Parallel power inputs?

Discussion in 'The Projects Forum' started by tracecom, Sep 11, 2012.

  1. tracecom

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    Apr 16, 2010
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    I want to have optional power inputs to a DC circuit. One input is a standard coaxial power jack for connection to an external DC supply. The other is a two conductor terminal block that might be connected to a battery. In the attached schematic, I think that I have it wired so that there is no conflict between the two power sources if they are both connected simultaneously. However, I would like more "eyes" to look this over, and point out errors. Thanks.
     
    Last edited: Sep 11, 2012
  2. elec_mech

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    Looks good to me.
     
  3. MikeML

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    What is D1 supposed to do?
    Do you want wall wart or battery to have priority?
    Why are you doing the switching in the negative lead?
     
  4. tracecom

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    Protect from reversed polarity input.

    Wall wart.


    The coaxial power connector doesn't allow switching the center conductor, which is the positive lead in my circuit (and in most wall warts.)

    Thanks for any suggestions.
     
  5. wayneh

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    Sep 9, 2010
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    I might be missing something but I don't see how this protects one source from the other. You need a diode from both sources brought to a common point, to prevent current going from one to the other under all circumstances. Power supply "OR"-ing.

    It sounds like it's not relevant to your operation, but you also need to worry anytime you connect one power supply "ground" to another. If they're both ultimately referenced to mains, you might set up a path for current to flow from one to another via the ground wire only. Bad surprise the first time you learn this the hard way.
     
  6. tracecom

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    I believe the coaxial power connector has a set of contacts that break when the plug is inserted. In my schematic, my intention is that when the plug is inserted, the negative side of the battery is isolated from the circuit.

    However, I will think through your comments about diode OR'ing. That may well be a more failsafe way.

    Thanks.
     
  7. wayneh

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    Yeah, I realized that after I wrote my post. But there is the issue that the two supplies might be briefly connected before the contacts open. That's probably not much of a problem, but something to contemplate.
     
  8. MikeML

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    Here is how I would do it...
     
  9. tracecom

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    I had drawn this up before MikeML's last post, but I have some BAT85's and may use those instead of 1N4001's. Technically, the break contacts in the coax connector are now redundant, I suppose.

    ETA: After thinking this over, the break contacts are not redundant. They disable the battery when the wall wart is plugged in.
     
    Last edited: Sep 12, 2012
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