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New build, hot end thermistor reading 800 degrees

Posted: Fri Feb 02, 2018 6:40 am
by IanCaz
I'm building a version of the "Dolly" prusa i3 clone build. I've got my cheap (generic Chinese) hot-end in & when I power it up, it shows it's sitting at 800 at idle (no heat applied).

Obviously it's not, since I'm sitting right next to it.

I'm trying to use the Marlin firmware & I'm going through all of the pre-set thermistors defined in the config file.

Any thoughts on how to configure this thing, since right now it keeps shutting down due to excessive heat.

Thanks,
Ian

Re: New build, hot end thermistor reading 800 degrees

Posted: Fri Feb 02, 2018 7:22 am
by chumm
What hotend did you buy? A store link could help. Most of the V6 clone hotends use the thermistor defined as 13 in Marlin, so your code would look like this:

Code: Select all

#define TEMP_SENSOR_0 13
If you have a working thermistor on the bed, you can swap the plugs on the board to see if you can get an accurate reading of the bed temperature on the hotend thermistor plug on the board, that'll let you know that the board is not the problem. If that works, and if you can't find a Marlin setting that'll work, you can get another of the same working bed thermistor and replace the one on the hotend with that. They can break pretty easily due to the super thin wiring and being bent into the hole, assuming it's the standard V6 clone. The good news is they're usually sold in packs of 10 so you'll have plenty of spares if you have a problem again.

Re: New build, hot end thermistor reading 800 degrees

Posted: Fri Feb 02, 2018 8:03 am
by IanCaz
Thanks for the advice! Swapping did give me good values, so I started looking at the thermistor.

I started digging and found an old ass post on the RepRap forums from uncle_bob that said:
Only when you have a thermistor that reads 100K at room should you proceed with anything else. You will damage things if you try to run with a bad / shorted thermistor.
I hit it with an ohm-meter & sure enough it's shorted, no resistance. Check each lead against the heat block, shorted. I unscrewed it from the heat block & when they screwed it in, they cut through the insulation on *BOTH* of the thermistor wires.

I put a message in to try to get a replacement, but I'll likely try some Kapton tape in the meantime.