Circuit for Deciding Power Source

Discussion in 'The Projects Forum' started by robsworld, Nov 5, 2015.

  1. robsworld

    Thread Starter New Member

    Nov 5, 2015
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    Hi, I'm working on a PCB and my idea is to have 2 different power sources go into the board and it decide which one to use. The PCB is an expansion to another board which has +12v DC on it, both are being connected via a USB cable. I'm going to send the 12v from the one PCB through the USB cable to the other board. This PCB has many uses so current draw will vary based on use. If a person only needs 500ma of power then the 12v from the USB will work. However if they require over an amp of power then they would plug in an external power supply using a barrel jack and at that point the board would use the power from it and not the power coming from the other board. Is this possible? Thanks
     
  2. Papabravo

    Expert

    Feb 24, 2006
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    Sending 12VDC through a USB cable would be inadvisable. Any device plugged into the cable or the port would be instantly toasted. BAD IDEA!
     
  3. robsworld

    Thread Starter New Member

    Nov 5, 2015
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    Sorry this has nothing to do with computers, the USB cable is only a cable for this project. I'm making an aquarium controller and one box is a power bar and the other is a box with more ports for different equipment. I connect both boxes together using a USB cable. The board needs 12v to function.
     
  4. Papabravo

    Expert

    Feb 24, 2006
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    Why are you so enamored with USB cables. They are not shielded, they won't carry much current, and they're not particularly cheap. The connectors are not particularly cheap either.
     
  5. robsworld

    Thread Starter New Member

    Nov 5, 2015
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    I've read good cables carry 1.5amp max so 1amp shouldn't be a problem. The project doesn't require lots of power but it can use probably close to 2amps if all ports were used. I want a cable that can be bought anywhere, shielding isn't important and I'm sure you can get them somewhere nowadays. I don't think ethernet patches carry any more current so I figure USB is a good choice. Serial cables with DB9's are to big. SMD USB jacks aren't that expensive and a cable is $10 so that's ok.

    EDIT: If you know of a better cable that's readily available can carry a 2amps and relatively cheap I'm ok with changing.
     
  6. Papabravo

    Expert

    Feb 24, 2006
    10,163
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    The cable may be able to carry the current, but how long is it and what is the voltage drop? The more current you pull the greater the drop and don't forget to include the length of the ground return.

    Get a spool of wire (AWG #18 for example) and some screw terminal connectors and roll your own. Male PCB connectors are cheap, and the matching Female connectors for the cable are similarly priced. As a bonus you can get screws that will prevent the cable from popping out.
    https://www.sparkfun.com/products/8084
    http://www.wago.us/products/pluggable-connector-systems/pcb-connector/overview/
     
  7. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,546
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    Yes. And, it can be done with no electronics. many barrel jacks have a single pole switch attached to the center pin contact. The whole idea is that one power source is disconnected when another is plugged in. Sounds like a perfect match to me.

    ak
     
  8. robsworld

    Thread Starter New Member

    Nov 5, 2015
    22
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    Thanks for the info guys, I appreciate it.
     
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