Greetings,

Maybe this is a dumb question. Not an electrical engineer here. Just dangerous enough to possibly fry my mbed. I'm prototyping my own baseboard for the LPC1768 mbed.

I've added a barrel power connector and hooked it up the mbed GND and VIN. The mbed board gets powered up nicely. Would it be a bad thing to plug in the USB cable to the mbed that also supplies power to the board while the barrel jack is powering the board?

I want to be able to communicate with the host via the USB. But I hope I don't have to make a special USB cable that disconnects the power from the USB plug.

Regards, Jim

Youre good: theres a couple of protection diodes on the mbed that make sure nothing bad happens when you provide power on USB and Vin. Just note that Vin should not exceed 9V or so depending on current you draw and note that VU (5V) is only present when USB power is supplied.

Thanks for the information Wim. I appreciate it.

-Jim

So if I plug in external power and USB power which source is used? Or both? Looking over the schematics it seems that Vin and VBUS go through the same voltage regulator providing +3.3V to the board.

Also, I guess I don't need VU (5V) as I'm powering the board with a 5V wall wart. Would anybody recommend that I supply the board with 6V and use a high quality regulator for clean 5V or should most 5V wall warts provide adequately regulated voltage?

Regards, Jim

When you supply both USB and Vin the highest voltage will be used to power the mbed and provide the external 3V3 out. External supply could provide clean 5V, but simple wallwarts are sometimes badly regulated. Make sure you have good quality supply like those used for cellphones and such. Also try to make sure that there is no voltage applied to mbed pins from external powered devices when the mbed power has been switched off. This can happen when you use different powersupplies for mbed and external devices and they dont switch on or off at the same time.

Hmm, I providing 5V to Vin which is the same as the USB provided voltage. Regarding making sure to to switch everything off at the same time... maybe it would be safest to just make a USB cable without the power hooked up and always power the mbed with the 5V barrel connector I've added?

This way everything is safe and I can still use the USB for serial to a PC?

Regards, Jim

Cutting the 5V USB supply will disable the USB port on the mbed. Two power supplies of 5V is ok. Just make sure that the supply for external hardware will also keep mbed powered up.

Got it. Thanks Wim.