Which board have you got again Pirvan ? Sanguinololu, Printrboard remake or a RAMPS/Azteeg board ?
And is it the stock heater bed you're running now ?

Wouldn't expect the MOSFET to deliver low-current in its usual failure modes... *shrug* maybe I just manage to incinerate them before such failures occur But do check the PCB Traces to/from the power connections on that MOSFET...

If its the stock 'original' SD Motherboard, it most likely has a pair of STP55NF06L's ( Datasheet http://www.st.com/web/en/resource/techn … 002690.pdf ). These are already 55Amp jobs, and following the usual tempreature based derating rules-of-thumb should have been more than capable of sustained 10A operation in a 'reasonably close to 25°C' environment.

For replacements, you can try the MOSFETs that come with the Botronicz Sang boards - they use NXP PSMN7R6-60PS MOSFETs (Datasheet http://www.nxp.com/documents/data_sheet … 6-60PS.pdf ) . These are good for a continus drain current of 92A .. so should be about 20% 'more durable' than the 55Amp ones... but realistically - your original 55A ones should still have been 'durable enough' to not prematurely fail

Anyway.. hope that helps

I have one of the original SD3 boards, the Sanguinololu 1.3a and the MOSFET is indeed a ST P55NF06L.   My Printrboard and it uses a FP30N06LE which are only 30A. 

I was looking at a bunch of MOSFETS at Mouser.com and the one you mentioned (PSMN7R6-60PS) looks pretty good.  I have this idea that if it's a higher rating, it might actually run cooler since we're not pushing it anywhere near its limits.

But ultimately, while I can order just about anything from Mouser, I'll try to see what I can find available locally (JDR Microdevices), so I can pick it up today and fix the board this evening.

Edit:

Today I checked JDR Micro and they didn't have any thing usable in stock, so I placed an order with Mouser for both the standard and the upgraded FET's  I should have them in a couple of days.

Meanwhile I don't know what to do with myself, the Doodle is down.... wait I have a Printrbot, maybe I can play with that

Wouldn't expect the MOSFET to deliver low-current in its usual failure modes... *shrug* maybe I just manage to incinerate them before such failures occur But do check the PCB Traces to/from the power connections on that MOSFET...

If its the stock 'original' SD Motherboard, it most likely has a pair of STP55NF06L's ( Datasheet http://www.st.com/web/en/resource/techn … 002690.pdf ). These are already 55Amp jobs, and following the usual tempreature based derating rules-of-thumb should have been more than capable of sustained 10A operation in a 'reasonably close to 25°C' environment.

For replacements, you can try the MOSFETs that come with the Botronicz Sang boards - they use NXP PSMN7R6-60PS MOSFETs (Datasheet http://www.nxp.com/documents/data_sheet … 6-60PS.pdf ) . These are good for a continus drain current of 92A .. so should be about 20% 'more durable' than the 55Amp ones... but realistically - your original 55A ones should still have been 'durable enough' to not prematurely fail

Anyway.. hope that helps

I was about to order a new heating pad from QU-BD, when I though about checking the MOSFET output, so I simply plugged the bed heater wire into the extruder heater  connector and turned the extruder heater on.  The bed heater heated all the way to 90°  without any problems.

So obviously the bed heater MOSFET needs replacement.  Since I have to do this anyway, I'm thinking about a higher capacity MOSFET.  Did anyone here upgrade their MOSFETs?  If so, which ones did you use?

So what going bad here?  The heater, the thermistor, or the board driver?

None of the above - you have a weak connection that has developed that has lowered the available current.
Check your terminations, and the wires leading into the bed itself... If its a silicone bed, there may be a break inside the silicone which happens with QU-BD's if you dont adequately brace the wire coming out from the bed (Z-axis movement alone can lead to broken internal wires if you leave it just 'hanging')...

But basically, for it to flat-line at a less-than-normal-temp it means that your heater is not outputting the same amount of power. Unless it is *obviously* damaged, then the other culprit will be reduced current availability - which is either because of bad/weak connections at a wire junction, or your PSU itself has suddenly started providing less current (heat beds use the most current of all things on an SD3 and thus have the highest demand for current...)

But theres no 'drivers' invovled here, the thermistor has nothing to do with it, and your heater circuit is only to blame if it has substantially changed - through obvious damage.

Loose wire/connection is all it'll be... I'd seriously doubt the PSU is to blame.

If that doesn't work; check all the wires running to your heatbed and make sure they're connected properly and not loose.  If any of your wires got pulled or jostled it's possible your temp sensor isn't getting a good reading on the heat bed.

So I'm at a loss.   I don't think it's the power supply, or I would probably see similar problems with the extruder.

If I were to connect a separate power supply to drive the heater, is there a way to control the temperature?

I usually set the print bed heater manually, rather than in GCode, and yesterday I noticed that it wouldn't go up above 66°.  Usually it takes about 6-8 minutes to heat up to 95°, but even after 15 minutes the temp was just bouncing between 65° and 66°.

So, thinking it might just be a software glitch, I clicked disconnect, quit and restarted and reconnected to the printer, the bed heater temps went steadily up to about 85°, then started dropping down to the low 70's.  So this time I turned off the printer for about 5 minutes, then turned back on.  This time the bed reached 92-93°, then started fluctuating by as much as 10°. 

I left it like that for about 30 minutes, the started a print which worked fine, but as I kept an eye on the bed temps I noticed it bouncing back and forth from as low as 80° to as high as 95°.

So what going bad here?  The heater, the thermistor, or the board driver?