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my device can work from external power source as well as from two AA batteries, so voltage in this case is 3V, but external supply is 5v.
I need to protect my circuit and power supplies in case of connecting all power sources together at a time.
I have restricted space in my device, so the circuit cannot be relatively large.
Please suggest what to do in this case. I would be grateful for any help.

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  • \$\begingroup\$ Many power sockets for battery operated equipment have a pair of contacts that are open circuited when an external plug is inserted. These are used to disconnect the internal battery when a power supply is used. \$\endgroup\$ – Russell McMahon Apr 8 '15 at 0:07
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This circuit should do what you want.

The two diodes steer either the output of the boost regulator or the external supply to the load. The typical V\$_{F}\$ of the diodes is 100 mV at a 100 mA load, which should not have much of an effect on your circuitry.

The two diodes are available in surface mount.

enter image description here

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  • \$\begingroup\$ Thanks for answer, but I don't need to have 3V output, my devices is running on 5V, so I have boost converter from 0.9 to 5V. Maybe there is any solution to come over this boost converter if I has 5V as input. \$\endgroup\$ – solderingiron Apr 8 '15 at 0:39
  • \$\begingroup\$ Please help to find best solution. \$\endgroup\$ – solderingiron Apr 8 '15 at 0:40
  • \$\begingroup\$ @solderingiron ok, your post was a little confusing to me, it said the device works on 3v batteries and there is also an external 5v supply. So I incorrectly assumed the device was working off the 3v battery voltage. I'll modify my circuit, it will just sort of be backwards from what it is. Really just a matter of adding the Schottky diodes to the output of the boost regulator and the 5v external supply. What boost regulator are you using? \$\endgroup\$ – tcrosley Apr 8 '15 at 0:58
  • \$\begingroup\$ @solderingiron I have updated my circuit to reflect your earlier comment. Since I didn't know the part number of the boost regulator you are using, I just through in a generic circuit which should be close to what you are using. Depending on the part you are using, there may be another way of doing this by driving the EN lead of the regulator via an inverter from the external supply. \$\endgroup\$ – tcrosley Apr 8 '15 at 1:29
  • \$\begingroup\$ Great, thank you for your help, you've helped me a lot. Have a nice day \$\endgroup\$ – solderingiron Apr 8 '15 at 4:17

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