I am looking for a circuit for one of mine typical application. I have transformer power supply with 230VAC/DC input with 12V /1A and 5V/500mA output.

Now I wanted to introduce another stand by power supply from battery which gives 12V supply & acts like standby.

My end device will work on either AC/DC power supply. Can let me know how it can be internally combine to form common source with protection circuit.

I would like to use recommend use of relay so it take care of such issue.

  • 1
    \$\begingroup\$ I'm voting to close this, OP is looking for recommendations without providing any "foot work". \$\endgroup\$ – Sorenp Oct 17 at 5:47

I think you can simply use a diode bridge and a capacitor to get 12V DC from your transformer and connect to the battery. Since the 12V battery usually charges to 14.4V maximum DC voltage, your AC power supply will not damage the battery by over charging it. If the bat voltage is low, it will be charged up to a little over 12V and that's it.

For better results, use a 12V linear regulator after the AC power supply in the following manner.


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ i wanted to monitor state of AC / DC power . I have arduino uno board .if its working on AC supply serial print should print AC /DC else it will be showing battery. \$\endgroup\$ – Ajit Oct 17 at 8:32
  • \$\begingroup\$ @Ajit: This answer is suggesting that instead of switching sources with a relay, you simply have the battery always in the circuit, charging when AC is available, and powering the load when it is not. Unfortunately, this is a bad design. The 7812 won't charge the battery at all (you need about 13.7V to float-charge a lead-acid battery), and the battery will destroy the regulator when the AC disappears (you need to add an external diode from OUT to IN). \$\endgroup\$ – Dave Tweed Oct 17 at 12:15
  • \$\begingroup\$ I had the suspicion that this will destroy the regulator. But I've come across many ups designs that use a 7812 for charging without a diode. And yes, 12V output won't be enough to charge the battery to sufficient level, my bad. As a workaround a resistor with a small value can be added between GND pin of 7812 and supply ground to get a slightly higher voltage. \$\endgroup\$ – Arosha Dissanayake Oct 20 at 3:37
  • \$\begingroup\$ Better yet, I edited the diagram with two diode instead of a resistor for the GND pin. This will increase the output a bit. You may add another diode in series if the open circuit output is still not enough. Also assuming the battery will damage the 7812, a low drop schottky diode was added. @Ajit If you want to sense if the AC is available, put a resistive voltage divider before the schottky and and measure the voltage using a Analog pin of your UNO. \$\endgroup\$ – Arosha Dissanayake Oct 20 at 4:05

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