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Strange Thermistor Readings

Posted by PimpKittah 
Strange Thermistor Readings
July 30, 2010 11:33AM
Ive been experimenting with a new type of Hotend piece based on the Alu block nophead uses on his mendel.

Strange things did occur when i tried to messure the temperature from the thermistor (dont have a datasheet for it). Ive pulled the thermistor from a cooking thermometer that supports 350C managed to make a "semi" working table for it.(by trial and error)

Ive hooked the items as following, Thermistor to board and a secondary Thermometer (same type where i took thermistor from) pressed onto the hotend, what i can see its kinda missaligned since the temperatures show 20C higher then it in reality.(not the big problem).

Once it reaches round 200C it behaves odd. temp moves from 198-202 but never higher MOSFET is ON and working. and my external thermometer shows higher values (the actual temperature) up to 225C.

When i shutdown the heater the really strange part occur, the Errorous readings reverse on temperature drop
let say i had 160C showing (in RR software) when it was actually 140C (Control Thermo) these values are actully inverted and i get 140C(RR Software) but it is 160C (Control Thermo).

Also need to note that ive added more entries to the lookuptable for more precision.(or that what i could understand from the code atleast)

I also use a IR thermo to verify the Temp of the Control Thermo

Anyone got any ideas what might seem to cause the trouble ?
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