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Ok,Silly question, what's the middle pin on a 3pin CB power plug?

Discussion in 'Communication (CB | GPS | HAM)' started by 73k5blazer, Feb 1, 2007.

  1. 73k5blazer

    73k5blazer Unplug the matrix cable from the back of your head Premium Member

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    I've a Cobra 29 CB Radio. It has a 3 pin power plug on the back.
    The cord supplied with it is a cigerette lighter style, with 3 wires going to the 3 pins on the plug. one side 12v, middle 12v, other side ground.
    Great.
    I've another 'universal' cb cord, just 12v on one side, ground on the other. Middle pin is not hooked to anything.

    The radio operates on either cord.

    Now, I'm wiring my K5 from scratch, I don't want the cig lighter, I want to wire it directly to my circuit breaker box, and am wondering, what that middle pin is for? Some cords have 12v on it, others have just a blank?

    The manual just labels the 2 edges 12v and ground. The middle pin isn't labeled.

    I've been searching and googling, but too much comes with searches like 3pin cb cord, cb wiring diagrm, cb power .

    Anyone know what that middle pin is supposed to be for?
     
  2. 88Silverado

    88Silverado 1/2 ton status

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    The middle pin is a dummy pin. It doesnt connect to anything and is only there as a "polarity" or "key" pin to keep the power cord from being installed backwards.
     
  3. 73k5blazer

    73k5blazer Unplug the matrix cable from the back of your head Premium Member

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    Really? The plug itself is keyed, you couldn't insert it the wrong way.

    The cord that came with it has 3 wires coming from the cig lighter, 2 12v and 1ground,I can't imagine they would run a whole seperate wire if it was just a dummy? :dunno: I guess stranger things have happened.
     
  4. 88Silverado

    88Silverado 1/2 ton status

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    If you look at the input schematic it doesnt show the center pin connected. However,..........

    Since the manufacturer knows people tend to tweak the power outputs and they can push some if those radios to 30 or 50W output, the single +12V input pin will be pressed to carry the current load. You can connect the third pin inside and run a pair of 12V into the unit for the additional current load.
     
  5. W7NB

    W7NB 1/2 ton status

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    I don't pretend to know anything about CB radios, but many Ham and commercial radios have a 3rd power pin that enables the radio to be turned off and on with the ignition. The main power wire is designed to draw the actual current of the radio, and the 3rd wire carries only power from an ignition switched source so when the key is turned on the radio comes on. If you don't want to use the feature you simply connect the main and ignition wires to the same hot lead and the radio is on all the time.

    Not sure it applies here but thought I'd throw it in.
     
  6. 88Silverado

    88Silverado 1/2 ton status

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    Yes, you are correct on the 3rd power wire from ignition and I completly forgot about that.

    Heres a pic of what the schematic shows. I really dont want to tear my rig out and open it up to look but maybe someday.



    Some additional info I found says it is a polarity pin but since the connector is already designed fir polarity, go figure. I wonder if its a spare hot pin to power something else in a different unit, could use common plug amoung several different radios :dunno:

    SCH.jpg
     
  7. 73k5blazer

    73k5blazer Unplug the matrix cable from the back of your head Premium Member

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    I figured it out.
    I was at a truck stop today that had a CB fix it counter, so I asked the guy. He said some radios need 12v constant to retain some things in memory.

    My radio doesn't need this pin as it has no memory.
     

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