I was curious about this. I've searched other similar questions but they weren't quite the same. One mentioned the use of diodes but I would like to know if this would work first before adding diodes.

Can I hook 1 device (12v) to 2 different power sources supplying 12v each.... Would this fry the device?

Scenario: I have a 12v led light strip. It's currently hooked up to a 4.2v li-on battery hooked up to a transformer that outputs 12v. This circuit has an on/off switch.

Now i want to add a dc input to the light strip also. The dc adapter will be supply 12v directly to the light strip (bypassing the transformer).

I was wondering, if this dc power source is already supplying the 12v to the light strip, and i turned on the battery power source, would this fry the device? or would it still be a total of 12v going into the device and is safe to use? Which power source would it be pulling from, maybe split between both? What if both power sources are identical in voltage and amps?

Thank you, i'm afraid to experiment with this to see what happens haha.

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  • \$\begingroup\$ What happens if the output of the "transformer" (which, by the way, isn't just a transformer) is 100mV less than the DC adapter? How much current will flow, and from where to where? \$\endgroup\$
    – uint128_t
    Feb 26, 2016 at 3:05
  • 1
    \$\begingroup\$ two diodes, one each at the output of the supply(one at battery output and another at the second source output) will do not harm. What is the purpose of the transformer? \$\endgroup\$
    – User323693
    Feb 26, 2016 at 3:05
  • \$\begingroup\$ great thanks, i guess i will use diodes just to be safe, do you have a recommendation on which diode? -- the transformer is to grab the 4.2v input of the battery to output 12v to the light. \$\endgroup\$ Feb 26, 2016 at 3:07
  • \$\begingroup\$ @unit128_t, from my research from other similar questions, the power source with the higher voltage will supply the device until it is no longer supply the highest voltage (or disconnected/turned off... fried) \$\endgroup\$ Feb 26, 2016 at 3:09
  • \$\begingroup\$ two diodes at the output of the transformer. Because the 12v from the adapter if its slightly less, then transformer and battery might act like a load and its starts to charging. And also you can add diodes with adapter as well (for more safety). \$\endgroup\$
    – Honeybee
    Feb 26, 2016 at 5:56

1 Answer 1


You can use ideal diode circuit LTC4357/4359. The 12VDC has to have a slightly higher voltage then your "transformer" if you want 12VDC to take over.

  • \$\begingroup\$ Bursic, thank you so much!! I will do that right now, but just to make sure, do the diodes have to be on both pos and neg leads? Also, what if i set the transformer to 11v, it won't try to charge the DC adapter would it? Meaning, do i need to place diodes on the DC adapter also? Thanks again for all your help! \$\endgroup\$ Feb 26, 2016 at 21:09
  • \$\begingroup\$ Yes of course, place the diode at each source. \$\endgroup\$ Feb 27, 2016 at 8:18

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