On Fri, 15 Dec 2006 05:35:38 -0000, Jim Land
wrote:

mm wrote in
:

I have a 2 inch Sony Watchman tv that runs on 4 AA batteries. I want
to use it in the car during a trip this spring.

Is it better to buy a "power supply" that plugs into the cigarette
lighter and generates 6 volts, OR

to run it with 4 batteries, measure the amperage, calculate the
effective resistance of the TV, and then make my own power supply that
plugs into the cigarette lighter, that would consist of a plug, a
jack, a wire and a fixed resistor. The resistance of the resistor
would be the same or nearly the same as the calculated different of
the tv.
....

Sounds like you have a good grasp of using a resistor to drop the
voltage, and the fact that if the car's battery voltage varies, so will
the voltage dropped by the resistor.

Better way: Use a voltage regulator IC to put out a constant 6 or 6.3
volts, even if the battery voltage varies (down to 10 or up to 15).

This sounds great but complicated. But something I would like to do.
OK, I looked into it during the intervening period and it's not
complicated. Mouser has the parts for under 3 dollars. Radio Shack
had the voltage regulator for 2.50, but Mouser wants about 90 cents
iirc. Shipping is free because I"m already ordering things.

Or,
easier, buy a power adapter that does essentially the same thing.

So far all the power adapters I've found with google are dedicated to
particular devices, like GPSes etc. I've seen them in the past.

If you are inclined to email me
for some reason, remove NOPSAM :-)