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For my circuit the primary Power Supply is a 6V Solar Panel (Note that for the most part the Solar Panel is usually well below 6V but it varies anywhere between 6V to 0V depending on light conditions.)

However there will be times when I want to power the circuit using a simple 5V USB Adapter.

Here are the design requirements:

1) I will NEVER remove the Solar Panel from the circuit. 2) I want the Solar Panel to automatically disconnect from the circuit when the 5V USB Adapter is connected to the circuit. 3) I'd prefer not to use any IC's or things of that nature. I wanted to accomplish this using MOSFETs (preferred), diodes if necessary, resistors, and capacitors/inductors if necessary.

I've seen similar posts to this and they've been moderately helpful but I haven't found anything that has helped me with a fully functioning circuit as I've described in the requirements.

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  • \$\begingroup\$ If you plug in the USB but there's enough solar power available to run the circuit, is it a problem to continue using solar? If so, just use the usual two shottkey diode supply sharing circuit. \$\endgroup\$ – The Photon Apr 21 '17 at 5:29
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    \$\begingroup\$ related. \$\endgroup\$ – The Photon Apr 21 '17 at 5:41
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    \$\begingroup\$ I'm pretty sure this has been asked at least 3 times now. \$\endgroup\$ – Voltage Spike Apr 21 '17 at 6:11
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    \$\begingroup\$ I think you're under the count @laptop2d I know I've answered 1 very closely related one & at 1 other that can be considered a duplicate. \$\endgroup\$ – Robherc KV5ROB Apr 21 '17 at 6:21
  • \$\begingroup\$ @laptop2d you're missing a power of ten or two there \$\endgroup\$ – Marcus Müller Apr 21 '17 at 6:41
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I agree with Photon, just ad a diode in series from your solar panels with the correct current rating and you're good.

If you're aiming to use the 5V USB psu as backup for when the light levels drop to low, you probably need to ad some circuitry to turn off the USB supply. Another option is to run the entire thing off batteries (if your power requirements allow) and just keep them charged with the solar cells.

If those related questions don't help, come back and state your requirements in more detail.

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