Leo Laporte on Twit.tv was recently talking about a new internet connected talking Barbie with the infamous “Math is Hard!” scandal.
Well.. math is not hard and this is simple arithmetic.

The first nice weekend to come along and the allsky1 camera system gets a reno.
Currently the heater (running on a timer), is 33 ohms at 24 vdc creating approx 18 watts of heat for a dome that is 14″ in diameter.

The new dome is 4″ in diameter, having a lot less air to heat up. So we want to reduce the amount of heat generated.

The volume of a sphere is
V=(4/3)πr3 where the radius is 7″ and 2″.
7″ gives 1400 in^3 but this is only 1/2 a sphere, so 700 in^3
2″ gives 33 in^3/2 or 16 in^3, a reduction of 700/16 =43 times the heated air volume.

So we should be able to pull down the heat wattage from 18 down to next to nothing, ie like 1 or 2.
But then again there are other factors, like the thermal mass of the dome itself plus the camera body (which has a cooled camera, so we do not want to heat that up if possible.

Let’s assume we use only one 20 ohm resistor and give it a variable voltage supply.. one of those radio shack 3, 4.5, 6, 7.5, 9, 10.5 and 12vdc wall adapters.
at 12 volts:
V=ir so i=v/r=12/20 = 0.6 amps @ 12vdc where w=vi=12*.6=7.2 watts
at 3 volts:
i=v/r=3/20=0.15 amps@3vdc w=vi=3*.15=0.45 watts
This gives us a good range
3.0v = 0.5w
4.5v = 1.0w
6.0v = 1.8w
7.5v = 2.8w
9.0v = 4.0w
10.5v = 5.5w
12.0v = 7.2w