technique for 'switching off' battery when DC power plug attached?

Discussion in 'General Electronics Chat' started by DaveH, Apr 16, 2009.

  1. DaveH

    Thread Starter Active Member

    Jan 1, 2009
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    I'm designing some audio frequency circuits that work off a 9V battery. A lot of commercial battery operated devices work with an AC to DC rectified supply whilst the battery is still in the device. The battery is electrically "switched off" (albeit with a minor current drain) in those situations.

    I think I can handle designing something along those lines using a JFET as a switch.

    Then I thought about logic controlled analog switches, but am not sure that's the right way to do it.

    I'm curious to know what design techniques are used in commercial equipment or by hobbyists for this kind of task. Is using a JFET a good idea, or are there other common and more elegant methods?

    I expect my circuit to have total current drain of about 50mA. It would be nice to have something where when the battery is "cut off" when the DC power is plugged in, the battery suffers only a current drain of only a few picoamps - or may be a bit more if that is a too demanding requirement.

    Any design advice appreciated.
     
  2. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Frequently the jack for the power supply has a switch built into it that disconnects the battery when the power supply is plugged in. The switched jack has 3 pins.
     
  3. DaveH

    Thread Starter Active Member

    Jan 1, 2009
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    0
    Oh right! You mean it's mechanical, I guess that's even better then there will be no drain on the battery at all. I was looking at some 2.5 mm DC sockets and there some were described as 3 pole and I was wondering what that was for. Thanks.
     
  4. Granz

    Member

    Apr 27, 2009
    15
    0
    What would be the best non mechanical way to do this?
     
  5. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    A schottky diode in series with the battery. As long as the external power supply voltage is higher than the battery voltage less the diode drop, there will be no conduction via the diode.
     
  6. Granz

    Member

    Apr 27, 2009
    15
    0
    But that situation is not always going to be the case. If the battery is fully charged, the external power supply may be supplying the same voltage as the battery.
     
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