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I am in process of building a solid aluminum bed, for higher temps and more controlled surface temperature, I will supply power from an external supply, to handle the current. It will have two sets of the standard heaters (8) per set and two thermistors. Does anybody have a procedure to connect the second heater loop in the rambo/Ardino? I,m not even sure it can be done.
Draw-Tech

I think you could control it with a SSR
Why two thermistors?

Maybe for the edge vs the center?

If you do not use dual extrusion you could use the second thermistor slot for.

But why use heating elements?

Get a 110 or 220v silicone heating pad and stick it under the aluminium bit. Some even have a thermistor built in.




Random example
http://m.aliexpress.com/item-desc/32424688342.html

This one is 59 cm in diameter. Has a thermistor and is 1500w operated directly from the mains. An ac ssr could drive it easilt.

I assume he is trying to have dual heating zones, I believe E3D have been doing this with the big box printer. It eats you have more consistent temperature across the bed, especially if the bed is large.
I don't know of any firmware out of the box that supports this, it probably wouldn't be very hard to add.
As I mentioned I believe E3D have been doing something similar, it might be worth pinging them and see what they are doing for firmware.

My new build uses one of the 6lb slabs of Mic6 aluminum from Sands Machine and a trio of 175W silicone heater pads. I've got some PID tuning to do (which is a 100% manual process on the DUET board), but after that I'll have a nice thing going.

[img]http://i.imgur.com/nDCE6Xk.jpg?1[/img]

[img]http://i.imgur.com/o44aqan.jpg[/img]

Nice Jim.
To bad it takes a couple 6 packs of cold ones to cool it down in a reasonable amount of time.