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Heated Bed Thermistor reading far too high

Posted by scubi 
Heated Bed Thermistor reading far too high
December 02, 2017 07:20AM
Hi
Please could you advise, solution

I have installed a Silicon Heated 300x300 230V AC 300w with a SSR solid state relay. ( 25A ) Specs on silicon bed is 100K NTC 1% thermistor.

In marlin ( 1.1.6 ) I have tried Thermistor no. 1 & 6 & 75 but they all read 90 degrees yet it suppose to be around 28 degrees ( Cold bed ) I plugged in a spare thermistor and reading was correct 28 Degrees ( room temp )

Why is my Thermistor from my silicon heater so high. I did add extension wiring , slightly thicker wiring ??

Please can you help if Marlin is holding it at 90 for some reason or something.

Thanks
Vaughan
Re: Heated Bed Thermistor reading far too high
December 02, 2017 07:29AM
Do you have a mutimeter, if so measure the resistance of the thermistor, should be around 100k ohm at 20c.

What board do you use , Arduino Due ?

Edited 1 time(s). Last edit at 12/02/2017 07:30AM by amigob.

P3steel DXL, with Due/RADDS/Raps128 dual Wade's extruder
Re: Heated Bed Thermistor reading far too high
December 02, 2017 07:35AM
Hi
I have a Multi-meter , using a Ramps 1.4 board . Must check how to measure resistance lol I check now
Re: Heated Bed Thermistor reading far too high
December 02, 2017 07:44AM
Hi
I just tested it and i put the multimeter on 200K and it come out at 6.8 reading ?? Not sure what that means ??
Re: Heated Bed Thermistor reading far too high
December 02, 2017 07:51AM
You measured when it was not connected to the board, and temperature around 20c ?


P3steel DXL, with Due/RADDS/Raps128 dual Wade's extruder
Re: Heated Bed Thermistor reading far too high
December 02, 2017 08:05AM
If it reads 200K not connnected at 20c then you could try thermistor type 2 or 52 in marlin.
I don't use marlin my self, but in the code these type seem to be for 200k thermistors

Edited 1 time(s). Last edit at 12/02/2017 08:06AM by amigob.

P3steel DXL, with Due/RADDS/Raps128 dual Wade's extruder
Re: Heated Bed Thermistor reading far too high
December 02, 2017 08:09AM
Hi
Sorry it reads 6.8 on the Multimeter
Re: Heated Bed Thermistor reading far too high
December 02, 2017 08:19AM
I didn't read careful enough :-)

Mhh, so it is 6.8k at 20c, I will check in the code.


P3steel DXL, with Due/RADDS/Raps128 dual Wade's extruder
Re: Heated Bed Thermistor reading far too high
December 02, 2017 08:22AM
If the surrounding temperature is warmer the 20c, then there is a possibility that it is a 10k thermistor.
That will be a thermistor type 4 in marlin.


P3steel DXL, with Due/RADDS/Raps128 dual Wade's extruder
Re: Heated Bed Thermistor reading far too high
December 02, 2017 09:58AM
Hi
The packaging etc says it a 100K NTC thermistor .

I set the multimeter to 200K because my multimeter does not have a 100K point. My room temp is about 28 degrees . The reading on the multimeter is 6.8 ?
Re: Heated Bed Thermistor reading far too high
December 02, 2017 11:50AM
That is 6.8 Kohm , I can't find any 10k thermistor that is 6.8Kohm at 28c.
6.8Kohm is about 35c.
At 28c they are about 8.5K to 9Kohm.

You can do one thing,, look at the resistance with the multimeter when you heat it up, put the heat on top of a cub of warm water.
The resistance should then decrease.


P3steel DXL, with Due/RADDS/Raps128 dual Wade's extruder
Re: Heated Bed Thermistor reading far too high
December 03, 2017 09:18AM
My suggestion would be to wire in a new thermistor. Thermistors break easily due to the thin wires, eventually you will have to replace it anyway.
Re: Heated Bed Thermistor reading far too high
December 05, 2017 02:39AM
Thanks guys for all the help. I selected the 10K thermistor & its reading the room temperature correct. I run a PID and entered the figures into marlin etc. Seems to be working fine. I would not know if my Temp on the BED is perfectly 90 when I do a print but it seems to be working.

Thanks for all the help
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