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Solved: I3 Mega fan 2 losing power when Hotend reaches 50°

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Dear all,

I have a small LED strip soldered to fan 2, the Hotend fan.
When I turn on my I3 Mega, the Hotend fan starts blowing at full speed and the LED strip lightens up.

If I now preheat, the LEDs turn off at 50° and the fan becomes much slower. If I stop it with a finger it cannot start spinning again of its own - I gotta give it a little shove.

Up to now it was the other way around.
When idle nothing would move nor the LEDs shine.
Once the 50° were eceeded the hotend fan ran full speed and the LEDs turned on.

I have Merlin 1.1.9 installed for a year now and haven`t changed a thing.

Do you have any ideas how I could resolve this?


The wire harness with the green connectors was broken. Replaced it.
The 3 small LEDs connected to the Hotend Fan 2 work again too now, btw.

My guess is that I fried the fan transistor by turning on the printer without a connected fan, hrere's the replacement part:
https://amzn.to/2Dpx6H3 (affiliate).

Under normal circumstances the hotend fan will start running first if the nozzle temp exceeds 50°C. If you use that fans current for the LEDs the fan will decrease its speed what is really bad in terms of coldend cooling.
Best is to wire the LEDs directly to a 12V output of the power source unit or a free 12V output on the motherboard.

The fans are used for cooling and not powering other stuff!

Yeah, well, that is the reason I have a Noctua Fan, to additinally power 3 small LEDs without losing cooling.
Thanks for pointing it out, though.

Anyhow, this is not the issue here. My problem is that the functionality is inverted. Full power while idle, much less when printing.

Problem 1: Fan and LEDs have 12V while the printer is idle.
Problem 2: Once the temp exceeds exactly 50, the power decreases to around 6,4V.

Expected behaviour would be almost the other way around, with 0V when idle.
Any ideas?


You sacrifice proper coldend cooling for some shiny LEDs? That is so wrong! Maybe you already damaged the electronics.
Install an appropriate fan again and connect the LEDs to the right output!

Dude, I tried to tell you politely. This is not my issue here. Thanks for pointing it out.
If you dont like to use fan power for LEDs, just dont. Fine with me. Don`t care, really.
Just keep it to yourself or create your own topic telling people why it is bad.

You have not contributed ANY informaton to my question and probably killed this thread.

The issue at hand is, that the fan would run while the printer is idle.
Any ideas how to solve that?
If not, do not bother to answer...