Here is the reply I got back from Robo:
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Yes, the heaters have a set resistance (which will change very slightly with temperature). I don't remember exactly what it is on the ones I sent you but you can put an ohm meter across the leads and measure it (~4 ohms maybe?).
No, they do not have to be powered from 40 VDC. That is the max voltage - go over that and they may overheat and burn up. Less voltage will just produce less heat - exactly what you want! You can set a specific heat generation (say 70 watts) by adjusting the voltage (the heaters will pull more and more current as the voltage increases) until you reach a point where measured voltage x measured current = 70 watts.
Say for example you adjust the applied voltage to 10.00 VDC and the heater pulls 5.0 Amps... 10 x 5 = 50 watts. Turn up the voltage a little more and maybe 12 VDC and 6 Amps... 12 x 6 = 72 watts. Hope that helps.
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What I don't understand is what is making the heater want to draw the amperage. If i set the power supply to 12VDC and 6amps what is making it want to use those 6 amps?
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Don't forget you will need to be able to monitor current and voltage with a pretty high res... This I guess gets expensive fast (but you suspected that right?)
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Yeah, I know this and it is something I am really dreading as I havn't yet done any research on this.