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For the past few days I've had a problem where as soon as I turn my printer on, the temperature on my second extruder starts climbing immediately. Doesn't matter whether there's a temperature setpoint or not, it just climbs to about 230 and stays there.

I've looked at the board and the LED that's mapped to that output is on all the time, although it's not on full brightness. It's not being visibly PWM'ed, but if I turn the setpoint really high (like 260), then the LED goes full brightness and the temperature does start to climb past its "resting" 230 temp.

The first extruder and heated bed still work just fine.

My pins.h in the firmware matches what SeeMeCNC provides except for the 2 lines that define the heater pin and thermistor pin for the second extruder, but I haven't changed those in months and this problem just started a couple days ago.

I can disconnect the hotend and the LED stays lit, so I think that rules out a possible short. Also, and this is the part that seems really screwy, I've tried uploading the eeprom_clear sketch that comes with the Arduino IDE and the LED stays lit even through that!

I'm thinking either A.) there's some kind of pin mapping that's stored separately from the eeprom (maybe, just guessing) and that got messed up somehow, or B.) my RAMBo just developed some kind of internal short and needs replaced.

Any other possibilities? Any advice?

I sent an email to Ultimachine. Here's their response (speedy!):

Hi Ben,

This definitely sounds like a hardware failure of the mosfet operating the Fan 1 circuit. Not a very common occurrence but I have seen this usually from accidental shorting of the circuit.

If you will provide me with the serial number and where you purchased the board and your shipping address we will send you a RMA label to return the board to us for repair or replacement as required.

I only need the bare board without any connectors.

If you have any questions please let me know.

Usually caused by a short, eh? Failure of a MOSFET, eh?

That led me to do some investigating.

While I was setting up dual extrusion, I had read somewhere on this forum in some other topic that all the MOSFET outputs in the green header at the top of the board were identical, and just labeled as heatX/fanX for convenience. That information turned out to be WRONG, but I didn't realize it at the time so I thought "well the build manual already had me put the layer fan on heat1 so I'll just put this new extruder's heater on fan1, if it's all the same anyway, and just sort it out in the firmware pins.h."

Turns out that according to the schematic at http://reprap.org/mediawiki/images/7/75 ... ematic.png the heater outputs and the fan outputs are controlled by different models of MOSFET, with the heater ones being much more powerful. Here are the spec sheets:

http://www.mouser.com/ProductDetail/Int ... 2FwA%3D%3D
http://www.mouser.com/ProductDetail/NXP ... jWjg%3D%3D

3.4A vs 63A continuous current, that is quite a difference. So by putting my new extruder on a fan circuit, it burned out the MOSFET slowly over time. If I'd put it on a heater circuit, it would have been fine.

Just posting this here so others can learn from my experience.

Almost right. Fan0 does use the heavyweight mosfet and is suitable for a heater. It's only Fan1 and 2 that use the lightweight mosfet.