After fixing the failed extruder thermistor on my Monoprice Select Mini 3D Printer, it didn’t surprise me at all when the heated bed thermistor also failed. Read on to see how I replaced it.

This is the bottom side of the heated platform. Wire leads from the controller are soldered to it at the rear, with some Kapton tape insulation. The thermistor is a surface mounted component at the center of the platform.

The heated bed thermistor is a surface-mount component in an 0805 package. I suspect that this part failed from mechanical stress, since the bed can flex quite a bit while removing strongly-adhered parts. This kind of component package isn’t suitable for applications where the circuit board will flex a lot.

These are some 100k ohm glass-bead thermistors that I originally purchased for the extruder. Their temperature curve isn’t quite right, so I ended up using a different thermistor for the extruder. The temperature curve difference won’t cause as much error on the bed as it would in the much hotter extruder.

I bent the thermistor leads to allow them to flex without breaking or applying a lot of stress to the glass bead seals. Then I added some Teflon sleeving to insulate them.

I carefully soldered the trimmed ends of the thermistor leads to the original component’s circuit board pads. I placed the thermistor bead over the thermistor traces instead of over the heating traces, to avoid measuring a local hot spot.

I put a tiny dab of thermally-conductive grease between the thermistor bead and the bed, and then I encapsulated the bead with Dow-Corning 748 non-corrosive silicone RTV sealant.

The wire lead connections to the bed could use some better strain relief. They will experience stress with Y axis movement.

I removed the original Kapton tape insulation, and replaced it with a small piece of Kapton tape to mask off the solder joints. If I ever need to get back in there to fix the solder joints, I don’t want the joints contaminated with sealant.

The second, much larger piece of tape is temporary. It will hold the wires in place while the new sealant cures.

I encapsulated the wire lead connections with Dow-Corning 748 non-corrosive silicone RTV sealant to provide electrical insulation and strain relief.

I used this non-corrosive silicone sealant to encapsulate the thermistor and the wire lead connections to the heated platform. Unlike common silicone sealants, it doesn’t release lots of acetic acid while curing.