wierd thermistor

hi i have a strange problem with my thermistor... when the thermistor approaches the target temperature and is 5 - 3 degrees close to it, it suddenly jumps its value by 10 to 20 degrees... the value that it displays then is false and because of that the heater stops heating to achieve the target temperature.... this makes the whole temperature graph oscillate because it never achieves the target temperature.

i am using a ramps 1.3 like board...

If it only does it at one particular temperature, it's most likely a discontinuity in the firmwares thermister table, which thermister, which Firmware and which table are you using.
I know there is an issue with Marlins epcos 100K table, but it's at about 50 degrees, so it's likely not the one your seeing.

Polygon, would you be so kind as to point me to (any) links that outline the issue? I wasn't aware that there was a flaw in the Epcos table (the thermistors I'm using are Epcos 100K, beta=4066). Best bet to regenerate, or can it be fixed? It's probably not an issue since 50C isn't a "working temp, but I'm a bit fussy about bad spots...

The Discontinuity is from my own observation.
The picture shows what I see as my heated bed warms up, I think it's unlikely the thermister resistance shows a commensurate jump.



Edited 1 time(s). Last edit at 10/13/2012 02:27PM by Polygonhell.

Ah...understood. thank you for the feedback and graph. I'll keep an eye open.

i have solved it for now ... i think it was because of the thermistor pcb track running too close to the mosfet... changed that and now the problem is gone