I'm currently creating a small device to operate on a CR2032 battery when it is disconnected and a USB power when it is connected. I want the device to be powered by USB when it is connected, without any interruption in the power feed. So my question is: how to switch from battery to USB one the device is connected?

I thought to connect two diodes and a PNP transistor to turn the power based on the battery as soon as the USB is connected. But I wonder if this circuit consume many current when the USB is disconnected (consumption must be very low)?

The circuit : Circuit with diodes and PNP transistor

If you have any solution which will consume less current please tell it to me !

  • \$\begingroup\$ There are several posts regarding this topic. You may want to search a bit. \$\endgroup\$
    – Rev
    Commented Apr 3, 2016 at 17:25

1 Answer 1


You don't need the transistor, it is sufficient to use one diode for each power source. Try to use Schottky diodes so the power wasted on the diode will be minimal

  • \$\begingroup\$ Thanks for answer. If i use only Schottky diodes, will the CR2032 be disconnected (no power waste) and will the power source be switched without interruption ? \$\endgroup\$
    – Thomas C
    Commented Apr 3, 2016 at 16:38
  • \$\begingroup\$ Yes to both questions \$\endgroup\$ Commented Apr 3, 2016 at 17:14

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