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Getting 12Vdc from a 48Vdc solar system

Discussion in 'Photovoltaics' started by jim w, Jan 9, 2005.

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  1. jim w

    jim w Guest

    I want to hook up a 12V transmitter at a remote 48V solar installation,
    so as to send telemetry down to a monitoring station.

    Any ideas on an efficient method to get 12V from the 48V system? I've
    considered just tapping at the 12V point in the battery series, but
    then, I'm pulling power from 2 of the (6V) batteries, and not from the
    others, which seems like a bad idea, as the "charge" would get unbalanced.

    Anyone know of an efficient 48V-to-12V dc-dc setup?

    - jim
     
  2. Some "AC" switchmode power supplies will accept DC voltages as low
    as that for 12V output.

    Switchmode _can_ be efficient, but it may not be. Some are rated at
    95% or better; until you look at the fine print and find that at
    your typical load, it's only 50% or so.

    If you don't mind a floating 12V, then I've got a mental picture of
    a "FET cross-bar" multiplexing 12V out of the 48V stack, switching
    between successive 12V "stages" at several kilohertz using MOSFETs.
    That should be nearly 100% efficient as it doesn't involve
    converting DC to AC and back to AC as per your typical switch-mode
    convertor.

    Don't regard that as an off-the-shelf item!
     
  3. Correct, but they do include a voltage regulator. :)
    Yup. They convert DC to AC, transform, and AC to DC. Shorthand/common
    name is "DC to DC converter". Some are not fully isolated (eg input and
    output might share a common ground) and some are isolated.
    Watch for deals at surplus places like AllElectronics.com, bgmicro.com,
    and etc. Often can pick up a converter for $10 that would be $80/ea in
    1000 pc lots.

    Just watch the output current you need, and input voltage that can
    handle the range your system produces. For example, a 48vdc nominal
    input might handle 44v-52v which isn't sufficient to attach to a 48v
    lead-acid battery. A better 48vdc nominal converter might take 40vdc to
    72vdc, which should do fine.

    Check if this one is fully isolated and has good input range:
    http://www.allelectronics.com/cgi-bin/category.cgi?category=200&item=DC-48&type=store
    At $6.50 for [email protected], if they are isolated I'd be tempted to hook
    three in series. If they have adjustable output (some do, usually +/-
    0.5v or so) then adjust all three to result in 14v output, otherwise put
    a power diode or two in series to drop down to about 14v. Of course, it
    is only 75% efficient at max load... Hopefully better at the load you
    need.

    Another option is to use DC-DC to charge a 12v battery, and let the
    battery meet the peak current demands.

    sdb
     
  4. Just about any of the Telco Type Dc/Dc converters would work for this.
    Most are SwitchMode PWM type and wpould have regulated outputs.
    We use a lot of these in Remote Radio sites in alaska.


    Bruce in alaska
     
  5. ptaylor

    ptaylor Guest


    I've got one from Vicor that is 48V -> 12V 100W in,75W out.
    I'm unsure of the part number,but I think it runs about $30-40.
     
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