dual power sources... voltages added? or higher used?

Discussion in 'General Electronics Chat' started by s_mack, Feb 24, 2013.

  1. s_mack

    Thread Starter Active Member

    Dec 17, 2011
    198
    6
    Stupid question I'm sure.

    I have two potential power sources (USB and battery) and I want certain parts of my device to work when one power source is active and other parts to wrok when the other is active.... so I have logic iCs deciding what signals go where depending on which power source is active. That's all fine... but the ICs themselves need to be powered at all times to do their job. So I figure I'll have each supply run through a common cathode diode and that way I have an output no matter which is on.

    But then I started to confuse myself (easily done) :)

    So yeah... V1 is +5v and V2 is somewhere from +4 and +7 (suitable for the logic IC) depending on battery charge. What is Vout? Is it the higher of the two or the addition of both? If the latter, then this won't work and I'm open to suggestions?

    - Steven
     
  2. s_mack

    Thread Starter Active Member

    Dec 17, 2011
    198
    6
    I'm thinking it'd be an average, now that I look at it. Right?
     
  3. crutschow

    Expert

    Mar 14, 2008
    19,357
    5,394
    The voltage would be the higher of the two, minus the forward drop of the diode. To minimize that drop you can use Schottky type diodes.
     
  4. s_mack

    Thread Starter Active Member

    Dec 17, 2011
    198
    6
    why can't I wrap my head around this? Why is it the higher?
     
  5. kubeek

    Expert

    Sep 20, 2005
    5,221
    976
    Because the diode of the lower one then becomes reverse polarized.
     
  6. s_mack

    Thread Starter Active Member

    Dec 17, 2011
    198
    6
    Ah, yes.... of course. Thanks.
     
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