enter image description hereI have designed a PCB. Now I am facing a issue that I want to use 12V PSU and 12V battery together so that if 12V PSU fails then 12V battery can supply power to PCB.Battery charging is not part of the project as i will remove battery when it is discharged and will attach to PCB after it is fully charged. Current requirement of my PCB is 3A. Now the issue is

How can I setup both power sources in my schematic that if PSU turns off then battery can supply power to whole PCB, Keeping in mind that my current requirement is 3A?

Basically, I need some switching circuit but with high current requirement.

  • \$\begingroup\$ How fast does the changeover need to happen, and what is the minimum voltage it can work with while the changeover is happening? \$\endgroup\$ Apr 20 at 13:53
  • \$\begingroup\$ I am operating a NIMA17 stepper motor and a 5V servo motor with the PCB using esp32-s3 module. Changeover should be fast enough maintain PCB operations without intereption.@AndrewMorton \$\endgroup\$
    – ROBERT
    Apr 20 at 15:00
  • \$\begingroup\$ You'll need to add a block diagram to the question so that we can see exactly where the power rails are going, including the charging for the battery. \$\endgroup\$ Apr 20 at 15:37
  • \$\begingroup\$ You will probably want a charge controller chip with automatic power path control and battery charging functionality built in, since you probably won't want to be unplugging and topping up the backup battery every few weeks. Specific recommendations are off-topic but this is an extremely common use case and there are products out there; I'm looking at one now. \$\endgroup\$
    – vir
    Apr 20 at 17:47
  • \$\begingroup\$ I dont want battery charging circuit. When battery is drain then i will simply remove it and will attach again with pcb after it is fully charged. I want to add the circuit in my PCB. I am attaching block diagram of system so that you can better understand \$\endgroup\$
    – ROBERT
    Apr 20 at 20:56

1 Answer 1


Probably the simplest solution is to use two diodes. However, in this case, it would be good for the PSU voltage to be slightly higher than the battery so that it has a higher priority for power delivery. enter image description here

  • \$\begingroup\$ My current requirement is 3A. Will i get any diode which has forward current ogf 3A? \$\endgroup\$
    – ROBERT
    Apr 22 at 20:40
  • \$\begingroup\$ I think that diodes with a current above 5A and a voltage above 30V would be the best. My favorite is a double diode from an old AT/ATX power supply, e.g. SBL2030CT, BYV118. \$\endgroup\$
    – nexus9
    Apr 24 at 19:58
  • \$\begingroup\$ There is one more question in my mind. My NIMA 17 Stepper motor is operating at 12V. So these diodes will drop about 0.6V. So i will get approx. 11.3 to 11.4V. Will NIMA 17 stepper motor work with 11.3V instead of 12V? \$\endgroup\$
    – ROBERT
    Apr 24 at 22:00
  • \$\begingroup\$ The voltage drop of 0.6 V for 12 V (5%) is negligible in most applications, and certainly for the motor power supply. \$\endgroup\$
    – nexus9
    Apr 24 at 23:02

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