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connecting two 12V power supplies together

Discussion in 'Electronic Design' started by panfilero, Mar 6, 2008.

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  1. panfilero

    panfilero Guest

    Hello,

    I'm trying to figure out a way to have two power signals coming into a
    circuit and the output remaining the same.... what I mean is that I
    have two inputs where I need to connect 28V DC, and I should have the
    same result whether only one of my 28V inputs is plugged in or both
    are plugged in... I'm going to try and feed these inputs into a 5 Volt
    regulator.... if I connect both the positive connections of these two
    inputs into the regulator.... would that be ok? Like connecting to
    battery or power sources in parallel? Something feels funny about
    that to me... and I'm not sure if it makes sense to connect two power
    sources together like that.... my gut is saying no. But I'm not sure
    why.

    Any thoughts on this is much appreciated,
    thanks
    J.
     
  2. Fred Bloggs

    Fred Bloggs Guest

    Power sources are not usually called "signals." You will be okay by
    diode ORing them...
     
  3. Greg Neill

    Greg Neill Guest

    It's unlikely that both 28V supplies will be
    precisely the same voltage (due to design
    differences, component tolerances, etc.),
    thus you will end up with an undesirable
    current loop between the supplies.

    What is the purpose of having two 28V supplies?
    Just to add current capacity?
     
  4. panfilero

    panfilero Guest

    I think that's what was bugging me... that the two power supplies
    aren't going to be clean regulated 28V sources... so I'm a little
    rusty on my electronics theory but there is something that leaves me
    uncomfortable about doing that.... just connecting them in parallel
    like that. I'm thinking a Diode OR type solution would be the way....
    basically I'm trying to pass the DC power on to somewhere else... but
    the DC power may come from two different sources and either source (or
    both) may be plugged in at any time, but I just need to pass that 28V
    on through regardless of which source is connected.

    much thanks
     
  5. Guest

    One way is to "diode or" the two 28V supplies. Connect the diode
    anodes to each of the supply outputs. Tie the cathodes together and
    to the input of the 5V regulator. Whichever supply has the highest
    output voltage will supply the load. This works best when the 28V
    supplies have current limiting. This ensures that when the dominant
    supply reaches its current limit, the other supply will take over.
     
  6. z

    z Guest

    yeah, if you're not concerned about sharing the load equally so as to
    maximize the output current available, just OR them and whoever's got
    the most voltage will carry it; being off constituting a very low
    output voltage.
     
  7. panfilero

    panfilero Guest

    DIODe Or Solution sounds best... the only con about using that is the
    voltage drop across the diode, if at all possible I'd like to get the
    whole voltage across...

    much appreciate the responses
     
  8. Guest

    Yeah, the diode drop can be a problem. You can minimize it by using
    Shottky diodes.
     
  9. z

    z Guest

    yeah, this is getting confusing. the title says two 12v supplies, the
    text says 2 28v supplies to be regulated down to 5v, now we're trying
    to avoid any voltage drop. ?
     
  10. panfilero

    panfilero Guest

    Thanks, for the relay idea.... a switch wouldn't work cause it has to
    be automatic.... and the power does more than just go to a regulator
    so getting the entire 12V passed is a concern, that's why I'm looking
    for something other than the Diode-OR.
     
  11. panfilero

    panfilero Guest

    Ahhh.... sorry about the 12V/28V confusion..... that part is a bit
    arbitrary to me, there will be situations where I'm passing 12 and
    other situations where I'm passing 28. And the 5V regulator.... it's
    in the circuit as well but the power does more than just provide the
    input for the regulator.

    Thanks
     
  12. Fred Bloggs

    Fred Bloggs Guest

    And what will you do for us if we tell you, other than disappear?
     
  13. JosephKK

    JosephKK Guest

    You could use some electronics and big power FETs.
     
  14. Fred Bloggs

    Fred Bloggs Guest

    View in a fixed-width font such as Courier.

    ..
    ..
    ..
    ..
    .. VPC
    .. .-----+-----+-----------------------------------.
    .. | | | |
    .. | | | |
    .. | |B1 |B2 |
    .. | --- --- L
    .. | - - O
    .. | | | N-CH2 A
    .. | | | VN1 D
    .. | | '-----+---D S ---. |
    .. | | | G | |
    .. | | | | | |
    .. | | \ ------- | VNC |
    .. '-----|-------------|ARBITER|--+----------------'
    .. | / ------- |
    .. | | | |
    .. | VN2 | G |
    .. '-----------+---D S----'
    ..
    .. N-CH1
    ..
    ..

    Depending on the application, the arbiter could take any one of several
    forms, and more than a few of those require detecting a low batt
    threshold, and that is complicated when the battery is relatively arbitrary.
     
  15. Jim Thompson

    Jim Thompson Guest

    Except that your circuit doesn't work. The devices need to be
    "back-to-back" in series, as in this previously posted circuit...

    http://analog-innovations.com/SED/PerfectDiodeForChargerIsolation.pdf

    (Body diodes _must_ be considered as sneak paths.)

    ...Jim Thompson
     
  16. Hi Jim,

    Fred's using N-ch mosfets with the drains on the negative inputs. The
    body diodes can't contribute to reverse current flow.

    The only thing he loses (by not having back-to-back fets) is input
    overvoltage protection and the ability to "turn off" an input.


    A similar arrangement would work for P-ch fets on the positive inputs
    with a controller such as the LTC4416.
    See, for example, figure 5 in this datasheet:
    http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1003,C1142,C1079,P31793,D17899

    Regards,
    Allan
     
  17. Fred Bloggs

    Fred Bloggs Guest

    The circuit is a standard diode-OR when the FETs are off. Start from
    there...
     
  18. Fred Bloggs

    Fred Bloggs Guest

    This would be the extreme case for a 24V system:
    View in a fixed-width font such as Courier.

    ..
    ..
    ..
    ..
    .. VPC
    .. .-----+-----------------------------------.
    .. | | |
    .. | | + |
    .. + |B1 |B2 |
    .. --- --- 20V L
    .. 29V - - O
    .. | | - A
    .. - | | VN2 D
    .. | '--------|<|-------. |
    .. | | |
    .. | | |
    .. | | VNC |
    .. | +----------------'
    .. | |
    .. | |
    .. | VN1 |
    .. '------------------------'
    ..
    ..
     
  19. Fred Bloggs

    Fred Bloggs Guest

    Here's a bad example of an arbiter, part of one anyway:)
    View in a fixed-width font such as Courier.

    ..
    ..
    ..
    ..
    .. VPC
    .. .-----+-----+-----------------------------------.
    .. | | | |
    .. | | | |
    .. | |B1 |B2 |
    .. | --- --- L
    .. | - - O
    .. | | | N-CH2 A
    .. | | | VN2 D
    .. | | '-----+---D S ---. |
    .. | | | G | |
    .. | | | | | |
    .. | | \ ------- | VNC |
    .. '-----|-------------|ARBITER|--+----------------'
    .. | / ------- |
    .. | | | |
    .. | VN1 | G |
    .. '-----------+---D S----'
    ..
    .. N-CH1
    ..
    ..
    ..
    ..
    ..
    .. ARBITER
    ..
    .. D1
    .. VPC>---|>|---.
    .. |
    .. .---+--------+-----+---+---+-----+--------+---.
    .. | | | | | | | |
    .. | | | | | | | |
    .. | | | | | | | |
    .. [680] | | [27k] [27k] | | [680]
    .. | | | | | | | |
    .. | | | | | | | |
    .. | - | | | | - |
    .. | ~~V D2 |IC1:A| |IC1:B| D3 V~~ |
    .. | - |/| | | |\| - |
    .. >| | /-|-----------+---|+\ | |<
    .. 2N3906 |-+-[2 K]-< | | | | >-[2 K]-+-| 2N3906
    .. /| \+|---+-----------|-/ |\
    .. | |\| | | |/| |
    .. G<--+ | +-------|-----|-----[2.7M]-+-->G
    .. N-CH2 | | | | | | N-CH 1
    .. +-[2.7M]-----|-----|-------+ | |
    .. | | | | | |
    .. | | [56k] [56k] | |
    .. | | | | | |
    .. [10k] '---------+---------' [10k]
    .. | | | | |
    .. | .-----' | '-----. |
    .. | | | | |
    .. VN2>--|------------+-----|<|-+-|>|-----+------------|--<VN1
    .. | D4 D5 |
    .. | |
    .. '----------------------+----------------------'
    .. |
    .. VNC
    ..
    ..
    .. D1,3,4,5,6,7 1N914
    ..
    .. IC1 LM339
    ..
    ..
    ..
    ..
    ..
    ..
    ..
     
  20. Jim Thompson

    Jim Thompson Guest

    Yep, Sorry, That does work for "loads". My approach is for
    charge/discharge isolation, as in LiIon battery controls.

    ...Jim Thompson
     
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