Bed Thermistor - failing - MINTEMP BED - known issue?  

RE: Bed Thermistor - failing - MINTEMP BED - known issue?

I put a piece of shrink wrap around the area that bends.  

RE: Bed Thermistor - failing - MINTEMP BED - known issue?

Posted by: @mgtarallo

Hi Guys - just curious how many of you have experienced bad thermistors?

I love Prusa's printers - I have 2 MK3s - both of which have had failed Thermistors at different times. BOTH replaces under warranty. Support has been great. I inquired if they are doing anything about this, being it seems to be a known issue. Even though the wire seems fine, testing with a multi-meter reveals it is not.

I even made a video here to send to Prusa:

https://fb.watch/2Y81Ln0PND/  

After replacing it - ohms read consistent 124 - and I have had 4 jobs complete already.

If you guys have some creative solutions to prevent this, I'd love to know about them

Thanks and stay well.

 

 

The problem is actually not the thermistor itself, rather the wires. I replaced most of the wiring in the moving area with silicone wiring (soldered and insulated with heat shrink). You have to be careful in the area of the heating block, as they are most sensitive there. Metal brushes for cleaning the heating block can have a destructive effect there.

RE: Bed Thermistor - failing - MINTEMP BED - known issue?

Just got an update from Prusa in case anyone is interested:

"Hello Mike,

your new heatbed thermistor should be dispatched today. 

The thermistor cable is quite fragile and easily damaged. It must be handled with care. We are familiar with this problem. We are currently collecting information regarding damaged thermistors and then it will be evaluated whether to change the cables or not.

S pozdravem/Best regards,
Viktor Olijnyk
Customer Support"

So they are familiar and aware of the issue of the cable / wire constantly breaking - and they do replace them under warranty.
FYI

RE: Bed Thermistor - failing - MINTEMP BED - known issue?

@mgtarallo

Thanks for the information and good luck with the replacement product 👍 !

RE: Bed Thermistor - failing - MINTEMP BED - known issue?

The good news is that thermistors are cheap and can readily be replaced with 30-60 minutes of effort.  Quicker if you are good at it.  

I treat thermistors as being expendable and keep 1-2 on hand at all times.  

RE: Bed Thermistor - failing - MINTEMP BED - known issue?

I got 2 months out of my first replacement sensor (compared to 1 from the factory-installed) by trying to keep the bundled cable from coming straight out of the connector.  The short in the first failed sensor appeared to be close to the 3d-printed cover/clamp exit and internally where it contacted the crimps for the heatbed power wires (I still have it, but no good tools to attempt even an amateur failure analysis.)

While I wait for the 2nd replacement, I'm going to forge ahead with my measuring tape, paperclips and stout twist-ties to see if I can maybe come up with a modified cable clamp/cover design with a bit of angled/curved conduit that doesn't get caught in the other wiring or smack against the back of a 17-inch^3 enclosure (and maybe learn Fusion 360 in the process?)

While I'm quite thankful for everyone at Prusa Support for their responsiveness and understanding, my under-developed engineer gene tells me to hack something together to help feel better about myself  😉

RE: Bed Thermistor - failing - MINTEMP BED - known issue?

Posted by: @mr-shaky

While I wait for the 2nd replacement, I'm going to forge ahead with my measuring tape, paperclips and stout twist-ties to see if I can maybe come up with a modified cable clamp/cover design with a bit of angled/curved conduit that doesn't get caught in the other wiring or smack against the back of a 17-inch^3 enclosure (and maybe learn Fusion 360 in the process?)

My crash-course in Fusion 360 resulted in a potentially useful mod that I've printed and installed on my MINI along with the new sensor.  In a nutshell, I lightly modified the covers for the heatbed connections and created an elbow to put over the wire harness and textile wrap as it exits the cover.  The original interlocking design of the covers was left intact, and I included a retaining ring.

I'm also evaluating a relocation of the wire exit-point from the Buddy board box.

The new parts:  https://a360.co/2Ycb4i5

Urban Grey PETG Prusament, installed:  https://photos.app.goo.gl/GhWLe98bBTpcNYxi7

Printing info:  .20mm Quality preset in PrusaSlicer.  Exported each F360 body individually, used the slicer cut tool to split the tube in 2 parts.

Other useful details: 

• - The 10.5mm ID retaining ring requires disconnecting the heatbed power leads from the heatbed.
• - The tube radius measures 5mm and 3.75mm (OD & ID, respectively.)  It extends just over 20mm from the cover, and another 30mm after the 90* bend.
• - A very slight (matching) taper is cut into the tube and replacement covers to replace the textile clamp in the original cover design--the end of textile wrap still needs to be quite close to the crimps of the power leads so there is some positive clamping load.
• - Getting the covers to slide into their locked position in this design is no less "fiddly" than the original.
• - The repositioned heatbed cable does make contact with the hotend/extruder wiring when Z at its lowest, but it doesn't appear to be cause for alarm.

RE: Bed Thermistor - failing - MINTEMP BED - known issue?

I repaired one a year ago and got about a year of it.  Toy wearing through them quickly.  If your printer in an enclosure?

RE: Bed Thermistor - failing - MINTEMP BED - known issue?

@cwbullet

The first failure happened without an enclosure.  The second failure was with an enclosure.

IMHO, it appears that the as-built cable orientation tended to focus its flex at or near the heatbed covers.  I'm testing the theory by making the bend much more rigid and reorienting the cable so it rolls/rotates more than it flexes.

RE: Bed Thermistor - failing - MINTEMP BED - known issue?

@mr-shaky

The number one reason in my opinion (based on the number posted on the forum) is from hitting the cable on the back wall of the enclosure.  Just make sure it is not hitting.  

RE: Bed Thermistor - failing - MINTEMP BED - known issue?

@cwbullet

I should have stated it in my previous response, but I did exactly that and it still failed.

RE: Bed Thermistor - failing - MINTEMP BED - known issue?

This has been a tricky issue.  I keep 3-4 of these thermistors in by parts inventory.  I am currently testing on made BIQU to see if a clone fares better.  

RE: Bed Thermistor - failing - MINTEMP BED - known issue?

@cwbullet

Indeed.  When I installed the second sensor and put the printer in an enclosure, I made sure to (very carefully) put a bend in the cable at the heatbed end and put the barest minimum of the textile wrap in the clamp on top of the buddy box.  Even with a maximum amount of slack in the cable, it failed.

Now that the 3rd sensor is installed with the mod, I'm in the middle of a pair of 30-hour PETG prints that use most of the bed, and Octoprint currently shows a relatively steady 90*C +/- .1*C bed temp.  A failing sensor could show a brief and semi-random dropoff of 5*C or more...

RE: Bed Thermistor - failing - MINTEMP BED - known issue?

@cwbullet

As with the MK3, I replaced all wires with flexible silicone wires and no longer have any problems with breakages or loose contacts. The wires and the silicone insulation is very durable.

RE: Bed Thermistor - failing - MINTEMP BED - known issue?

@karl-herbert

I did the same on one printer - it lasted almost a year and a half and failed.  It goes its worth.  

RE: Bed Thermistor - failing - MINTEMP BED - known issue?

@cwbullet

I have also considered using energy chains, but this may put too much strain on the wiring harness. The textile hose may be the best (better) solution.

RE: Bed Thermistor - failing - MINTEMP BED - known issue?

A short video showing the cable movement in the new test orientation:   https://photos.app.goo.gl/BNMJwSR8smnbsPan9

This was about 12 hours into the second 30-hour print, and I put an old mousepad under the printer between prints.  The print bed tapped the enclosure door when I tested Y range of travel after placing the pad, so I may need to remove up to 1cm of the extension tube...

After some thought, I'm going to reprint the buddy box lid.  I'll put an opening in one vent riser to pass the heatbed power connector and proceed without the cable in a clamp.

RE: Bed Thermistor - failing - MINTEMP BED - known issue?

The long prints completed without major incident, so I set about making a new lid:  https://photos.app.goo.gl/TkJBXPcRgTf3ZSvE8

No huge modifications made, just a quick sketch and intersect/cut to separate the correct part of the vent, then add a small lip to act as a retainer.  It was easier just to print the cap as-is and use side-cutters to remove the support.

I also printed the tube 1cm shorter (to again fit within a 17"/43.18cm cube.)

RE: Bed Thermistor - failing - MINTEMP BED - known issue?

Ho ho.  Mine's just gone after just under 4 weeks of ownership & use.  Of course, prints failed twice this week - both of them 20 hours into a 24-hour print 🙁  (I must lookup if there is a way to restart a print that has failed like this).  Cue a long call to tech support, plus my using a multimeter to diagnose a wiring fault for the second time since I got the thing 6 weeks ago.  A replacement is on its way out to me, and when I next put an order in I'll order a spare for each thermistor, but in the meantime I've splinted the cable (which is partially successful).
Long-term, I'd like to stop it from happening again; How do I download those replacement parts from Fusion360?