My friend has an old MP3 player (one of the first ones ever made; a Diamond Rio PMP300 32MB) that he plugs into his car stereo. The MP3 player runs off a single AA battery. Which is a pain if he ever forgets to turn the MP3 player off when exiting his car, cause the AA dies quickly. So, with a perfectly good 12V source sitting right there, it got me thinking...

I found some tech specs on this particular MP3 player here. Since we know that a AA is 1.5V (on a good day) and since those specs say that the player consumes 100-150mA of current, would it just be as simple as wiring up a cig lighter plug with a resistor (or several resistors) and attaching the leads to the battery contacts? Seems to me it would, but I don't wanna overlook something stupid and fry the player.

[BORING CALCULATIONS]
Here's what I've found/figured so far:

Car Supply Voltage:	12V
Player Required Voltage:	1.5V
Player Current Used:	100-150 mA (depending on mode)

So to find the resistance needed to drop the voltage from 12V to 1.5V at those currents, Ohm's law would give:

R = (12V - 1.5V) / 0.1A = 10.5V / 0.1A = 105 Ohm, for the 100mA current flow, or

R = (12V - 1.5V) / 0.15A = 10.5V / 0.15A = 70 Ohm, for the 150mA current flow.

Since the voltage drop is so big, the resistor wattage rating would have to be pretty high, at the maximum (10.5V * 0.15A) = 1.575 Watts. And the general rule is to pick a wattage 2/3 higher than you need, which would make it about 2.6 Watts. All I've got right now are half-watt resistors. So if I used six of them in parallel, it'd let me handle 3 watts.

So using six resistors in parallel, the resistance of each one would have to be 6x the total resistor value I'd need. Using that 105 Ohm total resistance figure, that'd give 630 Ohms for each, which I'd round up to the next common value of 680 Ohm.
[/BORING CALCULATIONS]


So, would this work? Did I calculate and round up correctly? Would I want to use a regulator circuit to "clean up" the power, or would this be good enough?

Or you could go to the Shack, get a cig lighter voltage adapter:

http://www.radioshack.com/product.as...Fid=273%2D1818

and only mod the end of the cord to replicate a battery.

Easier than to figure it out.

Thanks for the link and the good thinking! But that one supplies a minimum of 4.5V -- I have yet to see one that'll do 1.5V. Even so, for $30, I might just give the resistors a shot.

Soon as I can give some out again, you got rep.

EDIT: P.S. Congrats on the 1000 posts! (Only 999 as of this post but I just wanted to be the first)

Well go here >>>>>

This "buck," or voltage reducing converter is designed to power devices requiring 1.5 volts, including battery powered devices that require one alkaline cell.

12 volt to 1.5, Volt 350 mW DC/DC Converter (Adjustable Regulator)