RepRap: Blog

As always, apologies to whoever first suggested this causing it to sink into my unconscious and then re-emerge months later as my own idea...

Above is how we drive the heater in the polymer extruder. The microcontroller feeds a PWM signal into the darlington, and that puts a biggish current through the heating coil - typically about 1.5 A.

Ages ago Vik suggested using the change in the resistance of the heating coil to measure its own temperature by passing the current through a sensing resistor. But there are two problems with this:

1. That sensing resistor would get hot too, and so not give a stable indication of the current, and
2. The sensing resistor would waste some of the power.
   

Here's a way round that:

You leave the PWM/darlington circuit exactly as it is. But you add a parallel darlington and sense resistor too. Then, every second or so, in the middle of the period when the PWM signal is at ground, you turn on the other darlington at "Temp" for a few microseconds and measure the voltage at "Sense". That way the sensing resistor doesn't have time to get hot, and you don't waste power through it in normal operation.

I've put the value of R3 as 1, but one wants to choose a value that will give about 3V at "Sense" when the heater is cold - probably about 3R3 - because the microcontroller A->D has a full range of 5V. The "Sense" voltage will drop as the heater gets hot and its resistance increases.

Labels: temperature, thermistor, thermocouple

I have made some measurements of temperature at various points around my extruder. These are the temperatures I get with my software set to 200°C :-



They should be fairly representative as my latest build is now pretty much faithful to the latest design.

One point of interest is that the heater, and the inside of the heater barrel, are quite a bit hotter than the nozzle, where the thermistor is controlling the temperature. That begs the question of what the actual temperature of the extruded filament is, as that is the important thing to control.

I seem to be getting similar results with temperatures that are 20°C below the values I was using with my single piece nozzle. That would suggest that the filament exits at pretty much the temperature of the heater barrel rather than the nozzle. Mounting the thermistor on the barrel rather than the acorn nut is probably a better option.

Labels: temperature