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