Heatbed cable cover burnt the cable  

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Pixel_Joel
(@pixel_joel)
New Member

Hello!


I'm an owner of one of your Prusa i3 MK3 printers and I'm having quite a problem with my heatbed.


Melted ABS Heatped-cable-cover

I noticed that the heatbed-cable-cover ABS plastic had started to warp a bit. Until tearing it apart, I noticed that most of the plastic had been melting on top of the cables which also made the tearing apart part quite demanding.


Burnt cable. Led has fallen off from the slot 

After that I noticed that the ABS had made quite a damage on the led wiring in the headbed (causing the led to just fall off) and the copper cable's cover was quite melted in the process also.

So it seems that the plastic cover has messed up the wiring of the heatbed pretty bad. What can I do about this to get it back to working? It seems quite hazardous right now.


- Regards, Joel

 

Posted : 18/12/2019 1:54 pm
Neophyl
(@neophyl)
Noble Member

I think you have the cause reversed, its the wires/connector heating up that has caused the cover to soften.  If the resistance between the wire and the connectors increases (due to corrosion/bad crimp/ a n other reason) then that area will produce heat.  Its why fitting the crimped wires to the Einsy and the bed etc is so so important.

Looking at that Id say that the problem spot is where the wire goes into the crimped connectors as thats where the wire insulation is most burnt.

Posted : 18/12/2019 2:32 pm
Pixel_Joel
(@pixel_joel)
New Member

You might be right. Shame I didn't get to save it in time, probably have to buy a new heatbed because of the burnt connectors and the broken led. (Don't think I can solder that back together as it's connectors had broken off.) I think that's the safest bet.

Posted : 18/12/2019 3:13 pm
Karl Herbert
(@karl-herbert)
Famed Member

If you haven't progressed yet:

Check the resistance of the heating bed - should have an internal resistance of 4.8 ohms (24V, 5A, 120W variant). The small thermistor could also be defective (100 KiloOhm).

Statt zu klagen, dass wir nicht alles haben, was wir wollen, sollten wir lieber dankbar sein, dass wir nicht alles bekommen, was wir verdienen....
Posted : 18/12/2019 4:03 pm
Robert-mm200
(@robert-rmm200)
Noble Member

Check with support. The picture does not show any damage to the heat bed, but they will at least replace the cable under warranty. Broken led might get you a new heat bed - but not clear how it broke.

The cable is clearly a defective crimp. It happens. If you want to replace the LED - a little surface mount unit like that is easy to fix.

Posted : 18/12/2019 4:41 pm
joan.t
(@joantabb)
Veteran Member Moderator

Personally , 

I have had the cable break due to flexing, on two of my printers. the Prusa Heatbed, power wires are better than basic seven strand 16AWG wire, if memory serves me well, there are 21 conductors in each wire. BUT they are not  as good as the Flexible wire used for things like radio control models... which IIRC has 252 conductors in  each wire. making it much more flexible! 
I  would  dump the original cable replace with  16AWG silicone insulated Flexible wire   (slightly longer than the original wires), I would crimp the ends and then I would clean up the contact on the heatbed. to remove the oxides and make the copper bright again. i would replace the old wires with the new wires and make sure the cable strain relief was correctly installed, then I would print the new covers for the connection and fit them to complete the strain relief.   I have done this on both printers that failed, and did the same modification on two more prusa's to prevent this happening in the first place... 

as the wire fails, through fatigue, there become fewer strands to carry the required current,  so the remaining conductors get hotter...   this is probably what happened, the other failure modes are 

insecure connector screw, this can cause heating between the connector and the board...due to the higher resistance of the loose joint
Poor Crimp, this can cause heating between the connector and the wire ...   
there should be a resister and an LED in series between the + and - terminals on the Heatbed. if these have come off, you coud consider replacing them, Regards Joan

 

 

 

I try to make safe suggestions,You should understand the context and ensure you are happy that they are safe before attempting to apply my suggestions, what you do, is YOUR responsibility. Location Halifax UK...
Posted : 18/12/2019 5:18 pm
joan.t
(@joantabb)
Veteran Member Moderator

Personally , 

I have had the cable break due to flexing, on two of my printers. the Prusa Heatbed, power wires are better than basic seven strand 16AWG wire, if memory serves me well, there are 21 conductors in each wire. BUT they are not  as good as the Flexible wire used for things like radio control models... which IIRC has 252 conductors in  each wire. making it much more flexible! 
I  would  dump the original cable replace with  16AWG silicone insulated Flexible wire   (slightly longer than the original wires), I would crimp the ends and then I would clean up the contact on the heatbed. to remove the oxides and make the copper bright again. i would replace the old wires with the new wires and make sure the cable strain relief was correctly installed, then I would print the new covers for the connection and fit them to complete the strain relief.   I have done this on both printers that failed, and did the same modification on two more prusa's to prevent this happening in the first place... 

as the wire fails, through fatigue, there become fewer strands to carry the required current,  so the remaining conductors get hotter...   this is probably what happened, the other failure modes are 

insecure connector screw, this can cause heating between the connector and the board...due to the higher resistance of the loose joint
Poor Crimp, this can cause heating between the connector and the wire ...   
there should be a resister and an LED in series between the + and - terminals on the Heatbed. if these have come off, you could consider replacing them, Regards Joan

 

 

 

I try to make safe suggestions,You should understand the context and ensure you are happy that they are safe before attempting to apply my suggestions, what you do, is YOUR responsibility. Location Halifax UK...
Posted : 18/12/2019 5:19 pm
islander2013
(@islander2013)
Active Member

@joantabb

could you state the correct terminals needed to crimp please? i cant find the exact ones needed for the heat bad. i found this wire on amazon which looks good. thank you

16 Gauge Silicone Electric Wire

Posted : 24/09/2020 6:06 pm
Karl Herbert
(@karl-herbert)
Famed Member

@islander2013

These crimp contacts should be suitable: https://www.ebay.com/itm/Prusa-i3-MK2s-MK2-5-MK3-MK3s-Heat-bed-wire-crimp-on-connector-/324122789024

Statt zu klagen, dass wir nicht alles haben, was wir wollen, sollten wir lieber dankbar sein, dass wir nicht alles bekommen, was wir verdienen....
Posted : 24/09/2020 10:03 pm
islander2013
(@islander2013)
Active Member

@karl-herbert

Thanks for that, which would I buy for the fork terminals? Also would that 16awg cable I linked above on Amazon be ok? Thank you again.

This post was modified 4 weeks ago by islander2013
Posted : 25/09/2020 12:53 am
Karl Herbert
(@karl-herbert)
Famed Member

@islander2013

16 AWG is sufficient for up to max.. 15 amperes and ok for the supply of the heating bed, but it also does no harm if you use 14 AWG: https://www.amazon.com/gp/product/B074N89QMD/

Terminals for the Einsy Rambo should fit this one: https://www.amazon.com/gp/product/B005GDG7US/ and a crimptool like this one: https://www.amazon.com/gp/product/B00OVF2AKI/

 

 

 
 
 
This post was modified 4 weeks ago 2 times by Karl Herbert
Statt zu klagen, dass wir nicht alles haben, was wir wollen, sollten wir lieber dankbar sein, dass wir nicht alles bekommen, was wir verdienen....
Posted : 25/09/2020 2:00 am
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