power divider or step down ???

Discussion in 'The Projects Forum' started by cablguy, Mar 11, 2009.

  1. cablguy

    Thread Starter New Member

    Mar 1, 2009
    5
    0
    I need help badly! My project, I have an existing string of three 12volt batteries in series being maintained by a UPS. I need to get power off of this existing string for a new device (12vdc and .3A). Any ideas or suggestion for doing this at the absolute minimum cost would be greatly appreciated.

    cablguy
     
  2. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    A linear voltage regulator fits your description. The LM7812 will set you back less than half a dollar from most reputable vendors.
     
  3. cablguy

    Thread Starter New Member

    Mar 1, 2009
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    Thanks for the quick reply. I will let you know how it works out
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Ahhh, wait a minute.

    You're maintaining three 12v batteries in series using a UPS?

    I think you probably have them wired in parallel instead - unless your UPS designed input was for 36v to 42v.

    Anyway, a 7812 regulator has a minimum 2v dropout across itself. That might not be so bad while the batteries are charging; the float voltage will probably be somewhere around 13.5v, so the 7812 output will be around 11.5v. However, when the UPS starts operating on battery power, the 7812's output will quickly drop to around 10.2v.
     
  5. cablguy

    Thread Starter New Member

    Mar 1, 2009
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    0
    yes it is in series, and at full charge status we might be looking at 42-44vdc. If the system went to stand by long enough to drain below approx. 11vdc we we would have higher priorities than the stat-mon data. I have also found a source for 90vac that i could use. I have not looked at it yet but was told that it should be square wave, is that something that we could work with and keep the cost-per-each low?

    Thanks again for all the help
    cablguy
     
  6. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
    2,039
    287

    Why not just tap off the "bottom" battery? Assuming you don't need super good regulation, this should work fine.

    eric
     
  7. Tahmid

    Active Member

    Jul 2, 2008
    344
    25
    Hi cablguy,
    You could use the 42V as the input source to a circuit built with the L4976 Switching Regulator and take 12V as your output. Since it can accept input from upto 55V, 42V is not a problem. It is a 1 Amp Regulator, so it will work fine.

    [​IMG]
     
  8. cablguy

    Thread Starter New Member

    Mar 1, 2009
    5
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    need more help!! can anyone come up with the pcb for Tahmid's circuit?
    Thanks in advance to all, My butt is in a jam!
     
  9. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    If you go to National Semiconductor's site:
    http://www.national.com
    you can use their Webbench tool (flash version preferred) to generate a regulator design, complete bill of materials list, evaluation board, and order it online.
     
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