# Voltage of In-car charger for MP3 player

Discussion in 'Electronic Basics' started by [email protected], Jan 21, 2006.

1. ### Guest

I have an MP3 player and the mains charger delivers 5V, 2.4A. The
manufacturer's "proper" in-car charger has the following specification:
Input 12V - 24V DC, Output 5V 2.5A . I recently picked up an in-car
charger (cigarette-lighter socket thingy) which can do multiple
voltages ... except 5V of course. It does 4.5V and it does 6V - am I
going to damage anything by trying to charge or run the MP3 player off
either of these voltages? If not, which one should I use? Also, how
do I determine which polarity to switch the charger to?

Many thanks in advance for any help,
Joseph

2. ### Rich WebbGuest

A direct approach to "which charger" would be to measure the actual
current and voltage delivered by the manufacturer's charger to the mp3
player under varying load conditions (idle, in use, flashing lights,
whatever it does). Then make a resistive dummy load that gives the same
current/voltage results with the real charger attached to it and test
the multiple output wall-wart to see which setting is closest in voltage
for the required current draw.

Somewhat more approximately: multiple output wall-warts are generally
not very "stiff." Under light load conditions, the 4.5 V may really be 6
V or more and the 6 V out, closer to 8 or 9 V. Think of the 4.5 V as

Assuming that the multiple-output wart is rated for at least 2.5 A, I'd
start with the 4.5 V setting and see how it goes. Probably (but no
guarantees) that will work. If not, bump it up to 6.

Without having measured the actual performance of the manufacturer's
device and also the wart, either alternate setting does carry a slight
risk, perhaps of not a high enough voltage for the mp3's regulator or of
too high a voltage, forcing it to dump excess as heat and possibly
damaging the player.

As to which polarity, the manufacturer's charger or the mp3 player
should have a diagram like +-Co-- or --Co-+ to indicate what's required.
To be sure, go to your closest Radio Shack and get one of their
autoranging pocket multimeters and measure it. They're pretty cheap and
they do come in handy ...

3. ### Pooh BearGuest

Those cheap chargers often have poor voltage regulation. I wouldn't chance
using one unless I knew *exactly* the likely consequences.

Polarity can be easily determined with a meter btw.

Graham

4. ### Jasen BettsGuest

measure the output of the provided charger. if it's exactly 5v using other
than 5v could cause problems.

5. ### ehsjrGuest

If the in-car charger you have truly puts out 6 volts
at at least 2.5 amps, you can put a 1N5401 diode in
series with it and the MP3. That will deliver ~5.3 volts
under load. You will need to determine the polarity of
things - use your meter to determine which is + and -.

If you have doubts about the in-car charger, forget
the whole thing.

Ed