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Gen7 thermistor read inconsistency

Posted by phord 
Gen7 thermistor read inconsistency
February 05, 2012 12:12PM
I have calibrated my thermistor tables by reading the R2 values carefully and generating new custom tables for both the extruder and the heated bed. When I turn on the 12v power I can read both thermistors and they both show reasonable and similar values (about 22C). Looks good so far!

Then I turn on the heater for the extruder. Immediately the temperature reading from both thermistors drops about 5C (22%). If I turn on the HBP, the thermistor value drops 2C more for total error around 33%.

I'm guessing there is some voltage-drop problem causing a variance in the Vref, but I thought that shouldn't matter. I'm not sure how to diagnose it. I've tried two different PSUs (one is brand new 600W, bought for this printer). I read a steady 12V across the power output from the PSU on my DMM during these tests.

Extrusion tests of measured values:
180C: I can extrude tiny bit of ABS, and it solidifies almost as soon as it leaves the 0.4mm nozzle.
210C: I can slowly extrude endless ABS
220C: I can extrude ABS by hand at a decent speed, I guess.
260C: I can extrude an easy stream of ABS. I sometimes here a 'pop'.

I think these values sound about right, so I guess it's measuring correctly. But how can I be sure? And why the initial drop?

I'm using Teacup firmware, if that matters.

Is this normal? If not, do you have any ideas where I can look for the source of the problem?
Re: Gen7 thermistor read inconsistency
February 05, 2012 12:43PM
Yes those temperatures seem right. 260C is too hot for most types of ABS.

It looks like on Gen7 the thermistor bias resistors are connected the 5V rail on the PSU, but the micro can be powered from USB. In that case when the 12v rail load increases the 5V rail will likely rise. If the micro is using the USB voltage as its reference it will read a higher voltage and think the temperature is lower.

The thermistor bias resistors should actually be connected the the AVCC pin.

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