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Barrel plug (electrical power) conventions?

Discussion in 'Electronic Design' started by Sam Kaan, Dec 26, 2003.

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  1. Sam Kaan

    Sam Kaan Guest

    You know those little barrel plug that normally comes with the electrical
    wall adapter to power your cell phone
    embedded computers etc? 5V, 9V, 12V etc.

    Is there a convention as far as their polarities are concerned? I have a
    board that I need to power using these
    plugs. The board takes 5V to be supplied via a barrel plug connector. I
    am unable to find the documentation
    as to what polarity the plug should be wired up. Whether the outer barrel
    is to be made negative while the inner
    is positive and vice versa. I took a look at several Walmart adapter and
    the likes, and notice that it doesn't follows
    a set pattern, sometimes its one way and sometimes its the other way.

    By the way, when you pick up an adapter/transform how can you tell if its
    regulated or not? Does "For Indoor Use Only"
    or "Class 2 Transformer" means anything in particular?
  2. Manufactures use whatever they want for those Coaxial power plugs. No
    rhyme or reason. You need to determine which side is grounded to the
    circuit before connecting the AC adapter. You may have to open the item,
    but there may be a grounded connector for an earphone, antenna, or
    similar that you can check with an ohm meter to find which part goes to

    Does it measure the rated voltage with no load? If it does, its

    Does "For Indoor Use Only"

    This is rather obvious, isn't it? You can't use it outdoors where
    rain can get into it, or across the AC power pins.

    24 VAC or less, or less, and current limited to prevent a fire if
    the output is shorted.

    Merry Christmas!

    Take care, and God bless.
    Michael A. Terrell
    Central Florida
  3. Sam Kaan

    Sam Kaan Guest


    I see. So I just put my multimeter to such an adapter to my surprise it
    19V. While the adapter has labeled clearly on it 12V. So this is not
    regulated right??

    I supposed the regulated adapter would keep a constant 12V as the circuit
    various current loads from it.
  4. James Meyer

    James Meyer Guest

    Well, actually there is a *little* rhyme and reason.

    Many coaxial plug sockets have three terminals. There's a connection to
    the inner, one to the outer, and a third that forms a switch with (usually) the
    outer connection. It's often a convenience when a device is to be powered from
    either internal batteries or a wall plug to have the act of inserting the plug
    automatically disconnect the batteries and let the wall power take over. The
    third contact is used and then that determines whether the outer or inner
    contact is one polarity or the other.

  5. Yes, that's what regulated means.

    Merry Christmas!

    Take care, and God bless.
    Michael A. Terrell
    Central Florida
  6. Jim, I agree it should make a difference, but I see quite a few
    center negative connectors with a switch, and they use it to disconnect
    the battery's ground to take it out of the circuit. It makes no logical
    sense, and I think that is why its done. Anything to help obscure the
    design, and to cause you to damage it when you plug in the more common
    center positive power supplies. Walk into most places that sell
    replacement wall warts. They want to start plugging in an adapter with
    no idea of polarity. After all, if it doesn't work, they will try to
    sell you another piece of junk.

    Merry Christmas!

    Take care, and God bless.
    Michael A. Terrell
    Central Florida
  7. Rich Grise

    Rich Grise Guest

    If you have the luxury of selecting your own connector, pick one
    that the outside is negative, if your circuit is "negarive ground."
    If you have the further luxury of being able to add a regulator
    to your device, then do that, and spec about a 9V wart. :)

    Good Luck!
  8. Leon Heller

    Leon Heller Guest

    I always use a series diode on any equipment using this type of PS.

  9. I read in that Sam Kaan>) about 'Barrel plug (electrical power)
    conventions?', on Fri, 26 Dec 2003:
    There is supposed to be: the centre pin should be positive. This is in
    an IEC standard for 'assessed quality' versions of the connector, which
    are mostly dimensionally incompatible with the ones widely used.

    But in the past, some manufacturers have used the opposite polarity. The
    best thing to do is to put a diagram on the product, showing which
    polarity it needs, AND put a protection diode in there to guard against
    reversed polarity.

    The diagram looks a but like this:
    - +

    The 'o' should be inside the 'C'.
  10. I read in that James Meyer <>
    Not really: you can put the switch in either battery lead.

    Keep the pin positive!
  11. Tim Shoppa

    Tim Shoppa Guest

    Even better than a convention, there are two conventions!

    (I'm ignoring those AC-out-on-coaxial-plug adapters here, as they comply
    with both of the above conventions, 60 times every second.)

    The wonderful thing about standards is there are so many to choose from!

  12. Tim Shoppa

    Tim Shoppa Guest

    Are you implying that it makes more sense to switch (+) as opposed to (-)?

    I will admit that I always put a bridge rectifier after my DC power jacks.
    (Put a filter capacitor there too and you can run the devices off of AC-out

  13. N. Thornton

    N. Thornton Guest

    Even better is to use a full wave rectifier in your product, then you
    can plug any of the 3 types of PSU in, centre -ve, centre +ve, or ac.

    Regards, NT
  14. James Meyer

    James Meyer Guest

    I've seen very few (no?) commercially produced devices that didn't
    include a little picture, sometimes embossed into the case, that showed whether
    the center pin is either plus or minus. It's a case of RTFM.

  15. James Meyer

    James Meyer Guest

    A good idea if you don't mind losing 1.4 volts of input.

  16. I do see items with no markings, or they had a sticker that dried out
    and fell off. RTFM? Most things I see have no manuals. Last year I
    scrapped over 30 junk cordless phones and only a couple were properly
    marked. Some didn't even tell you what voltage, just the usual cryptic
    warning to only use the proper power supply.

    I do keep the manuals for anything I buy new.

    I strip junk electronics for hardware and a few parts. There is a
    local program in Ocala to teach people to solder so they can get low
    paid assembly work. They use scrap boards in the class. Then, the boards
    are sent to a refinery where the boards are burnt so the metals can be
    recovered. If I could just find a place to take all the scrap plastic
    cases there would be nothing left over for the landfill.
    Merry Christmas!

    Take care, and God bless.
    Michael A. Terrell
    Central Florida
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