trouble with heated bed - smoking Q2

[doug]

I have a Printrbot simple metal with factory extended x axis and heated bed upgrade, F4 board, many, many hours of successful printing. All of a sudden on power up Q2, the MOSFET controlling the heated bed, gave up the smoke burning up the traces and what I assume is part of the ground plane on the back of the board. I could use some help figuring out how to fix this. I measure about 1ohm resistance in the heating element and can not find it shorting anywhere. The thermistor measures 93 ohms. I am using a 30a 12v power supply. I did not have the right MOSFET on had to switch out. I had a RAMPS 1.4 board that I got for less than shipping on a new MOSFET would cost so I decided to connect it knowing I might smoke it. I was right. As soon as I powered it up the hot bed MOSFET got hot enough to loosen the solder joints. Just to make sure the bed heat plate wasn't somehow shorting to the print bed, I removed the bed and put it on a wood surface on nylon standoffs. Same thing exact thing. I'm not sure what to test next. Any help is appreciated.
thanks

[RetireeJay]

The fact that you have overheated the MOSFET on two different boards strongly suggests that the bed heater - or the wires going to the heater - are shorted.

I think the bed should measure no less than one ohm, probably closer to two or maybe even three. But measuring very low resistance with typical VOM's is tricky. Basically, you have to start by shorting both probes to each other (or to a solder pad) and see what resistance you measure in that condition. Let's call that R0. Then when you measure the bed resistance, you hope to see R0 plus another one or two ohms of bed resistance. If you still see R0, then the bed is shorted out.

Take your measurements with the wires going to the heated bed disconnected at the control board. Measure both by probing both at the heated bed terminals and at the wires where they would connect to the board.

At room temperature, the thermistor should measure close to 100k ohms, not 100 ohms (but maybe you missed the "k" on the readout). If the thermistor really was 93 ohms then if you look at the bed temperature in your favorite user interface program you would see a very hot temperature, not room temperature.

[doug]

Thanks Jay. I agree it almost has to be the bed heater or wiring. I am consistently getting around 1 ohm measuring the heater at the pads on the heater and from the wire ends. This appears to be within spec so I am stumped. I think my next course of action is to replace the overheated MOSFET on the RAMPS board connect the heater. If the problem persists I may just replace the bed heater to eliminate it as variable.


My error on the thermistor. I dropped the k when I was typing this up. I am reading 93k ohms. Between the component tolerance and the cheap DVM I used, I am considering it to be good.

[doug]

It was the heater. I replaced the heater and the new one works.