Hotend thermistors - why 100k ohm?
I am debating making my own hotend, and it seems that most hotends use 100k resistors. Is there any particular reason for this?
I ask because I can get a 10k, 300 degree C thermistor from digikey for almost the same price as the 100k, 200 degree C thermistor.
http://www.digikey.com/scripts/DKSearch/dksus.dll?vendor=0&keywords=B57560G0103F000
You’d have to change out the pull-up resistor and generate a new thermistor table.
There’s no particular reason for 100k ones besides the typically used resistor. You’ll get the best resolution when the pullup resistor is the same value as the NTC at your desired printing temperature. Many suppliers include resistance tables in their datasheets, which make it easy to select the proper pullup.
Use createTemperatureLookup.py to create the thermistor table needed for your firmware.
Thanks guys! I thought maybe that value was chosen to maximize ADC sensitivity at operating temperature or something.
It maximizes ADC sensitivity at operating temperature when used with the standard pull-up resistor. If you change that resistor, it changes the range of maximum sensitivity.
100K resistors reduce the power through the sensing circuit and the potential for self-heating error.
A 10K, B=3947 thermistor at 185C would have a resistance of about 100 ohms. The max sensitivity pullup resistor at 185C would be the same 100ohms, and with a 5V VRef/ADC, the thermistor would then have an out-of-spec 63mW of self-heating.
Nophead’s http://hydraraptor.blogspot.com/2007/10/measuring-temperature-easy-way.html is the main reprap reference for this.
(Nophead has an update at http://hydraraptor.blogspot.com/2012/11/more-accurate-thermistor-tables.html )