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Power supply 'return' confusion

Discussion in 'Electronic Basics' started by Igmar Palsenberg, Jan 27, 2004.

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  1. Hi,

    I've got a piece of equiptment lying around here which I need to bring
    alive. However, the naming on the power supply confuses me.

    It says :

    -48 volt
    -48 volt return
    Ground connection to supply

    The -48 volt return confuses me : What is that ? The PSU that I have
    supplies -48 and +48 volt, and a ground. Can this PSU power that piece
    of equiptment ?

    Any hints are welcome.


  2. Why are those labelled this way ? The reason I didn't hook it up is that
    I can't be for sure it's wired OK, and i'll blow things up.
    The PSU does have a real grounded common, so that should be OK. I'll
    check for sure with a scope, not having a proper grounding should be
    easely spottable with one.
    Partly. I just wonder why they don't put +48 and ground on it instead of
    -48 and 'return'.

  3. krikkit

    krikkit Guest

    It might be less confusing if the terminal labeled '-48 volt return' were
    thought of as being 'common'.
    On some power supplies, the 'common' terminal is at the same potential as
    'ground' (chassis ground),
    on others, like the one you have, it is isolated or floating from

    The replacement supply you have with +48, -48 and 'ground' is a dual supply.
    That is.. +48 volts between the ground and + terminal, -48 from the ground
    to the - terminal. There would be 96 volts between the + & - terminals. The
    'ground' terminal is the common (return) and may or may not be actually
    connected to chassis ground. You can use this supply for replacement by
    using only the - and ground terminals taking into consideration that you may
    have to physically isolate the 'ground' on it if it would cause a conflict.

    Is that any hhelp or did I just confuse you more?
  4. w_tom

    w_tom Guest

    Phone system does not use +48 volts. It uses a -48 volts
    relative to ground.
  5. JeffM

    JeffM Guest

    It might be less confusing if the terminal labeled '-48 volt return'
    True. Other ways of saying this are
    "zero volts" or "the return path for the -48 volt supply".
  6. In other words : just -48 volts and ground. Now let's start the hunt for
    the right connectors :)

  7. DarkMatter

    DarkMatter Guest

    It's positive 48 volts with the positive leg grounded, if one wants
    to be technical about it. It all about reference points.
  8. grahamk

    grahamk Guest

  9. DarkMatter

    DarkMatter Guest

    With OUR HV supplies, the "black" lead is always ground.

    That means that if one sees an HV supply with a red led, it is a
    positive supply.

    If the output lead is clear, it is a negative supply.

    Black is always assumed as ground, and is never elevated in
    potential with respect to ground. It can be negative or positive,
    just not with respect to ground. ONLY with respect to the PS output
  10. This Naming convention often is used to indicate that the -48V Return
    is not tied to ground (or cannot be reliably thought of as being tied
    to ground)
    Telco equipment often keeps the 48V supply and chassis/shield Grounds
    seperate so that the Telco can implement their own grounding regime.

    Richard Freeman
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