Hot end suddenly not heating up

I have a home-built MendelMax 1.5+ that's been running great for around a year. RAMPS 1.4, 24V PSU with a 12V stepdown to run the hot end and fans.

After a long print on Saturday, I turned the printer on on Sunday but found that the hot end would not heat up. Here's what I've observed:

1) The hot end thermistor is fine. I measure a good resistance across it, the software is reading room temp and touching the hot end warms it up enough to register the difference.
2) I get 12V across the terminals of the stepdown output before turning on the heater. I have a fan over the extruder stepper that is hooked directly up to the 12V terminals, so simply turning on the power turns this fan on. It's running fine.
3) When I turn on the heater via the software, the fan I just mentioned stops running, I hear the PSU fan adjust slightly, but the hot end simply doesn't heat up.
4) Last night while measuring with my multimeter, I don't know if it was coincidence or something I touched, but the hot end suddenly kicked into life. I turned the printer off and on again, but it was no longer working. I haven't been able to reproduce.

Just to be clear about my setup, it's like this figure: [farm9.staticflickr.com]
The only addition is that I have another fan connected directly to the 12V terminals, as described in #2 above.

I'm out of ideas. Can anyone suggest some more troubleshooting steps?

Thanks!

What's the voltage when you measure between D10 ground and your power supply ground? It should measure 12V when you aren't calling for hot end heat and near 0v when you are.

My money would be on a bad connection at a screw terminal, bad mosfet, or bad solder joint.

I agree - it sounds like a poor connection, especially when you said you think you touched something and the hotend worked.

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Waitaki 3D Printer

Thanks. I measured the D10/ground voltage as suggested, and I do get the 12V off and 0V on measurements that you described. I really don't see any bad joints around the heater itself, and like I said I've been using the printer for a long time (100s of hours or printing). More likely to me is something on the RAMPS board got fried but I don't know how to verify that's the case.

Hey guys,
Have a similar problem. My machine is only a couple weeks old but I have done about 100hr of prints already. Anyways I had a 16hr print fail last night and the nozzle was all goo-d up this morning. Took it all apart and when I put it back together the high temp sheaths on the heater slide back and the wires touched and shorted. Is there a fuse on this circuit. I get the 12v from D10 ground to power but it stays the same when I turn on the heater? Sorry not trying to hijack the thread just similar issue.

Thanks,
Seth

Well I figured out my problem. When the heater end shorted it burned a trace on the back of the board directly behind the D10. Soldered the trace back together and she is printing again. Since the hot end draws so my I and going to add a fuse inline to prevent this in the future. Also as a failsafe if the heater ever just decides to heat itself to the moon it will blow the fuse rather than burning the shop down! Maybe this will help with you problem as well? The trace is on the back of the board directly behind D10 which means you have to split the two large boards to see it and get to it.

Seth

I checked the terminals and everything is well connected. I don't know how to check if the RAMPS MOSFET has blown - can anyone advise on that?
The lights come on when I send the signal to the board to turn on the heater so I get the sense that the RAMPS board is generally okay, but obviously some aspect could have shorted that would not cause the heater to get hot.

If the LED on the RAMPS board lights up, the mosfet is at least passing minimal current. Take the mosfet out of the equation, just hold the ground from the hot end to the power supply ground. If it heats up, then it's either D10's connector, the mosfet, or a trace/solder joint. A continuty test and reflowing the joints should eliminate all but the Mosfet as the issue.

I'd also check your hot end to make sure that it doesn't have a short in the heater/resistor. Measure the resistance between the positive lead at the step down and ground. It should measure a few ohms (depending on what you use as the heating element) When D10 turns on (grounds), it may allow a direct short that trips your step down (why the fan shuts off) and puts a different load on your power supply (why the PSU fan changes speed). When it momentarily worked again, that short to ground might have been broken allowing it to work again...for a short period).