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Looking for a power supply/storage system

Discussion in 'Electronic Components' started by [email protected], Jul 5, 2007.

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  1. Guest

    Heya -- does anybody know whether there is such a thing as a
    rechargable 9V power source that can be hooked into a 28V circuit for
    application on a plane?

    The long story is like this: I have a couple pieces of equipment (most
    notably a tricked-out non-standard GPS) on a plane that run off those
    little 9V bricks. The batteries are good for about a week, then they
    need to be exchanged - which is kinda annoying. There's a power-supply
    (run-off-the-mill wall-wart) that I run off a little 110V inverter
    circuit that'll supply power while the plane is actually running, so
    the batteries won't poop out during flight - but that has power only
    while the engines are running (say 5hours or so per day during flying
    season). Now the problem with changing batteries is that the system
    loses configuration when I do it and I have to go set it up when it
    comes back on (just a few minutes, but it's annoying).

    So I figured there must be some kind of gizmo that'll accept either 9V
    regulated or (better) 28V unregulated on the input side when available
    while recharging some kind of battery; and supplies 9VDC on the output
    side whether the input-side is powered or not. It's not a whole lot of
    current (think two or three 9V batteries lasting for a week or so) and
    I'd be happy if it just pooped out after a week of sitting unpowered -
    but I'd like to have the 9V available continuously as long as there's
    daily (or, say, every-other-day) a couple hours worth of input. The
    problem is that I have no idea where I'd start looking for that kind
    of thing. Boating folks typically run off the boat-batteries which are
    available 24/7, not "when the engines are running".

    Anybody have an idea?
  2. ehsjr

    ehsjr Guest

    Here's a simple circuit you can use.

    +28 ---[75R]---+---Vin|LM317|Vout---+
    | ----- |
    | Adj [6.2R]
    | | |
    [.1uF] +----------+
    | |
    | [D1] IN4001
    | |
    | +----->|--------> ~8.9V to GPS/etc
    | | D2 1N4001
    | [9.6V NiCd]
    | |
    Gnd -----------+---------+

    This will give you a constant current ~200 mA charge while
    +28V is available on the input. The LM317 will dissipate
    ~ 1 watt, so put it on a small heatsink. Use 8 2100 mAh
    NiMh cells to make the 9.6V battery pack. A typical 9V
    alkaline battery is about 625 mAh, so the 2100 mAh cells
    should give you more than 3 times as much.

    Use a 10 watt resistor for the 75 ohm R, and 1/2 watt for
    the 6.2 ohm resistor.

  3. mpm

    mpm Guest

    Or you could get rid of the LM317 and use a switcher, which would
    produce MUCH less heat.
    For example, an LM-2575T-ADJ. This would require a couple extra parts
    to implement (an inductor, a Shottkey Diode, and an output cap, and
    two small resistors to set the 9.6V reference.)

    Also, I'm wondering if you could just use a small solar panel to float
    charge a 12-volt gel cell and regulate 9.6V from that. Seems to me
    you could get a reasonable size panel given the amount of charge
    you're seeking. That way, it would "never" die...??

  4. Franc Zabkar

    Franc Zabkar Guest

    My favourite method is to adapt a cheap mobile phone car
    adapter/charger. Many of these are switchmode types based on a
    Motorola MC34063 IC. The IC's rated current output is 500mA and it is
    designed to tolerate 40V inputs. I believe it shouldn't be too hard to
    reconfigure it as a regulated current source. You may also need to
    replace the caps with higher voltage types.

    - Franc Zabkar
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