I saw a circuit on a website to make a backup battery circuit for a rechargeable 12v battery circuit. Circuit 1

However, I don't anticipate regular power outages, so I believe that one non-rechargeable 9v battery will last for more than the life of this project. I came up with the following circuit modeled after the first circuit (And a voltage regulator circuit I saw):


simulate this circuit – Schematic created using CircuitLab Is it dangerous to mix different voltages like this? I heard it could be for transformers, but what about a battery? Would the circuit only use the battery if the transformer is disconnected, and finally, does it look like it would work?



Your circuit looks fine to me !

The diodes are a bit over specified though, the 1N5406 can handle 3 A, in your case 1 A should suffice (the LM7805 is not going to pull more than 1 A). So you could use 1N4001 instead. But if you have the 1N5406 lying around, you can use those, it will work just as well.

Just build it and enjoy :-)

  • \$\begingroup\$ Thanks for the recommendation! I had already ordered the 3A diodes before I made the circuit, since I mistakenly thought that I needed 3A because the transformer is 12V 3A. If I ever build more of the circuit though, I will go with the 1A diodes as recommended! \$\endgroup\$ – DarthCaniac Jul 6 '15 at 18:48

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