I'm building a small project where I have 12v @ 8amps batteries and on some occasions, I would want to plug in an adapter so I can still use my motor board in the absence of not having a fully charged set of batteries. I want to isolate both power sources too.

What options do I have available? I'm currently looking at some power multiplexer chips (I might have the terminology wrong) but I cannot find any that will meet my requirements.


1 Answer 1


Use an electromechanical relay. When the power adapter is not plugged-in the relay is unpowered and the normally-closed contact can route battery power to the load. When the adaptor is plugged-in the relay activates and disconnects the battery power in favour of the adapter power: -

  • NC contact to battery positive
  • NO contact to adapter positive
  • Common contact to load
  • Relay coil powered by presense of adapter
  • \$\begingroup\$ Ah right! I like this idea. Just a quick question? Would the time between switching be quick enough to not affect other components like MCUs. I have one connected to monitor the rotary encoders. \$\endgroup\$
    – nasher128
    Sep 15, 2017 at 15:55
  • \$\begingroup\$ You need to ensure you have enough power supply capacitance on your load so that the 10 ms changeover won't cause a significant dip in the power rail when the relay switches between A and B. It's easy enough to test. Try without capacitance added and if it fails add 100 uF etc.. \$\endgroup\$
    – Andy aka
    Sep 15, 2017 at 16:13

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