Quick 9v power question

Discussion in 'General Electronics Chat' started by Fastmerc, Feb 22, 2015.

  1. Fastmerc

    Thread Starter New Member

    Feb 20, 2015
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    For a quick little Sunday project, I just took an old pair of powered computer speakers and made a little guitar amp out of it by adding a 1/4 input jack. It is powered by a 9V ac adapter. I wanted to also make it portable so I soldered in a 9volt batter clip directly to the ac adapter jack lugs. My question is do I need to unplug the 9 volt battery if I am using the adapter to power the speakers?
     
  2. #12

    Expert

    Nov 30, 2010
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    Probably. Wall warts are notoriously unregulated. They would likely try to charge the battery. That's bad unless the battery is rechargable, and if it is, it needs a regulator for the charging current.
    There is a way to use the power jack to automatically disconnect the battery when you plug in.
     
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  3. Robartes

    Member

    Oct 1, 2014
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    13
    Yes. If you don't, you will have whichever of the battery or the adapter is at the highest voltage trying to push current into the other one. If you don't want to unplug the battery every time, you can build a simple switch with a FET or a transistor BJT.

    Update: or you could follow #12's even better suggestion to automate the switching
     
  4. Fastmerc

    Thread Starter New Member

    Feb 20, 2015
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    I know I could just mount the batter on the outside of the speaker so I could easily unplug it if I need to or I could add a little mini switch to the case, is there another way?
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Although the ones I have purchased in the last 5+yrs have been SMPS type, it may pay to check the type, a give away is the non-SMPS are usually much heavier.
    Max.
     
  6. #12

    Expert

    Nov 30, 2010
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    There is also the idea that TS said, "AC adapter". Those don't mix at all with batteries! :eek:

    Does that mean AC input or AC output?
     
  7. Fastmerc

    Thread Starter New Member

    Feb 20, 2015
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    How would I do the automated switch?
     
  8. Robartes

    Member

    Oct 1, 2014
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    On a standard DC connector, pin 2 and 3 are disconnected when you insert the plug:

    dc_connector.png
     
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  9. #12

    Expert

    Nov 30, 2010
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    Something like this:
     
  10. Reloadron

    Active Member

    Jan 15, 2015
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    That's what I would use. Disconnects the battery when you plug in external power.

    Ron
     
  11. Fastmerc

    Thread Starter New Member

    Feb 20, 2015
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    I think how I have it is what you are suggesting but I am uploading pics now.
     
  12. Fastmerc

    Thread Starter New Member

    Feb 20, 2015
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    Here they are
    [​IMG]

    [​IMG]

    [​IMG]
     
  13. Robartes

    Member

    Oct 1, 2014
    57
    13
    I'm assuming your AC adapter outputs DC with positive on the center pin? In that case, your circuit should work.

    Update: that's wrong. See below.
     
    Last edited: Feb 22, 2015
  14. Fastmerc

    Thread Starter New Member

    Feb 20, 2015
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    yes the pin is positive
     
  15. Robartes

    Member

    Oct 1, 2014
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    13
    Hmm, please disregard my remark in post #13, it was wrong. Pins 2 and 3 of the connector are linked by the PCB trace, so your battery would still be connected, even when the adapter is plugged in.
     
  16. Robartes

    Member

    Oct 1, 2014
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    13
    Additionally, are you sure the adapter outputs DC? The first thing after the connector on the PCB seems to be a bridge rectifier, which suggests AC.
     
  17. Fastmerc

    Thread Starter New Member

    Feb 20, 2015
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    1
    Adapter output is DC 9v 150ma
     
  18. Fastmerc

    Thread Starter New Member

    Feb 20, 2015
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    could I break the trace contact between the pins (scratch through it) or does the circuit need those to be bridged?
     
  19. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Yes the input socket goes to the bridge, at least it will be steered to the correct polarity via the bridge.
    Just that you will get ~ 1.5v drop over the diodes.
    Max.
     
  20. crutschow

    Expert

    Mar 14, 2008
    13,014
    3,234
    Note that a standard 9V transistor battery will likely not last long (perhaps a few hours for an alkaline type) in powering your amp.
     
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