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Thread: External power and USB

  1. #1
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    External power and USB

    I've put together a few projects requiring that I cut the 5V pads as shown here. There have been times when I would have liked to temporarily reapply usb power, for example, when uploading a sketch while the external power is not present.

    One workaround is to include a jumper pin on a breadboard that allows usb power to temporarily be reapplied (via the VUSB pin on the Teensy). However, it would be really convenient if there were adjacent VUSB and VIN pins on the Teensy so a jumper could be added directly to the board. This would also allow a Teensy with cut 5V pads to be moved to another project requiring USB power, without resoldering the pads back together.

    In fact, the idea of an on-board jumper could conceivably already be done on the current design since the VUSB and VIN are diagonally adjacent. Maybe with an extra-wide jumper...

    This is just a thought for future board designs. I recognize it may not be practical, since these boards are super crowded already, and there is some desire to not move the pins around for backwards compatibility. But it is an idea to put out there.

  2. #2
    Senior Member+ Theremingenieur's Avatar
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    3 Schottky diodes should do it:
    Click image for larger version. 

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    The upper diode from Vusb to Vin is not forcibly needed, it is thought as a helper for those who expect to feed peripherals connected to Vin from the USB supply

  3. #3
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    Thanks! This looks like "Option #3: Two 1N5817 Diodes" on the "Using External Power and USB" link I referenced in my post, with the additional diode on the top. Yes, that does look like a good solution. I do tend to rely on the 5V power for other peripherals, and as long as I can afford the additional diode drop this would appear to be a good solution. Wouldn't there be some risk that Vusb-Vd > Vin, where Vusb would start driving Vin?

    Other boards have solutions to this issue (e.g., see Uno schematic). I figure there must be some really good reason the Teensy does it the way it does. It really is just a minor inconvenience for me, but I would prefer not to have to think about it at all.

  4. #4
    Senior Member+ Theremingenieur's Avatar
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    The risk of Vusb driving Vin is there. But as always in life, there are no 100% fool proof solutions. In my designs, this third diode does not exist. So, either everything is powered from Vin, or Vusb is applied, just for flashing the Teensy. Vusb is never used to power external peripherals.

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