One approach would be to measure the current the device normally uses when
connected with the AAA cells and then calculate the value of the resistor
you would need in series if running it from a "12V" supply.
  For example, if it uses 60 mA when connected to 6V and the automotive
voltage will be about 13V, you need a 7V drop across the series resistor.
  So calculate the resistance like this:  R = V/I = 7V / .06A = 117 ohms

  In this example, the power used in the resistor would be: P = I X I X R =
.06 X .06 X 117 = .42 W   So a half watt resistor should be fine.

If you have two identical 6V devices, you could probably just connect the
two in series and run them both from 12V with no problem.

Larry A.

On 9/17/07, Ed Duntz <eduntz@hotmail.com> wrote:
>
> I would like to add some interior lighting to my '85 Westy. I have found
> some battery powered LED lights that would work nicely, but they run on AAA
> batteries at either 4.5 or 6 volts. I don't like the idea of changing
> batteries and would like to run them off the van's battery. I know that I
> could hook them up to a selectable voltage dc power supply plugged into the
> cigarette lighter, but I'd like to find a way to run them off of 12 volts.
>
> Is there an easy way to run 4.5 or 6 volt LEDs on 12 volts? Some sort of
> resistor inline? These lights are inexpensive as opposed to ones I've seen
> online designed for 12 volts which are three to four times the cost.
>
> What would happen if I ran them on 12 volts instead of 4.5 or 6?
>
> Can anyone point me to a good web site which explains these questions?
> I've found some LED sites, but none that answer my questions.
>
> Thanks,
> Ed in CT
>