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Share negative between two power supplies?

Discussion in 'Electronic Basics' started by [email protected], Sep 18, 2005.

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  1. Guest

    I have a small 12VDC (130mA) fan that will normally be powered with an
    AC adapter. However, sometimes I want to be able to run it off a
    battery with the flip of a DPDT switch. Since a DPDT does not have
    enough connections to switch both the positive and negative from the
    power supplies, I am wondering if I can tie together the negatives from
    the battery and the AC adapter, and use the DPDT switch to only move
    between AC adapter positive and battery positive.

    Will this work, without adverse effects to the battery?

  2. Bob Monsen

    Bob Monsen Guest

    Yes, it'll work.
  3. Guest

    Yes, it'll work.

    Cool, thank you for the quick answer. :)
  4. kell

    kell Guest

    As Mr. Monsen says, using a common ground will work. Strictly
    speaking, however, this is not necessary; you were mistaken to think
    that the DPDT switch does not have enough connections.
    You may simply connect the "poles" (center terminals) of the
    double-pole-double-throw switch to the fan terminals. Then for each
    pole, connect one of its "throws" to the adapter and the other to the
  5. John Fields

    John Fields Guest

    Yes, but if you do it that way there's no need for a DPDT switch.
    If you already have the switch, you only need to use half of it,
    like this:

    +------------+ +---------+
    ACIN>---|~ +|------> | <------|+ |
    | AC ADAPTER | | | BATTERY |
    ACIN>---|~ -|--+ O +--|- |
    +------------+ | | | +---------+
    | +---+---+ |
    | | FAN | |
    | +---+---+ |
    | | |

    I've shown a SPDT switch with a center-off position in case you want
    to turn the fan off, but if you have some other way to do that you
    can use a simple SPDT toggle switch. Just to avoid any
    complications, make sure it's a "form C" (break-before-make) switch.
  6. ehsjr

    ehsjr Guest

    With a DPDT switch, you can wire it like this:

    --------- | |
    | | ----|---- --------- |
    | AC |-----o o o----| | |
    | | | Switch | | Battery | FAN
    | Adapter |-----o o o----| | |
    | | ----|---- --------- |
    --------- | |
  7. Jasen Betts

    Jasen Betts Guest

    if it's a DPDT it does have anough constacts, maybe you have single pole
    dual throw (SPDT)
    yes, definately.

    but most sockets for connecting dc adaptors have a switch built in that
    can be used to select battery/adaptor power instead, this is how most
    battery powered appliances are built. but you'll probably still want a
    switch to turn the fan off :)

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