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How do you use a 12v 10a capacitor as a momentary backup battery for bridge

Discussion in 'Power Electronics' started by Brad Hruska, Feb 7, 2017.

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  1. Brad Hruska

    Brad Hruska

    Jan 26, 2017
    I first like to thank everyone for all there help thus far.

    I have completed a collapsing circuit it can run on 12VDC, 110/220ac to 12VDC transformer, or 12v Lipo batteries. There is 1 power supply line out. Thus using relays Lipo batteries are the primary power supply, then I can add a lead acid battery, or ac transformer to dc. when I add the AC line it automatically changes from Lipo to AC transformer.

    But here is the problem, there is a momentary break when the unit switch between ac transformer and batteries. So on the power out line, I would like to add a capacitor on the power out line, so to continue to supply power to the equipment while it switches over from one source to another.

    Thus, How large of a capacitor would I need the system is on 12v, and uses up to 10a for a .5 second break?

    Please include a part number or size please.

    Please include a schematic of what I need, I was thinking of just cutting the line and solder the 2 ends to the capacitor legs, but something tells me that I will need something to limit the voltage out to 12v on the line to the equipment.

    Please let me know.
    Last edited by a moderator: Feb 7, 2017
  2. duke37


    Jan 9, 2011
    dV/dt = I/C so C = dt/dV * I
    C = 0.5 * 10 / 1 if a 1V drop is allowed.
    C = 5 F = 5000000μF
    This is getting a bit big:)
  3. TCSC47


    Mar 7, 2016
    As Duke says!
  4. davenn

    davenn Moderator

    Sep 5, 2009
    it would be better to do some delayed switching between the 2 power sources
  5. Brad Hruska

    Brad Hruska

    Jan 26, 2017
    This is my schematic of what I have done, it works well except it needs a make before break connection.
    Please remember that all 3 power sources go out on one master wire to the different components.

    Daven you said to use delayed switching, I was thinking of using a capacitor and diode on the coil trigger but I don't know how this would work. The relays are Omron ly4 and LY2 12vdc 10a.

    Please let me know how to do this. Thank you all my relay set up.jpg
  6. Brad Hruska

    Brad Hruska

    Jan 26, 2017
    I was thinking of adding a momentary switch to bridge the connections, but I still prefer something automatic incase I forget to press the button.

    Possible relay set up.jpg
  7. Alec_t


    Jul 7, 2015
    Might be simpler to diode-OR the three supplies , if the load can tolerate a supply slightly less than 12V (say 11.4V).
  8. BobK


    Jan 5, 2010
    He already has them diode or'ed. The only question is which one would dominate if 2 or 3 are connected.

    Since they are or'ed, he is correct is saying that all he needs is make before break to make it work.


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