Feeding the mbed with a battery

11 Oct 2012

I am supplying power to my mbed with a 9V battery. It works fine. However the "5.0v USB Out" pin does not supply 5.0 volts but much less. Then I cannot power my lcd 16*2 display. Does anyone know the reason and how to fix it?

11 Oct 2012


This is because the VU pin is powered from the USB connector, not from VIN. It is only +5v when the mbed is plugged into your computer.

The residual voltage you are seeing is caused by small leakage currents through the protection diodes in the power circuit.

To supply your LCD you will need and external 5v regulator to make a 5v rail from the 9v battery. You can do this easily with something like a 7805 or LD1117S50 linear regulator. If you want a challenge you can investigate using a more efficient "buck converter" which steps down DC voltage at 90% efficiency.

Hope this helps!

Thanks, Chris

11 Oct 2012

It helps. Thanks a lot!

11 Oct 2012

I also successfully operate my mbed + LCD directly on 4xAA cells which have a much higher energy density than normal 9V layer types. Re Chris's switching regulator comment, I'm waiting for this months Elektor magazine where they are publishing a switchmode 7805 circuit in a similar size to one of the tab mount packages.

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