Thermistor problem?
Hello
We are proud owners and users of a Prusa i3 MK3, a very nice printer.
We have had a mishap during one of our prints that lead to us having to dismantle the hot-end to do cleanup and since then we have had thermal runaway problems and mintemp errors pre-printing (also our prints look disastrously bad when we manage to get to the end of the print).
Essentially, the hotend reports 14 degrees at rest where the bed reports 23, we measured using a FLIR camera the temperature of the hot-end around 27 degrees at that moment. if we jump-start the heating by bringing the hot-end above the mintemp temperature, and we set the hot-end to 200 degrees, we measure 250 degrees with the flir camera. This leads to thermal runnaways when the print fan kicks-in (obviously the printer can't maintain 250 degrees on the hot-end when there is air-flow around) and this leads to catastrophic oozing of the PLA filament and other issues...
I have really tried to reassemble the hot-end carefully when we did the maintenance.
I am hesitating on ordering a full new hot-end or just a new thermistor, would any of you have other suggestion?
Thanks in advance!
Sébastien
Re: Thermistor problem?
I would check the thermistor wiring, ive had to repair mine already, the wire is very thin and breaks very easily. Ill bet yours is partially broken and causing issues.
Re: Thermistor problem?
I agree with Nathan0876. These thermistors are "negative temperature coefficient" which means that when they are cold, they have a high resistance and when they are hot they have a low resistance. So when your thermistor is reading BELOW room temperature when it's actually at room temperature, that means that the total resistance of the circuit is higher than it should be (fairly significantly in this case). So you have either a connector or a cable that is not doing its job. Most likely a broken wire that is just barely making contact (a wire can have an outer insulating jacket that is fully intact, but broken copper wires inside). Unfortunately, given the history you describe, the damage was probably done while you were cleaning up the hot end, meaning the damage is very close to or right at the edge of the thermistor body. This is not an easy place to splice a wire! So the best thing to do is buy a new thermistor.
There are thermistors on the market that may be less expensive to buy - or to ship - than the ones from Prusa, but if you want the thermistor to come with the right length of cable and the right connector pre-installed, buy it from them. You could also go directly to E3D and check out what they offer for thermistors; theirs would at least have the right body type and the right resistance/temperature curve even if they don't have the right cable length or compatible connector.
Re: Thermistor problem?
Thanks a lot for your input, I will look to order a new thermistor. Is it available on the prisa shop? Didn’t find it yet...
Re: Thermistor problem?
Thanks a lot for your input, I will look to order a new thermistor. Is it available on the prisa shop? Didn’t find it yet...
Yes it is i have already ordered a replacement to have on hand just in case.
However at first i could not see the spare parts in the store page, i had to have support add the spare parts page to my account so i could then view and purchase the spare parts, so if you cannot see them in the store contact support they will be able to assist you.