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CAP sizing

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

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  1. ltj

    ltj Guest

    I have a 3.3V module that draws 60mA when idle and jumps to 1A about every
    second. I had a 10uf tantalum on the output of the LM1086 regulator but
    that doesn't seem to be enough. How do I figure what caps to put on the IN
    and OUT of the regulator?
  2. John Fields

    John Fields Guest

    You need to know what the input voltage to the LM1086 is when you put a
    60 mA load on it and what happens to the input voltage when you change
    the load to 1A. You also need to know how often (with a little more
    precision than "about") the load changes from 60mA to 1A and how long
    the 1A load lasts before it goes back to 60mA. You also need know how
    the input to the regulator is being generated. That is, do you have a
    transformer connected to the mains driving a full wave rectifier, or
  3. ltj

    ltj Guest

    The 1A loading occurs every 2.2 seconds (or longer). Best I can tell it is
    actually a series of ten pulses, 3mS each, lasting a total of .3 seconds.

    I need to run this from a battery pack. Initial attempts with 4xAA alkaline
    dropped to near zero volts when pushed to 1 amp. I know alkalines are the
    worst for high draw circuits so I'll try a NiMH pack when I get home. I'm
    not concerned with battery life since this is a portable test instrument
    that will only be used occasionally for an hour at a time. I'd like to have
    caps provide this current but will go to C or D cells ($$$ in NiMH) if that
    is the only solution.

  4. Nick Hull

    Nick Hull Guest

    How about putting 2 Alk "D" cells to power the 3.3v module directly and
    use a very simple charger circuit to keep the D cells charged? I did
    something similiar with an electric clock that would only work on a
    really fresh cell; I kept the cell charged so it worked for years
    without discharging.

    If you want to go the cap route, I use a similiar circuit to run
    electric fencers. They take a big current surge every second. I
    usually use a big (about 2000 mf) cap to provide the surge. A quick
    test to see if it is big enough is to disconnect the power source (in
    my case a junk 12 v auto battery) and see how many pulses I get out of
    the stored energy in the cap. If I can get about 3 pulses then I figure
    the cap is big enough.
  5. ltj

    ltj Guest

    NiMH did not help. I scoped a voltage drop down to 3.0V (not 0V) when the
    surge occurs.
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