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centre pin negative question

Discussion in 'Electronic Design' started by Sean Bartholomew, Nov 26, 2004.

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  1. what is the purpose of having centre pin negative power transformers
    like those commonly used in audio gadgtry eg. guitar FX pedals?
     
  2. The center pin of plug packs can be positive or negative. You may have
    noticed some sort of pattern, but I wouldn't like to make any rules.

    I think the reason for the apparent random assignment of polarity is that
    there never was a standard.

    If the connector has switchover contacts which switch from battery to
    external power, then it may be important which polarity the switch breaks.
    I wouldn't think that would matter when using a "wall wart" supply which is
    isolated.

    Roger
     
  3. Keith

    Keith Guest

    Maybe not RULES perse, but I've found the bulk of the smallish electronic
    gear which uses those "wall-warts" to be center positive. Center positive
    is popular enough, that you can "guess" something is CP and be right
    probably 95 times out of 100. The most common wall power supplies are 9v
    and 12v, although you'll run into the occasional 5/6v models. 3Com, makers
    of networking equip, were famous for throwing everything away and using
    something goofy like 15 volts *AC* into their mini hubs and switches. Some
    small cisco equipment uses 5v and 12v, but uses a molex plastic connector
    with 4/5/6 pins instead of the DC barrel connector that most of the other
    devices use.

    Keith
     
  4. Rich Grise

    Rich Grise Guest

    The absolute worst power supply connector I've ever seen was on some
    off-brand 8-port ethernet router. The supply was about 3 times the bulk of
    a typical wall wart, but had its own AC cord, like a printer supply, but
    it was +-15 AND +5, the DC cable was about the bulk of a hand-drill
    power cord, but stiff, and the plug was a f***ing MOUSE connector! The
    keying on the connector was f***ed up from previous owners trying to shove
    it in - it took an extraordinarily delicate touch to feel when the pins
    were aligned with the holes, while manhandling that f***ing cable - once I
    bent one of the pins, I gave up on it and just tossed the whole thing and
    wrote off the twenty or so bucks.

    I guess the moral to this story is, if you need multiple supplies, use a
    Molex.

    Thanks,
    Rich
     
  5. Tam/WB2TT

    Tam/WB2TT Guest

    Probably goes back to those bitty Japanese AM radios that used PNP
    transistors, and grounded the positive side. I don't know what it is, but
    there is a sort of standard for the coaxial power connectors. You will find
    that for the same OD, there is a choice of center pin diameter. People have
    tried to correlate that to polarity.

    Tam
     
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