a likely cause of the issue. I will power the Arduino with a external, stable power supply. It was being powered by a RPi3, and yes there was a LCD also on the board.
Thanks for the help!

If you want to use an external 5V supply connected to anything else than USB, you have to modify the RAMPS board. ( Disconnect D1, IIRC )
Sometimes people have to do this MOD, because they want to use 5V servos powered by RAMPS.
OTOH, a powerful USB PSU doesn't help, as soon as you connect 12V to the RAMPS. Then the onboard 5V regulator on the Arduino will be used and the USB PSU is cut off. ( without the D1-mod )

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o_lampe
If you want to use an external 5V supply connected to anything else than USB, you have to modify the RAMPS board. ( Disconnect D1, IIRC )
Sometimes people have to do this MOD, because they want to use 5V servos powered by RAMPS.
OTOH, a powerful USB PSU doesn't help, as soon as you connect 12V to the RAMPS. Then the onboard 5V regulator on the Arduino will be used and the USB PSU is cut off. ( without the D1-mod )

Is this the case if I use a 9V power supply on the barrel jack connector?

Yes, that's actually the same as using 12V from the RAMPS. It only feeds the weak onboard regulator from Arduino.

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o_lampe
Yes, that's actually the same as using 12V from the RAMPS. It only feeds the weak onboard regulator from Arduino.

Ok, so if I were to clip D1, use a power supply on the Arduino, and also connect it to my RPi, would that improve the voltage to the heatbed MOSFET?

Or do I need to use a strong 5V power supply? If so, where is the ideal location to connect that?

Thanks for all your help so far. I have a external MOSFET board with heatsink on the way, but I am concerned that even with this, the low voltage to the MOSFET trigger will cause it to not work right due to the LCD drawing a lot of the USB power.

And maybe replace the hotend and bed MOSFET on the RAMPS board? I have 3034 MOSFETs on the way.

Edited 1 time(s). Last edit at 01/11/2017 07:17PM by beercity.

Replacing the MOSFets with 3.3V gate voltage types sure helps. Didn't check if 3034 MOSFet is such type...
Yes, you'd have to use a strong 5V supply. Connect it to the Servo header or an unused endstop connector.

Seems, you're on the right track