Mintemp Error and Breaking Heatbed Thermistor  

RE: Mintemp Error and Breaking Heatbed Thermistor

A picture would help much.

RE: Mintemp Error and Breaking Heatbed Thermistor

I will try and post a picture at some point. Otherwise it is breaking near/at where the cable crimps on the heatbed. I think i am going to try and splice a thicker gauge cable (stranded cable) for the thermistor. I also see some other mods out there for different shaped clamps.

RE: Mintemp Error and Breaking Heatbed Thermistor

Good luck, and  welcome to the club. I had similar problem with the bed thermister wire(s) breaking at the clamp. I chatted with support and the second I told them I bought a kit from one of their resellers the person on the other end was reluctant to help me. I see so many posts about the thermister. You did the right thing by letting PR know. 

One time -oh well. 2 times - hmmmm. 3 times - there is probably something wrong. In this case, I think there is a major design issue with the cover/clamp.

When I built my kit I had someone double check the wires were routed correctly. I tried soldering a thicker piece of wire in, but at the slighted movement the very thin thermister wire would break at the solder joint. I ran it without the clamp for a short time and it broke again. I had bought one and this time I had to replace it.

Warranty or not, users should let PR know and they should "man up" and fix the problem.

Your solder fix should get you by for a while.

Kenn

RE: Mintemp Error and Breaking Heatbed Thermistor

This last round I did buy two so if it fails I can go back to a new unmodified one. But I dont have much faith of a lasting fix.

RE: Mintemp Error and Breaking Heatbed Thermistor

Mintemp error appeares when you have the nozzle temperature below the set mintemp threshold. I guess it is 15°C. It does not need to be the cause of fault as it can be also a consequence either of broken thermistor or broken thermistor wires, which could be your case. Higher temperatures leads to lower thermistor resistance and vice versa. Broken wires mean infinite resistance.

RE: Mintemp Error and Breaking Heatbed Thermistor

This is a mintemp bed error. You are correct that it shows up when the temp drops too low. In this case it shorts and states 0C. If you wiggle or bend the cable coming out of the heatbed you can watch as the temp cuts back to room temp. It is very much a short. I just installed the 4th sensor this time re-enforcing it with heat wrap and running it outside of the clamp as I feel like its breaking near the clamp on the heatbed.

RE: Mintemp Error and Breaking Heatbed Thermistor

Now you have me worried. Tomorrow, I will get some replacements....before I need them.

I was thinking of heatshrink, but I could not find any of mine. Let us know how it goes.

RE: Mintemp Error and Breaking Heatbed Thermistor

The cable MUST NOT flex at the heated bed.

Prusa provides a nylon rod to insert in the heatbed connector. If you do it right - there is no flexing at that point.

Mine does a gentle bend over the full length of the cable.

RE: Mintemp Error and Breaking Heatbed Thermistor

I always point to the construction manual as a reference. It keeps me from making dumb mistakes.

I did not do that this time... The heat plate only has the nylon rod in the new design. The old heat bed cable uses a stiff fabric wrap.

Key point here is it has to be grabbed by the connector. Flex at this point will break wires.

See Step 5:

https://manual.prusa3d.com/Guide/7.+Heatbed++&+PSU+assembly+(spiral+wrap)/512?lang=en

RE: Mintemp Error and Breaking Heatbed Thermistor

@robert-rmm200

I just checked the manual and that IS NOT the one that was up when I built my printer. Even the instructions for the the OLD design are misleading ( at best ). The OLD design that I got DID NOT  have a nylon rod, spiral wrap, .or cover with a place for the nylon rod. Maybe WE should chat with support and see if WE can get the updated parts.

RE: Mintemp Error and Breaking Heatbed Thermistor

I found link to another assembly manual

https://manual.prusa3d.com/Guide/7.+Heatbed++&+PSU+assembly+(Black+PSU)/1153?lang=en

RE: Mintemp Error and Breaking Heatbed Thermistor

Step 11 in the manual you linked shows the twisted cover inserted into the connector 5-6mm. That is critical to clamp it.

As updates come along, the updated STL files are available on the Prusa site. Probably quickest to print your own.

I did that anyway for my heatbed connector. There was not room in the Lack enclosure for the stock one,

so I grabbed one that has the cable exiting at a 45 degree angle.

RE: Mintemp Error and Breaking Heatbed Thermistor

We will see how me bypassing that goes. I honestly believe where it is clamping is where its failing, but I dont have the tools to test that. I have the newer design that just uses that cloth wrap that was in those directions.

RE: Mintemp Error and Breaking Heatbed Thermistor

Posted by: @christopherbrosz

This is a mintemp bed error. You are correct that it shows up when the temp drops too low. In this case it shorts and states 0C. If you wiggle or bend the cable coming out of the heatbed you can watch as the temp cuts back to room temp. It is very much a short. I just installed the 4th sensor this time re-enforcing it with heat wrap and running it outside of the clamp as I feel like its breaking near the clamp on the heatbed.;

Hi Christopher.

I agree the thermistor wire is faulty... I believe the problem is stress fracture of the conductors due to repeated flexing, (this also happens to the heatbed power cables)

I have a small correction to your comment, as above...   the thermistor is a negative temperature coefficient resistor, what happens in the MINTEMP case, is either:

the temperature is actually low so the thermistor goes very high resistance and reports MINTEMP, correctly, Or,

the THERMISTOR wires BREAK and  go Open Circuit (almost infinite resistance)  and the printer interprets that as 0 degrees Centigrade AND ERRORS   MINTEMP

if the thermistor or wire, goes Short Circuit, the printer errors MAXTEMP 

i replaced my heatbed power cables with 16 awg, flexible silicone insulated wire (it has 252 very fine conductors in each wire, as opposed to the 22  thicker conductors in the prusa wires, you could use 14 awg or 12 awg (both have even more filaments) if you wanted to but so far I have not had a failure with 16awgflexible silicone wire...

you could replace the flexing section of the thermistor wires with flexible silicone insulated wire making insulated joints under the heatbed, and inside the einsy case, or crimping new connectors directly to the flexible wires...  either will prevent the joints being a weak point due to flexing!

regards Joan

RE: Mintemp Error and Breaking Heatbed Thermistor

Hello,

I also encountered this problem last weekend, the cause was a broken cable in the middle of the wiring harness from the Heatbed to the mainboard.

Since there is a long waiting time for parts now, I decided not to neglect and try to fix it. I didn't want to remove the sensor from the board and after measuring the cable I found a bundle break.

I replaced the thermistor cable with a wire from the FTP CAT6 cable and inserted them into the TPFE tube. I started the exchange before moving under the Heatbed and ended it after entering the mainboard case.

I used a hollow tin with rosin inside for soldering.

So far, I have completed another 60 hours of printing and everything without the slightest problem, I think that now the cable will not break. Nevertheless, I will order a new original.

I also enclose photo documentation of the replacement / repair.

Printing forever 🙂

With Regards

Jirka

RE: Mintemp Error and Breaking Heatbed Thermistor

@j-bergl

Using ethernet cable never occurred to me. Ill keep that in mind if I break it again. Thusfar wiring that line outside the clamp has worked well for the last 100s of hours.

RE: Mintemp Error and Breaking Heatbed Thermistor

Cat5 Ethernet Patch cord is ok for this sort of job, the multi core wire will last quite a while during flexing

Cat 5 ethernet permanent  cable is not appropriate, the single core will fracture with little flexing. 

regards Joan

RE: Mintemp Error and Breaking Heatbed Thermistor

Hello,

yes I know that soft (knitted) CAT5 would probably be more suitable but I didn't have it at home.
That's why I used a hard wire from FTP CAT6.
however, I have another 200 hours of printing behind me and so far without the slightest problem ..

With regards

Jiri Bergl

RE: Mintemp Error and Breaking Heatbed Thermistor

Good to hear you sorted it!

regards Joan