Thermistor resistance and "def"

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Iamarobot
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Thermistor resistance and "def"

Post by Iamarobot » Wed Mar 15, 2017 6:38 pm

I've just built up a new V3 kit, and am getting a reading of -50 on the hot end followed quickly by the dreaded "def". I've checked the resistance of the thermistor and it's spot on at 100k Ohms at room temp from the hot end up to the Rambo now that I fixed a poor connection in the supplied wiring and some other issues I was responsible for...

My question is: If the thermistor is reading the correct resistance, can it still be damaged? Or, if I'm reading the correct resistance at the Rambo, could the wiring still be suspect? Obviously there's an issue, but I'd like to be able to pinpoint it as either the wiring (which I think is ok at this point) or the thermistor.

Since I'm pretty sure about the wiring, my next step is to replace the thermistor, but I'd rather not if it's ok and it's still a wiring issue.

Thanks to anyone who can clear this up for me!

Polygonhell
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Re: Thermistor resistance and "def"

Post by Polygonhell » Wed Mar 15, 2017 7:01 pm

If the resistance is correct, the Rambo should show a reasonable temperature about 20C for 100KOhms.
You are I assume measuring the resistance with the thermistor disconnected from the board?
If you plug the hotend thermistor into the Bed connection does it read reasonably? they are identical parts.
Have you checked the resistance of the bed thermistor? it can cause def too

It is possible to damage the thermistor inputs on a RAMBO, but it is unusual to do so, so until you're certain you've eliminated the possibility I'd assume it's bad wiring.

Iamarobot
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Re: Thermistor resistance and "def"

Post by Iamarobot » Thu Mar 16, 2017 1:10 am

Thanks for the speedy reply!

I was measuring the resistance at hot-end thermistor leads where they are soldered into the hot end pcb, and then I measured at the receptacle for the 8-pin connector, and at the 2-pin plugs that go into the Rambo. The resistance was ~100k Ohms at each spot. When I started up the machine I would see -50 degrees for the nozzle and 25 for the bed, and then both would switch to def.

Now for the weird part - after letting the machine sit and ponder its sins overnight, as you recommended I just swapped the connectors for the two thermistors in the Rambo and they both read 27.5. I've swapped them back, and they're both still reading just fine. So... Apparently the thermistors and the wiring are fine, but for some reason there was an issue all day yesterday. That makes me think there might be a mediocre connection somewhere in the wiring that fixed itself overnight, and the thermistors are a-ok.

It would be very convenient if one could measure the resistance of the thermistor leads at the hot-end pcb, as it doesn't require removing the thermistor. In my case, it read correctly at that point, so I'm following up my original question with wondering if the resistance is going to be the same at those leads or if it could vary which would make that test of the thermistor unreliable.

I'm going to calibrate the machine and see what happens...

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