PT100 daughter board reading 2000C on start up

mrk

Sorry. Long post alert!

The PT100 daughter board is reading 2000C at start up.
First i thought the problem was the molex connectors on the E3d-pt100.
Soldered on new leads and heat shrunk them, and still reading 2000C.

Then i thought there was too much resistance in the leads or maybe interference on start up... or the jumpers on the daughter board itself.
Removed the jumpers and soldered a CAT7 cable for a shielded 4-wire connection all the way up to the "umbilical cord" for the hot ends.
The "umbilical cord" is a 20cm 2-wire unshielded run to the sensors. Fans, heaters, steppers and a 12V LED supply also runs in the same cable loom. All of witch is unshielded.

Still reading 2000C.
Also measured all my grounding and double checked that none of the heaters or PT100's had a short to ground somehow. Everything checks out like it should be.
Both the PT100 reads about 110ohm at the terminal for the daughter board in room temperature.
Unsoldered the PT100 and checked with the included 100ohm resistor at the very end of the whole cable run. The values was -0.3C and -0.4C

The only thing that works is disconnecting the PT100 after start up, and then reconnecting them. Then it all works just fine.

So now I'm out of ides for things to test or improve.

I'm in much need of some expert help.

Im exited and im looking forward for your reply.

Veti

did you try the 2nd channel as well?

whats your M305 line?

mrk

Yes. This is on both channels. It’s a dual extruder printer.

dc42

My guess is that a startup transient is causing the MAX31865 chips to go into error mode. Have you tried grounding the hot end metalwork to the PSU negative output?

mrk

@veti
M305 P1 X200
M305 P2 X201

mrk

@dc42
Just tested it now.
Heater 1 went on full. But I could se the correct and rising temperature in the web interface. So the start up problem was gone, but the new problem was way worse.

After this test I measured 24v between the chassis and Hot end metalwork.

Took the hot end apart and found that the ceramic insulation on both the PT100s had deteriorated so much that they made somewhat intermittent connection to the chassis of the sensor itself.