Sunday, 19 August 2007


I made a start on my extruder controller, on breadboard, as it is a bit experimental. As you can see it's is a strange mixture of surface mount and though hole technologies!

The little PCB on the far right is a 3.3V regulator which I hacked out of a scrap PCB complete with all its decoupling caps. It had three handy vias for the three connections I had to make. The rest of the parts are the heater controller. The circuit could not be simpler. The heater is switched with a BTS134 protected MOSFET. Even with only a 3.3V gate drive its on resistance is so low it does not even get warm when switching about 1.5A.

The thermistor is just wired to a potential divider which gives 0.6V with an impedance of 100Ω. That makes a voltage that varies almost linearly with temperatures between 20°C and 200°C that can go straight into an analogue channel on the MSP430F2013. The micro can also measure its own supply voltage so that can be used to null out the supply tolerance.

Here is a graph of voltage on the input, inverted as it is an NTC thermistor, so it is roughly a graph of temperature :-

The heater was driven with a fixed 50% PWM drive. The temperature rises exponentially until it reaches equilibrium after about 12 minutes. I then turned on the motor and let it extrude some plastic. You can see the temperature drops significantly and rises again when I stopped the motor. This is because the hot plastic leaving the extruder carries heat away with it. Because plastic has a very high specific heat capacity this effect is significant. Finally the temperature falls exponentially when the heater is switched off.

The graph shows why closed loop control is necessary. The rise will be much faster because full power will be applied until the target temperature is met. That will reduce the warm up time considerably. I also hope to reduce the sag that happens when extruding which will make the extruded filament more consistent.

So a little bit of software now to close the loop.

1 comment:

  1. Its not just sensible heat that's been carried away by the extruded plastic, but also latent heat.

    That is, the energy required to break the secondary bonds with the polymer is provided by the heated extruder head. This energy (heat) is then released when the plastic re-solidifies after extrusion.

    This is not really important here, but an interesting side note none the less.