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The PRUSA i3 MK3 is a 3D printer designed and manufactured by PRUSA Research, a Czech 3D printer manufacturer. It is the successor to the PRUSA i3 MK2 and is one of the most popular and well-regarded 3D printers on the market.

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Turning on my Prusa i3 mk3 hotend causes printer to restart

tl;dr I have a printer that keeps restarting everytime I turn the hotend on (thing that heats the material). After taking it apart, nothing looked wrong with any wires or connections. I'm suspecting the power supply to be failing, but I have yet to check it (how would I even do that?). I want to make sure I get everything right before I go and buy a new hotend or power supply.

Hey there! I have a Prusa i3 mk3 that I've been using for quite a while now- at least a couple of years. Iirc, it has about 50+ hours of print time. For context, I am pretty experienced with these 3d printers and have a couple of years of fixing things in general. I also have all of the basic tools any good repair person would need: a soldering iron, multimeter, plenty of screwdriver bits, pry tools, etc.

Just recently, I was running a calibration when the printer suddenly restarted itself. I went to check it, and physically, nothing looked wrong. So, I decided to run the calibration again, and the same thing happened. This time, I noticed that it only happened after the hotend (the thing that heats the filament) was turned on. After manually turning on the hotend, I confirmed that it was the problem.

I found a guide on how to replace the hotend and followed it so I could get the part out. Throughout the guide, I didn't find anything wrong. There were no blown parts on the Einsy board, the wires weren't cut or crimped, and they were plugged into the right ports.

After finally getting the hotend off, it didn't seem like anything was wrong. For reference, the only things that go to the hotend are the heating element wires and a thermosister to check the temperature. The metal block looked good, and the heat sync looked perfect. There was only 1 thing I noticed that could be weird. The heater wires at the hotend kind of looked like they were touching, which could have been shorting, but they were stuck in their place when built, so nothing could have moved to suddenly short the printer. I wish I knew exactly how it heated the block, but that seems like it probably isn't the problem.

I then unplugged the heating element and not the thermosister, set it to turn on, and it didn't immediately restart. This tells me it isn't a software issue with the thermosister saying that the printer is overheating (again, the printer would restart as soon as the temp was set, not after a few seconds of heating).

My main (and honestly only) suspect at this point is the power supply failing. Is there a way to check if a power supply is failing or not? Or is there something else I should check/do that I haven't done? I want to make sure I know what the problem is and if I can fix it before I go replace the hotend or power supply.

Sorry for the long read btw. I'm happy to give more details if needed. Any responses are helpful!

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Update: After opening the bottom part of the PSU, the part with all the screws holding the wires in place, I can confirm the the PSU is not the problem. When I turn the hotend on, the power LED does not turn off and then immediately turn back on, it stays on. This means that the PSU is not suddenly shutting down when under load and then causing the printer to restart. This is kind of annoying, because it means that the problem very well may be the hotend.

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Stop! 1st - that is a VERY good question - lots of detail - thank you.

2nd You need to do a little more troubleshooting b4 you jump to buying a new HE.

It is possible the HE is causing a power spike, but it is also possible there is a problem in the control board.

What happens if you disconnect the HE? Does the 3DP still restart?

If it does, you may need to replace your control board - or do some serious troubleshooting.

There are a couple of Heater ports on that RAMBo PCB. You could try the other port.

I think you would have to reconfigure your F/W to use the other port unless you have 2 HEs.

You can find the full schematic of a RAMBo 1.2g bd here. This is a typical heater driver

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Only the heater drivers have the Zener in the 12V feed. You should check those.

The MOSFET also has a reverse diode which might be bad. Not sure how to test that.

But it is driving a resistor, so there shouldn't be any voltage spikes for it to suppress.

Bottom line - my hotend (not a Prusa) uses a resistor that measures 3.55Ω for 112V supply.

Check that. It should be an easy replacement if it is bad. You don't replace the whole HE.

Good luck.

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Thanks! Glad to know my rambling is worth something.

To start, no, the printer does not restart when the HE is unplugged (I think I mentioned that)

Secondly, I don't believe there are any other ports for the HE heating part (not the thermosister). For reference, this is the Prusa i3 mk3 Einsy board, doesn't seem to be any other heater ports unless I'm reading this wrong. I'll add an image of one to the post.

As for checking diodes and resistors, I'll have to do that. I never really got around to doing it on the board due to the number of wires. I should be able to test most of them.

Thanks for the answer! I'll have to test the HE with my multimeter (I believe high resistance means it's bad, right? I'm not making that up, am I?) to see if it's the problem. If it isn't, I'll look at the things you've mentioned here.

Thanks again!

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Did you try to separate with a small screwdriver the two wires of the thermistor on the left side of the hot end that were touching and basically shorting?

I know this is weird, but it's also worth checking the power supply voltage setting, if it's set to 230V power and you're in a 120V country the printer may power on but fail when the hotend starts.

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Hey there! I'm going to go ahead and answer my question, since I think I found the problem. If you want to get straight to the point, it was the HE that was the problem.

After reading what @ruggb said (and a couple of Google searches), I got the idea to check the Einsy board more, and to test the resistance of the HE as well. Thankfully, I had another Prusa i3 mk3 that I could check against. Nothing was wrong after moving some wires out of the way and checking the board-they both looked the same. I then decided to check the resistance of the HE on each of the printers. This is where I found the problem.

The broken printer's HE read: 0.05 ohms

The good printer's HE fluctuated, but read about: 15 ohms

This was a surprise to me. I triple checked to make sure the variables were all the same. Thermosister plugged in, both been unplugged for a little bit, and haven't printed anything in a day. At this point, I am almost 99% certain that the HE is the problem. I wish I had a broken printer that I could take the HE out of, but I don't.

I'm not entirely sure why the HE broke, but it might have something to do with the heater wires (not thermosisters). They looked close at the heating end, almost touching each other, and could have been shorting out. I would try to fix it, but I would need more tools, materials, and also confidence that shorting wires were the problem for that.

As for what I'll do, I'm going to try looking for a new HE to replace it with soon. Although if you think it was something else that's broken, not the HE, please feel free to comment on this answer. I will update this answer if and when I replace the broken HE. Thank you to everyone who helped me with this problem!

Cheers!

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0.05Ω is lead resistance. Your heater has a dead short.

High resistance isn't good either. The resistance depends on the supply voltage.

Hotends produce about 40W. So, with a 12V supply that would require a resistance of about 3.33Ω.

@24V it would be 2x that.

So, mine is about 42.6W @12V.

@15Ω & 12V you get about 9.6W. I would say that isn't enough to melt chocolate. So, it is probably trash also.

Can you just replace the heating element?

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@ruggb It wouldn't surprise me if that reading I gave is probably bad, but I'll look into it. However, this, non-broken HE works great, so bad readings are probably why it seems to not work well, even though it does. I'll keep an eye on that HE. Thanks for the info though!

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