Help - I need a design for a 6 volt, 700 mAmp dc power supply

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jcsealock - 28 Apr 2007 13:19 GMT

I want to build a sturdy, portable dc power supply to plug into my MP3
player that has issues with charging its internal battery. It works
just fine with the AC/DC adapter but want my player to be "portable"
again.

Since I'm just now learning basic electronics I would rather build a 6
volt, 700 mAmp power supply powered off of one or more rechargeable
batteries than wait until I know enough to repair the mp3 player (if
ever).

Any pointers/designs anyone wants to share?

Thanks.

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Chris - 28 Apr 2007 14:13 GMT

> I want to build a sturdy, portable dc power supply to plug into my MP3
> player that has issues with charging its internal battery. It works
[quoted text clipped - 9 lines]
>
> Thanks.

Hi, JC. If you can take apart the mp3 player, you should be able to
replace the internal battery. It's very likely that's where your
basic problem lies. But most small mp3 players are meant to be
disposable, so getting it apart and successfully putting it back
together might be really tough.

Given that, a quick fix might be to put five "C" or "D" size Ni-Cad
batteries in series to give you your portable 6VDC (5 X 1.2V ea.).
That should keep you rolling.

Good luck
Chris

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jcsealock - 28 Apr 2007 22:07 GMT

> > I want to build a sturdy, portable dc power supply to plug into my MP3
> > player that has issues with charging its internal battery. It works
[quoted text clipped - 22 lines]
> Good luck
> Chris

Hi, Chris.

Thanks for the tip.

I've acutally been making repairs like that for a while on this unit
and I've had it apart and together a few times. (and it shows). The
battery hasn't been the problem. The problem for this unit is that
the internal works that do the charging aren't doing what they should
and I don't have the know-how to fix that issue - Currently, I'm just
able to find loose or broken connections and resolder them.

If it is a simple as putting some together in series, that's great. I
figured I would also have to control/adjust the amperage. Are amps
constant when batteries are connected in series?

Thanks again.
John

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Chris - 29 Apr 2007 02:18 GMT

> > > I want to build a sturdy, portable dc power supply to plug into my MP3
> > > player that has issues with charging its internal battery. It works
[quoted text clipped - 42 lines]
>
> - Show quoted text -

Hi, John. When you plug in a wall wart, it's made to supply a
voltage. The device you plug in will accept power from that voltage
source, and the current will vary depending on how much power your
device wants. For instance, during the battery charge cycle, current
need will increase, and after the charge cycle is over and it's on
trickle charge, it will be much less. But as long as the current need
of your mp3 player doesn't exceed 700mA (or whatever's listed on the
wall wart), it's fine. If you exceed 700mA of current draw, the wall
wart will get hot and eventually burn out.

Batteries are similar. Five Ni-Cads in series will supply a well-
regulated 6VDC for your mp3 player. "C" or "D" sized batteries are
made to supply 700mA at least. As you increase the current
requirement, all batteries will get warmer and the voltage will
decline a little, but their output voltage will remain relatively
constant until they've been mostly discharged.

Do yourself a favor. With the mp3 player open, plug in the charger
and leave it on for a while. If the battery isn't getting hot, your
idea of putting in an external voltage source to extend the life of
the player will work just fine. If the battery is getting hot,
though, you're going to be wasting most of the power of the Ni-Cads in
warming the mp3 player battery.

Be sure you know the output polarity of the charger (i.e. which side
is +), and make sure to hook up the batteries the same way. Reversing
polarity will let out the mp3 player's magic smoke. ;-)

Cheers
Chris

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ehsjr - 28 Apr 2007 23:34 GMT

> I want to build a sturdy, portable dc power supply to plug into my MP3
> player that has issues with charging its internal battery. It works
[quoted text clipped - 9 lines]
>
> Thanks.

You can buy a 6V sealed lead acid battery like part
CAT# GC-613 from Allelctronics and use that. You'll
need to buy/build a charger for any rechargeable battery
you use. Here's a simple trickle charger schematic
for the battery mentioned above:

-----
+12 ---[10R]---+---Vin|LM317|Vout---+-----> to battery +
| ----- |
| Adj [240R]
| | |
[.1uF] +----------+
| |
| [1075]
| |
Gnd -----------+---------+----------------> to battery -

You can use a 1K and a 75 ohm resistor in series to make
the 1075 ohm resistor. For the 10 ohm power resistor,
you can use 2 20 ohm, 7 watt resistors from Allelectronics
in parallel (their 10 ohm 25 watt resistor is sold out).
The others can be 1/4 watt. You can use any 12 vdc wall
wart capable of 300 mA or more, like CAT# DCTX-1234 from
Allelectronics.

Ed

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Electronics Forum / Basics / April 2007

Help - I need a design for a 6 volt, 700 mAmp dc power supply