Designing for multiple power inputs.....

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1

Hi All,

I have a project that is currently powered by a +12VDC wall wart power supply that is then converted to +5VDC by a linear regulator on my PCB. I’d like to make this project so that it can be powered either as described above, OR by connecting it directly to +5V via a USB connection.

It seems like I would need to diode isolate my onboard +5V regulator from the +5V coming in from the USB connector. That would mean that when powered by the +12V wall wart, the +5V on my board would actually be +5V minus the diode drop of about 0.6V, or 4.4V. I don’t think that’s going to work!

Is there any way to do this as I describe? The best I can think of is to use a Schottkey diode with a lower voltage drop, or to install a small switch to select the desired input power source.

Thanks,

John

2

Hello.

A Schottkey diode after the 5 volt regulator would be the easiest way to do this. You’d then inject your 5 volt USB power after the diode. That will prevent 5 volts from backing up into the regulator.

It’s a bit more complicated, but you could use a MOSFET to select the power source too. Have a look at how the Arduino Uno switches between the DC jack and USB power for an example.