Hi Simon. I will look into that, although I'm doing something similar. The plan is to switch the dew heater through a mosfet and cycle at long intervals and low power, in laymans terms - HIGH and LOW from an Arduino output. Not sure how effective that will be? Just a matter of getting the current through the element at an appropriate level. Can be switched in response to humidity as well.
It will be effective, but you shouldn't need to manually set the current, just run the heater at 12v, let it draw what it needs(which for most heaters is minimal) and the PWM does the rest.
By setting the PWM duty cycle somewhere between 0 and 100%, you are effectively adjusting the current(averaged over time).
Say for example the heater draws 1A for 2 seconds, then is off for three seconds(40% duty cycle), that is essentially the same as running the heater constantly at 400mA. PWM is much easier(and likely cheaper) to implement than an adjustable constant current source.
Starting bottom left, the 17.6R, 4 x 12R, 4R and corresponding current values, represent each appliance at max load and are not part of the circuit as such. That way, I can get an accurate reading of the total current demand on the power supply.
The dew heater element, 17.6R is switched between HIGH and LOW through a mosfet, at this stage - the Arduino digital pin voltage and setting is represented by the 5v supply and switch.
