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Easy way to get 12DC from 78 VDC ?

Discussion in 'Electrical Engineering' started by me, May 9, 2007.

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  1. me

    me Guest

    I've got a project that has a 84VDC unregulated power supply on it. I
    need 12VDC,3A for another part of the project.

    Is there a simple way to get 12VDC from that supply ? Regulated ? Like
    maybe a PWM circuit driving a chopper transistor with a capacitor to
    smooth out the waveform ? I say regulated because the 84VDC supply is
    heavily loaded and it droops.

    Thanks.
     
  2. Palindrome

    Palindrome Guest

    The usual solution is a SMPSU configured as a "buck"
    http://www.maxim-ic.com/cookbook/powersupply/pdfs/CB100.pdf
    http://www.maxim-ic.com/quick_view2.cfm/qv_pk/3939

    You will need to derive a low current, lower voltage supply for the chip
    itself.
     
  3. me

    me Guest

  4. ehsjr

    ehsjr Guest

    There's a simpler way - buy a separate power supply.
    But setting that possibility aside:
    Does your 84 VDC supply have the "guts" (current capability)
    to provide an additional 36+ watts beyond what it is
    providing now? If it doesn't, you need a separate supply.

    And if it has lots and lots of "guts" you could go simple
    with a linear, but it would waste a *lot* of power. As
    Sue recommended, and assuming the existing supply can provide
    an additional 36+ watts, a buck regulator is the way to go,
    even though you don't find it simple.

    Ed
     
  5. Try a switch mode power supply that has a wide input voltage range of
    nominally 85-264 VAC. These have a full wave bridge input and will often
    operate on lower voltages. You will need one rated for more than the
    3A because the input diodes are rated for the input current for normal
    operation in which the diodes carry current only half of the time.

    There are also solar system controllers that convert higher voltages down
    to 12 or 24 volts for battery charging.
    Bill Kaszeta
    Photovoltaic Resources Int'l
    Tempe Arizona USA
     
  6. ehsjr

    ehsjr Guest

    Bill, you just gotta read the thread. I'm not
    the op - I'm not searching for the solution.
    You should post to him, not me.

    Also, his supply provides 84 volts, not 85, and it
    droops under load, so a supply with an input range
    of 85 - 264 vac is a non-starter.

    Ed
     
  7. me

    me Guest

    Yes, it has the guts.
    How do you make a simple linear PS that has an input of 84V ? I
    redesigned things and now I need about 50 ma instead of 3A. Is that
    getting into Zener diode territory ? All I need to do now is power a
    single sided op amp.
     
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