extruder setup and thermistor problem

The setup page on the PIC section isn't as detailed as it probably should be.
Firstly it isn't updated to the UC board 1.2.1 which has been out forever.
The tip110 direction is only hinted at in the photo and is backward from what you would expect for pin 1 (the square contact on the board).
There is also no clear definition on wiring the fan or the dc motor. I'm guessing I should just follow the photo and wire it to where the stepper is supposed to go.
ok, enough of that.


My main problem is how to figure out my thermistor capacitor codes.
I got a thermistor from mouser rated up to 300C because I knew I'd end up using HDPE. The data sheet gives me a spreadsheet of R to temp ratios. (25C=10k, 100C=R946.6)
Pluging that into the thermistor beta calculator (http://reprap.org/bin/view/Main/MeasuringThermistorBeta) I get a beta reading of 3497 and a Rz of 29256 then plug that into the capacitor calculator (http://reprap.org/bin/view/Main/ExtruderIO) and get a required capacitor of 0.025uf which looks kinda accurate and I guess that would be a 253 marking on a ceramic disk cap.

Anyone who has actually done this and gotten it to work accurately, please let me know if I have the right numbers and have done it right.

thanks.

I just did some resistor tests with my thermistor and the chart in the pdf and the numbers I mentioned seem to be fairly accurate. I used boiling water (obviously 100C) my fridge and freezer and it also seems to pick up the difference in tempfrom one room to the next (my computer room is always hotter than the others.

If the numbers and the calculator on the wiki is accurate then I guess it should work with that cap and I suppose I have to set the number in the pref file.

The PIC can't measure above about 170C with a 10K thermistor, because the resistance becomes too low for the PIC output current to be able to drive it. You need a 100K thermistor for HDPE.

Yes a cap marked 253 would be 25000pF = 2.5nF = 0.025uF, but you are more likely to find preferred values of 2.7nF or 2.2nF.

I think the best cap value for accuracy is about 10 times bigger.

---

[www.hydraraptor.blogspot.com]

I guess that makes sense on the resistance for high temps. Is the arduino more capable of that kind of sensitivity? (found one on mouser [www.mouser.com]) what are you using?
A bigger cap for more accuracy? ok.
I guess I could go maybe for a 0.1uf or a 0.33 uf I have a bunch of those laying around. Then just change the cap setup in the prefs file until I get the actual air temperature.
thanks

The Arduino uses an ADC to measure temperature so will be more accurate and doesn't run into the problem with drive current. However, a 100K is probably better with the Arduino as well because you get less self heating which can introduce a few degrees error in the mid temp range.

More details here: - [hydraraptor.blogspot.com]

I actually use a 10K thermistor with a micro with a more sensitive ADC so self heating is much less of a problem.

The mouser one you linked to is only rated for 300F not 300C. I would use the 100K from Epcos in the same series as the 10K mentioned in the blog article.

---

[www.hydraraptor.blogspot.com]

I'll probably just get it from the rrrf store. For my current version I am just trying to get a sloppy machine together.

I found the page on your bog with the part number in case zach won't have it when I get that far.

thanks