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I have to work on a circuit that basically goes like this. I have two voltage sources, a 9V battery and AC. The AC is the main source, however, when this is unplugged, the 9V battery must come to the rescue after 5 seconds. For this work, I truly have no idea of where to start with, so it would be great if your could guide me. I'm not asking for anyone to solve my problem, but if you could help me with a starting point that would be very welcome.

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    \$\begingroup\$ (1) Why do you want a 5 s delay? Most people would want instant transfer to keep the circuit alive. (2) What current? (3) What voltage comes from the AC source? Is it greater or less than 9 V DC? (4) What are you powering? \$\endgroup\$ – Transistor Nov 11 '17 at 0:01
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    \$\begingroup\$ Often backup uses transistors, diodes or FETs to act as logical OR analog switches for sourcing voltage. "High man wins" w.r.t. voltage \$\endgroup\$ – Sunnyskyguy EE75 Nov 11 '17 at 0:26
  • \$\begingroup\$ @Transistor Hello. The 5 second delay is just part of the requirements for this work. I believe I know how to make that part. I'm not sure of the current, the AC source gives 120V at 60Hz, and I'm powering a basic bulb. It's a uni work so I guess it's not meant to be super efficient. \$\endgroup\$ – Fernanda Nov 11 '17 at 2:55
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Use something like the Linear Tech LTC4420

enter image description here

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