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Battery pack - regulating the current?

Discussion in 'Electronic Basics' started by James Martin, Dec 12, 2003.

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  1. James Martin

    James Martin Guest

    Firstly I haven't done any electronics in a very long time - so please
    bear with me.

    I have an old portable DVD player that I can no longer find spare
    batteries for in the UK, they were specialist just to that player. So
    I thought I'd construct an external battery pack for it that I could
    plug into the mains socket.

    The mains adaptor outputs 9 volt 2 amp. So I got a pair of 3LR12's in
    series which gets me the 9 volts. But how do I regulate the current?
    Presuming that the player needs those 2 amps then it would seem that I
    need to put a 4.5 ohm resistor in series. Is this correct?

    Thanks for any help.

    James.
     
  2. ling

    ling Guest

    I don't think you need that resistor in series to create a regulated 2amp.
    What the DVD set needs is a regulated 9V supply which can provide up to
    2amp.
    So just make sure that the 9V can do that and if you want to be doubly safe,
    connect a 2.5amp rating
    fuse in series.

    hope this helps...
     
  3. John Fields

    John Fields Guest

     
  4. James Martin

    James Martin Guest

    Thanks for the replies.

    The 3LR12 battery is also known as an MN1203. Here is a description:

    http://www.cellpacksolutions.com/Search_Data_Sheet.asp?ID=MN1203

    Its a very long life battery which I was hoping to use to keep my dvd
    player running on a long flight over the Christmas holidays.

    Acording to a meter the voltage from my battery pair is correct but
    when I tried running the dvd player off it the picture was very
    unstable and occasional the screen went solid white (which was a bit
    alarming!) When I tried charging the dvd players from my battery pair
    I got a "charge error". So something is out. I was presuming that
    the player was drawing more current than it needed - but from what you
    say it sounds I was wrong.

    If you have any suggestions about what I might be doing wrong they
    would be gratefully received.

    Lee
     
  5. What exactly do you mean by 'according to my meter'? Did you measure the
    voltage when the DVD player was connected and in operation or just a no-load
    voltage? If it was no-load, try again with the player on. If the voltage is
    approx. 7.5V or less, you've only overloaded the batteriy pack (it can't
    supply 2A). If this is the case you will need 4 instead of 2 batteries (2
    series strings of 2 batteries connected in parallel) or (better) an
    appropriate rechargeable battery pack. Note that rechargeables have a much
    smaller internal resistance and thus provide the device with a more constant
    voltage.

    Dimitrij
     
  6. James Martin

    James Martin Guest

    What exactly do you mean by 'according to my meter'? Did you measure the
    Thanks for the helpful suggestions.

    The measurement was at load. I tried out the 2nd pack I made at the
    same time with identical batteries and connectors. It was producing a
    higher voltage 9.5V I thought I'd give it a try and the player was
    much happier. The picture was stable and everything appears to work
    fine except that the players battery refuses to charge from it. I
    then tried the meter on the mains adaptor and found that was
    generating 9.7V with the player on.

    I didn't know that rechargeables had a smaller internal resistance.
    Thats very useful information. I'd much rather use a rechargable
    battery pack. There is a pack for dvd use on sales in the US, Amazon
    has it but refuses to ship it to the UK. I might have a hunt in the
    shops while I'm over there.

    Thanks

    James
     
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