Thursday, January 23, 2014

Controlling a chamber heater

My lab is in the basement and it is pretty chilly there, usually about 18C. I have trouble with prints warping, even with PLA. A few weeks ago I borrowed my wife's small space heater and put it in the chamber with the printer, raising the temp to about 40C.  This seems to help the prints a lot by both slightly preheating the incoming filament and also reducing the shrinkage of the plastic during the print.

I gave her back her heater and she got me my own, a 1200 watt ceramic heater for $15 from a local store. The problem is that it had no thermostatic control as hers did, so even on low, it got so hot that it melted some of the parts on my printer (70C - my parts are mostly PLA). I thought about getting another heater with a control, but I decided that if this was going to run unattended, I didn't want to trust the cheap mechanical control that's likely to be on those things.

I had a 25 amp AC solid state relay sitting around and I decided to try using the printer to control the heater. I cracked it open and determined the best place to hook up. You really only want to switch the heat on and off, not the fan, because if you turn the heat and the fan off at once, you're likely to eventually damage the heating element.

I then hooked up a battery to the relay to just hold it on, switched to the highest heat range and watched it run for 30 minutes as a worst case test.  I don't expect to run it on high but I didn't want to put something together that might melt or catch fire if left unattended.

The result was that the relay got a little too hot for my liking, creeping up over 110C and still going (though slowly).  So I opted to put a small heat sink on it.  Since the relay reached 110C with no heat sink at all (the back panel was up against an insulator) I figured a couple of aluminum fins would work nicely.  I cut a bit of aluminum, bent some tabs up and stuck it to the back of the relay with some heat sink compound.

After assembling with the heat sink, a subsequent test only reached 76C which is within my comfort zone.

I then put a thermistor in open air in the chamber and hooked the relay up to the bed control - I don't currently have a heated bed so that's the easiest way to deal with it for now.  Eventually I'll recompile the firmware to control chamber temp on the 3rd MOSFET channel.  If I ever want to mount dual extruders again, I will probably just toss a separate Arduino on it to control it as a standalone.

It still needs some insulation on it because the bottom terminals are live 120v mains, but it works very well in testing.

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