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Thread: Cutting VIN to VUSB trace - or not

  1. #1
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    Cutting VIN to VUSB trace - or not

    What if I don't (didn't ) cut the trace from VIN to VUSB when powering from an external 5V source? I made a dual CAN board with its own 5V supply powered by the vehicles 12V. This supplies the CAN transceiver and also the Teensy 3.6 when USB isn't connected. I'm using the SD card to log the CAN data. However, there are times when I would like to log via serial. So what's going to happen when I have competing 5V supplies? It's handy having the trace still in place as I'm still developing the code, but when it's loose in the wild I'd hate to blow anything up. I may still be able to get a hook in there and cut the trace as the Teensy is raised up from the CAN board.

  2. #2
    Senior Member PaulStoffregen's Avatar
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    Quote Originally Posted by DaveAK View Post
    So what's going to happen when I have competing 5V supplies?
    Usually "not much" happens, especially if both sides are powered up. Perhaps a slight increase in power consumption can be seen sometimes. But if your PC turns off, or if both of your power supplies have very unusual design, then the result could be "something bad", possibly even damage to your computer.

    Another option is to cut the pads apart, then solder a diode between them so Teensy can still get power from USB, but power can't flow back into your PC. The downside to this is VIN will be about 0.7 volts lower when getting USB power, which isn't ideal if you connect a CAN transceiver chip or other stuff which wants 5V power.

  3. #3
    Senior Member+ defragster's Avatar
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    Another option is to lay waste to a USB cable. Skin it somewhere between the ends and find and cut the 5V power line and safely insulate it … without damaging the other wires. Using that cable when externally powered would have the same effect.

    I got a pair of Digistump dongles that have a swiitch on the PCB that diverts the PC USB 5V to an LED showing no power but leaving the GND and Data lines usable. These are not available anymore - but similar units may be.

  4. #4
    Senior Member xxxajk's Avatar
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    Cut the trace, insert a diode.
    Use another diode on the other side.
    The source with the most current wins.
    As an additional bonus, if you lose power on either side, the Teensy will remain powered.

  5. #5
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    Quote Originally Posted by PaulStoffregen View Post
    Usually "not much" happens, especially if both sides are powered up. Perhaps a slight increase in power consumption can be seen sometimes. But if your PC turns off, or if both of your power supplies have very unusual design, then the result could be "something bad", possibly even damage to your computer.

    Another option is to cut the pads apart, then solder a diode between them so Teensy can still get power from USB, but power can't flow back into your PC. The downside to this is VIN will be about 0.7 volts lower when getting USB power, which isn't ideal if you connect a CAN transceiver chip or other stuff which wants 5V power.
    I was thinking about the diode option. This would probably work fine because the CAN transceivers are powered directly from the 5V regulator on my board so I would only be powering the Teensy with the reduced voltage. (CAN would only be operational when connected to the vehicle.) The 5V supply is just the recommended capacitors and resistors from the data sheet, so hopefully not that unusual! Alas though, it's all soldered up now so this would have to be on the next iteration. I have some 0603 diodes that could probably be soldered directly to the pads.

    Quote Originally Posted by defragster View Post
    Another option is to lay waste to a USB cable. Skin it somewhere between the ends and find and cut the 5V power line and safely insulate it … without damaging the other wires. Using that cable when externally powered would have the same effect.
    This is a great idea. I have plenty of cables I can sacrifice.

    Thanks guys!

  6. #6
    Senior Member+ MichaelMeissner's Avatar
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    I've seen this on tindie.com that gives you a switch for the power:


    You can also build your own version with some DIY USB connectors:


    If you do do surgery on a cable, be sure to label it, so you don't wonder why you can't power some other device with it.

  7. #7
    Senior Member+ defragster's Avatar
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    Quote Originally Posted by MichaelMeissner View Post
    I've seen this on tindie.com that gives you a switch for the power:


    ...
    If you do do surgery on a cable, be sure to label it, so you don't wonder why you can't power some other device with it.
    The tindie 'USB Switch Buddy Kit' is about like what I got as a kit to solder, very handy. I put a female header across the switched 5V - I use that to connect DMM to measure current draw with meter completing the circuit when the switch is open.

    Depending on how the cable is edited it could use tape to seal the cut - or if peeled open will need strain relief loop and a tie wrap ... something making it obvious in either case.

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