How can I power one 12 volt outlet (feeding a XM radio transformer) from two separate 12 volt sources on a RV (ignition and coach power). Is it feasible to use 2 diodes?

  • By connecting one of them?
    – Eugene Sh.
    Jun 15, 2015 at 19:53
  • Trying to power it by either 1)turning the ignition switch on or 2) using the coach batteries to power it on. The radio has a dual power input. Don't want to back feed.
    – Jim Samack
    Jun 15, 2015 at 19:58
  • Could you clarify your question a little? Is it from a source which provides 2-12v? Is it DC or AC?
    – George
    Jun 15, 2015 at 20:22
  • In an RV, it's from a 12V lead-acid battery bank or 14.4V automotive alternator. Both systems exist simultaneously and independently. One will come and go at random, as seen from the equipment; the other is more reliable but can still go away.
    – AaronD
    Jun 15, 2015 at 20:33

1 Answer 1


The diodes should work, but be aware that they will drop ~0.7V or so, so you'll actually get ~11.3V instead of 12V. If the radio is okay with that, and if you can dissipate the heat caused by the diode drop * current drawn (may be small, but must be known), then you're okay. It won't backfeed either source.

If you don't want the voltage drop, then you can use a relay like this:


This has a disadvantage of dropping out the load momentarily if In2 turns on while In1 is already on, but it does avoid a diode drop.

  • Looking to power the plug in transformer for the XM Radio from a cigarette lighter type adapter. Thanks!
    – Jim Samack
    Jun 15, 2015 at 20:40

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