rbbb powers and runs off usb but not off external power

Questions about the Really Bare Bones Board
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Joined: Sun Jan 10, 2010 12:46 am

rbbb powers and runs off usb but not off external power

Post by cain8885 » Sun Jan 10, 2010 12:50 am

So I have an RBBB built up and off of usb it runs fine. I plug the cable into the programming header and everything works great.

In assembly of the board, i purposely left off several parts that seemed only related to the power jack/ power regulation assembly. This includes the regulator, the diodes, the 2 large caps, and R1. So basically the bottom half of the board is empty. As I said earlier, it does work fine when using USB.

So I program the board via usb, and disconnect the programming header and hook the board up to my external power. I have +5v and gnd going to both sets of +5v and gnd pins on the top of the board to the left and right of the programming header. Yet, nothing happens. My power source is coming from a 12v wall adapter that is being fed into an LM7805 that is powering the rbbb.

Is there something obvious I am missing as to why this will not work? I don't understand what is stopping the arduino from powering on?

Any assistance would be greatly appreciated.


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Joined: Mon Jul 28, 2008 6:52 pm
Location: Western New York, USA

Re: rbbb powers and runs off usb but not off external power

Post by floresta » Sun Jan 10, 2010 11:27 am

If it works when you feed 5v from the USB cable it should also work when you feed 5v in from the pins you specified since they actually provide a shorter path to the IC.

There is a possibility that your 7805 is oscillating. You didn't specify whether your external 7805 has the required input and output capacitors. If these are missing you might try adding them back. Also, you didn't say anything about C3 which is part of the power supply but also bypasses the power pins of the IC. You really should install C3 if it is missing. If you need the real estate you can tack it on under the board between the +5v and Ground lines.


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