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Thread: External Power Newbie Lesson

  1. #1
    Senior Member pictographer's Avatar
    Join Date
    May 2013
    San Jose, CA

    External Power Newbie Lesson

    I have an externally powered Teensy 3.0 project. When attempting a recent software upgrade I noticed it would no longer work with external power unless the USB serial cable was also attached. Since VIN and VUSB were cut, I didn't expect the project to work with only the USB cable. At this point, I think the experts already know what happened here, but it wasn't immediately obvious to me.

    I checked the external wiring. Tested the power source I was using. Uploaded the Blink sketch to rule out my software. Poked around checking continuity and voltage.

    Then it occurred to me to try connecting the USB cable and the external Vcc lead, but not the external ground. Ah ha! It started blinking. Indeed, my external ground connection was broken.

    Lesson: the USB cable still provides a ground connection even when power is not supplied by the USB cable.

  2. #2
    Senior Member xxxajk's Avatar
    Join Date
    Nov 2013
    Buffalo, NY USA
    The easy thing to do is place a diode across the pads, and to diode protect your external source too.
    After you do that it can be powered from either, automatically!

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