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24vdc to 12 vdc for remote deployment?

Discussion in 'Electronic Basics' started by [email protected], Jan 24, 2005.

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  1. Guest

    Hi,
    My first post so excuse me if I sound a bit basic-I have a 12vdv comms
    device that I want to put in a remote cabinet.The cabinet has
    24vdc.The device will draw 750mA to 1000mA at most-the specs say 9
    watts power for the device.
    Space is a preminium so I'm looking for 2 things
    1) how to get from 24v to 12v
    2) how small can be it, either in unit or PCB form, as to not vastly
    increase the footprint of the original device-which is currently 10cm
    by 13cm.I've been doing a bit of Googling and it looks like I need a
    7812 stabaliser-is this correct and what else do I need for the
    "complete" unit?

    Thanks

    Sandy
     
  2. A 7812 is a linear voltage regulator, so it takes the 24V, and drops it
    to 12V. However, at 1A, that means the 7812 will need to dissipate 12W,
    which is far beyond it's capabilities.

    A better solution would be a prebuilt dc-dc converter. You can get these
    all over the place. A quick search brought up this one:

    http://www.v-infinity.com/adtemplat...atky=328060&subcatky1=248699&subcatky2=876307

    However, there are lots to choose from. Size is an issue for you, so
    you'll have to be picky.

    --
    Regards,
    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
     
  3. cabsandy

    cabsandy Guest

    Richard,
    I found this page whilst doing some research but does it not go
    againnst what Robert write above? .i.e. the 7812 couldn't dssipate the
    power of the 9 to 12W that would occur?

    Thanks

    Stewart
     
  4. My data sheet says the thermal resistance from junction to ambient,
    without a heatsink, is 65C/W, so 12W gives a junction temperature of
    800C with 25C ambient, so that's out. You need a heatsink.

    For a good TO220 heatsink, you get about 22C/W at 25C. We need to keep
    the junction temp < 125C. The resistance to case is 5C/W for the device
    itself. Thus, at 25C ambient, 25 + (5 + 22)*W < 125, so the power
    dissipation must be limited to 3.7W. This is assuming a perfect junction
    between the case and heatsink. That means that the maximum voltage input
    for 12V out at 1A is 15.7V.

    Thus, unless I've added wrong, at 12W, a typical 7812 in this
    application, with or without a heat sink, will shut itself off in a few
    seconds.

    You can get beefier linear regulators, of course, or use an external
    pass resistor to increase the maximum power dissipation, but then you
    need to deal with the heat.

    A DC-DC converter is more efficient and thus cooler. It's more
    expensive, but for a single project, it is probably a better choice.


    --
    Regards,
    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
     
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