Although the M2’s heated build platform works well enough, somebody who knows what he’s doing (you know who you are: thanks!) sent me an improved version. It’s a PCB heater, laid out to compensate for the usual edge cooling, firmly attached to a tempered glass plate with genuine 3M thermally conductive tape:
They designed the heater around the 30 VDC power supply used in their other equipment. Although I had
high moderate hopes that a boost power supply would convert the 24 V supply I already had for the stepper driver bricks into the 30 V for the heater, it was not to be. So there’s a 36 V 9.7 A 350 W supply arcing around the planet that (I think) should work better: adjust the voltage down as far as it’ll go, soak up another few volts in the solid-state relay, and Things Should Be Close Enough to 30 V. One can buy a genuine 30 V supply, but it costs surprisingly more than either 24 V or 36 V supplies on the surplus / eBay market and won’t really provide the proper voltage without upward tweaking anyway.
I replaced their standard 0.156 inch square terminals with Anderson Powerpoles, soldered a length of shielded cable to the 100 kΩ thermistor pads, and gimmicked up a connection to the 24 V supply; it delivered 23.7 V at the PCB terminals. The thermistor is 100 kΩ at 25 °C and 11.4 kΩ at 77 °C. The PCB heater is 5.9 Ω at 25 °C and 7.3 Ω at 77 °C; it dissipates 77 W at 77 °C (no, that’s not a typo).
The ultimate temperature looks to be about 90 °C with a 24 V supply, which isn’t quite enough for ABS (which I’m not using in the M2 right now, but probably will eventually). The time constant, assuming the 1-e-1 point is 66 °C, works out to about 9 minutes; it’ll be up to final temperature in half an hour. Those numbers aren’t quite as accurate as one might wish, because the heater power drops as the temperature rises and the copper resistance increases.
A 30 V supply would dissipate 120 W at 77 °C and rumor has it that the ultimate temperature is around 125 °C, which would be fine for ABS. Goosing the power a bit would produce more heat, but I’v been running the Thing-O-Matic at 110 °C and that’s good enough. More power, of course, gets it to the temperature setpoint faster, which is probably a Very Good Thing.
Obviously, you need PWM to control the temperature; given a 9 minute time constant, a bang-bang controller will work perfectly well.
The original data, including the thermistor resistance after I got my act together, plus a cute little temperature-vs-time graph:
The colored flyspecks are part of the paper; I salvaged a stack of fancy menu cards from a trash can and padded them up as geek scratch paper.