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I have two power supplies that share the same load, an LED string. The one power supply (Circuit A) comes from the AC Mains which is rectified and with the help of a switching mode driver IC, it supplies a constant current to an LED string.

enter image description here

Should there be a power failure in the AC mains, I would like the second power supply (Circuit B) to AUTOMATICALLY kick in to power that same string of LEDs. The second circuit is powered from a 12V lead acid battery which with the help of another switch mode IC it provides a constant current to the LED string. Should the power in AC return, then it must switch back to the AC mains circuit.

My question is: Will I be able to perform this automatic switching using a relay or relays? If so, could you please guide me as to how I can go about this? If this is not a good idea, may I please ask for a better suggestion. In the third diagram I drew my thought pattern on how I was thinking to switch between the two circuits.

I apologise in advance for anything that might be unclear or vague, I will gladly explain more if need be.

Your help would be greatly appreciated! Thank you!

PS. The circuits I drew are not the full circuits, I just drew them roughly so that you can get the idea behind them.

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Generally the idea will work. You will need 2 relays that toggle between output A and B. 

A----o\    o-----B
       \
        |
        C

In the unpowered state the relay will connect the common pin to output A. This will form your switch state for the 12V backup supply. The relay is powered by your 230V Circuit A, if the relay is powered it will connect the common pin with output B.

What could create a problem is the moment the transition occurs. You may need to make sure your buck/boost circuits will not create peak voltages, especially if the switch condition is bouncing.

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    \$\begingroup\$ One question might be why you do not use a single driver to supply the LEDs, charge the 12V batteries with your 230V main circuit and also supply driver/LED with 12V. In that case you would continue to supply your LEDs from the battery without switching between two driver circuits. \$\endgroup\$
    – optronik
    Commented Oct 27, 2015 at 0:40
  • \$\begingroup\$ Thank you very much for your answer @optronik. Ideally, it would be much better to have used one driver I know, but I was struggling with that when I was designing my circuits, so I opted to do them seperately with two different drivers and then try integrate them with a switching mechanism. I am not too sure what you meant in your explanation about having two relays, could you please elaborate? Or are you saying I can use one relay that has two switches internally, one for the top node of my LED string, and one for the bottom node of the string? Then the two will switch simultaneously \$\endgroup\$
    – Sisqo9
    Commented Oct 27, 2015 at 22:09
  • \$\begingroup\$ There are relay that switch 2 or more independent circuits with one action. One example is the type "56.32" in the following data sheet (sorry, it is in German): cdn-reichelt.de/documents/datenblatt/C300/FIN56%23FIN.pdf \$\endgroup\$
    – optronik
    Commented Oct 28, 2015 at 18:41

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