# 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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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 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.

Last edited by a moderator: Feb 7, 2017
2. ### duke37

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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

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Mar 7, 2016
As Duke says!

4. ### davennModerator

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

60
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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

60
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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.

7. ### Alec_t

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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

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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.

Bob

Bob