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I'm new here and my knowledge in electronics is basic, so what I'm trying to do is to switch between two power sources, one at 5V an other at 12V, I already made a circuit that kinda works, but I have to use a diode on the 5V source to lower the real voltage and I don't want that. Is there any way to do that without draining the 5V source?

It would be no problem if the draining would be on the 12V source. Circuit

L.E Looks like that I was wrong, the 3V source does nothing there, I've tried other configurations and I've got nothing, seems that the easiest way to do it is with a relay.

enter image description here

Also I'll try using a Schottky diode as @gbulmer told me in the comment.

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    \$\begingroup\$ A simple solution is to use a relay to select the input voltage. A problem with a MOSFET is it conducts with sufficient reverse voltage. To reduce the voltage drop fron 5V use a Schottky diode; there will still be a voltage drop, but it will be significantly smaller. \$\endgroup\$ – gbulmer Mar 6 '15 at 13:10
  • \$\begingroup\$ @gbulmer Thank you, i'll try using a relay and a Schottky diode. \$\endgroup\$ – Sinjuice Mar 6 '15 at 17:14
  • \$\begingroup\$ if you use a relay, there is no need for the Schottky diode. Assuming a Single Pole Double Throw Relay with the two voltages connected to the 'Throws', switching the relay on or off should select between the two voltages. In this case, a diode is not necessary as only one input voltage can be connected to the rest of the circuit. \$\endgroup\$ – gbulmer Mar 9 '15 at 19:52
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The problem with your +3V source is that it is ground referenced.

A MOSFET turns on based on a voltage differential between its Gate and Source pins. If you have access to a 15V supply, that'd turn on the FET if its datasheet calls for 3V Vgs.

You can use a schottky diode to prevent a voltage drop in the +5V rail.

If you're using a physical switch on the 3V, why don't you just use that switch on the 12V with a schottky?

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Using a p challel mosfet on 12Volt and shotty on 5V is sensible BUT if you cant have the volt drop of the shotty and don't want a relay you do have options A diode connected germanium PNP popwertransister das worked for me on two jobs one of which got published BUT if your junkbox doesn't have these you could use a mosfet solid state relay

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