Thermistor voltage divider resistor question

Hi all

I'm in the process of building a Prusa i2 using my CNC router as a repstrap. The router works well but is really slow compared to a dedicated printer as I've got a maximum speed of 1500mm/min (much more powerful though of course). It is taking 9 hours to print the x-end-motor!

To get the extruder working I built a little pic based controller for the hotend temperature and a heated bed.

I used a 390ohm resistor for the hot end and 1K for the bed. These give about 5 ADC counts /degree in the relevant temperature ranges and with simple on/off control I still get control within about +/-3 degrees.

I don't understand why RAMPS uses such a high resistor for the voltage divider (4.7K). This significantly reduces the measurement accuracy compared to my values, particularly for the extruder.

I'm going to be using sprinter firmware and understand that I will need to change the thermistor tables if I change the resistor.

Can anyone suggest why the 4.7K value was chosen and what if any downside there is to going to a lower value which will up the accuracy?

Thanks

Peter

Edited 1 time(s). Last edit at 05/20/2013 09:57AM by matherp.

4K7 is the resistance of a 100K thermistor at the mid temperature range. It gives the most linear response curve across the whole range. If you make it smaller you get better resolution at the top but worse at the bottom. If you make it small, 390R then the thermistor will start heating itself so may give a false reading.

The resolution in Marlin is increased by oversampling giving four more bits, so 4K7 works fine and the temperature swings by only fractions of a degree with PID.

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[www.hydraraptor.blogspot.com]

Nophead

Thanks for the input.

I certainly accept the argument about self heating and on that basis 1K is probably as small as I would want to go for the extruder and still gives considerably more resolution than 4K7. I'm using a 10K thermistor for the bed so I can use 1K for that as well.

The 4K7 seems to be a compromise to allow the use of the same resistor for both bed and extruder which doesn't really seem best practice so I'll continue to have a play with the lower values

Thanks

Peter

JUST FOUND: Marlin already has support for 1K resistor with 100K epcos - perfect:-)

Edited 1 time(s). Last edit at 05/20/2013 02:49PM by matherp.

Yes but what problem are you trying to solve by changing the resistors. As I said it works perfectly well with 4K7. Temperature agrees with a thermocouple within a degree or two and is stable to within a fraction of a degree.

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[www.hydraraptor.blogspot.com]