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Hot end fan not working - please add another step in the assembly checklist

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I just wanted to share my story about a non-working hot end fan.

When I received my Ultimaker 2, I noticed that it would only run for a few minutes before it jammed. After some reading on this forum I checked the hot-end fan on the back. This was not running at all.

I tried to power the fan separately and it still would not run, so it was confirmed that it was dead. I replaced it with a new one, but it still would not run. Just to be sure that the cable from the mainboard to the fan was ok, I tried to power it with a separate power source using this cable. It worked.

So the mainboard was the problem here. I got hold of a new board from the dealer, but the fan would still not run. I could power the fan from a separate power source but not from the board. I tried to measure the output from the board, and surely it gave correct voltage, but only as long as the fan was not connected.

Then I noticed something on the old board, which was in the machine originally; the transistor controlling the fan had a dent on it. It was burned out.

Then I removed the cable from the machine and noticed that the two leads were switched. This explained a lot. I switched them back, replaced the broken transistor on the board, and now the fan worked as expected on both the boards.


My conclusion is that I suspect that the board was fine when it left the factory, however when connecting the fan with reversed polarity, there is nothing in the circuitry that protects the mainboard. There is probably a protection diode inside the fan that shorts when the polarity is reversed and thus destroying the transistor on the mainboard.

I hope that someone from the Ultimaker team can add a step in the assembly guide to check for small errors like reversed polarity on cables. I guess these cables are pre-assembled and assumed to be ok. This was a real pain to figure out and I spent a lot of hours switching boards and debugging electronics and I hope my experience can save other people from this same thing. The Ultimaker is a really nice piece of engineering and it would be a shame if people’s impressions of it were to degrade because of sloppy QA.


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