This is a very niche question but I'm wondering if there is a general princinal that can be applied to help answer it.

Here is my prediciment. I am making an Arduino on a breadboard. For the power supply, I am using this Breadboard Power Supply Stick. For the USB to serial I am using Board for RL USB to Serial. I have got everything working, sort of.

The issue is, that when the breadboard power supply is at all attached to the board, even if it is not plugged in, it makes the USB to serial connection unable to upload programs to the ATMEGA328P. If the power supply is switched off and even unplugged from the wall, the USB to serial connection does not work. The only way that the USB to serial connection works is if the power supply is physically removed from the board.

Why is this? How can I fix it? Ideally, what I would like to do is be able to power the mC and a few servos using the breadboard power supply(the USB alone can't power much), while still being able to program and read serial out from the chip on my computer while the program is running.

Thank you.

Picture of breadboard

Another picture Related Documents

Here is the schematic for the USB to serial connection http://dlnmh9ip6v2uc.cloudfront.net/datasheets/BreakoutBoards/FT232R-Breakout-v36.pdf

Here is the underside of the USB to serial board http://arduino.cc/en/uploads/Main/arduinobb_usbback.jpg

Here is the schematic for the bread board power supply http://dlnmh9ip6v2uc.cloudfront.net/datasheets/Prototyping/Breadboard%20Power%20Supply%20-%20SMD%20v13.pdf

  • \$\begingroup\$ Removing the red Vcc link form the USB interface to breadboard +5V rail shold have done the same thing. \$\endgroup\$
    – Russell McMahon
    Jun 18, 2012 at 16:32

1 Answer 1


The probable problem is that the USB VCC and BPSS (Breadboard Power Supply Stick) 5V output are hard connected together and the regulator on the BPSS acts as a load to the USB 5V supply. See "Vcc back-feed" diagram below.

Breaking the 5V feed between the two power supplies MAY be all that is required.
This may cause "issues if the two nominal 5V supplies are substantially different in voltage but will probably work OK.

Care needs to be taken in choosing which items work on which supply.
An active switching arrangement that connected the two supplies only when both were powered may be useful but should not be necessary.

enter image description here

  • 1
    \$\begingroup\$ Thanks that fixed it. I moved the USB to serial to another board and things are working together now. Thanks! \$\endgroup\$
    – Alexis K
    Jun 18, 2012 at 5:12

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