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Thread: confused again. Cutting VIN from VUSB - Teensy 3.0

  1. #26
    Sorry for bumping this old thread but it has quite a high ranking in the search for the issue of using USB with an external power source. I wanted to document the approach I frequently use in case it is useful for others.

    First I cut to seperate VIN and USB as shown by Russ:



    I then add a connection to the VUSB via, if it's for a breadboard project I'll add a pin like so:



    (If it's for something more permanent, I'll attach my connection directly to the via or neighboring pad)

    Then if I want to power it via USB I can just jump from VUSB to VIN:



    If I want to power the teensy from an external source, I can just connect its VIN and ground pins to the appropriate source:


  2. #27
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    Does anyone have a surface mount diode part number that will solder directly onto the pads that connect VIN and VUSB?
    Does anyone know what that footprint is?

  3. #28
    Senior Member onehorse's Avatar
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    FYI, this would solve the problem too.

  4. #29
    Senior Member+ MichaelMeissner's Avatar
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    If you are putting the Teensy in a breadboard/perfboard, you could always not use a pin on the AGND pin, and put one on VIN pin. Then you can use a simple wire to connect the two pins on the breadboard (or diode). The lipo shield mentioned above by onehorse should also work (mine is being delivered today).

  5. #30
    Senior Member onehorse's Avatar
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    Michael,

    Since you are the first customer I hope it works as advertised! Please let me know if you have any trouble.

  6. #31
    Senior Member+ MichaelMeissner's Avatar
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    onehorse: ok, but I might not get to it until next week. Note, since I do things via breadboard/perfboard, I would be mounting it on top of the Teensy and not below it like the board is designed for. This means the LED might not be visible when mounted on the top, and I might need longer pins to mount the JST connector.

  7. #32
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    Trying to understand why the cut has to be done between those two pads.....
    Click image for larger version. 

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    I may be wrong but I would suggest to cut as indicated on the following schematic view:
    Click image for larger version. 

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    which is here on the board, between the two legs of the diodes ? Unless Paul has a better idea as I don't have a PCB picture without components on it to see clearly where to cut.
    Click image for larger version. 

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    In that manner whatever is connected to VIN or VUSB the highest voltage would be fed to VREGIN

    Just my 2 cents bright idea.

  8. #33
    Still confused after all this time . . . . I cut the trace as described above, reconnect the USB and the Teensy 3.2 continues to run and power the graphics display.

    I did not connect an external power source (in my case a battery)!

    What am I missing??

  9. #34
    Senior Member+ defragster's Avatar
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    First pic in preceding POST #26 shows where to cut the trace on the bottom of the Teensy. It is also shown on the reverse of the Teensy card.

  10. #35
    As I mentioned in my post I cut the trace per instructions. What I don't understand is why the Teensy is still powered. I don't want to connect the battery until I understand this. Is the Teensy still powered after this trace is cut and the USB Is plugged in without any connection to the Vin?

  11. #36
    Senior Member+ defragster's Avatar
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    opps - wasn't sure which description of trace cutting you used - the post #32 is a bit off.

    Unpowered - Do a continuity/resistence check across the two 'resolder' pads you cut between. If that is not open - you need to cut better. If cut the USB should not power the Teensy - and only talk to the Teensy with proper external power.

    If cut through and connected to USB - you should see VIN on one pad and not the other against GND.

  12. #37
    Senior Member PaulStoffregen's Avatar
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    Quote Originally Posted by Reefhermit View Post
    As I mentioned in my post I cut the trace per instructions. What I don't understand is why the Teensy is still powered.
    I don't understand why either. Unless you've got some other power source connected somehow, or some strange setup none of us can anticipate, the only plausible explanation is the pads aren't really cut apart.


    I don't want to connect the battery until I understand this.
    I don't want you to connect that battery either. Something is definitely not right here. Batteries usually can deliver a *lot* of current if things are connected wrong.


    Is the Teensy still powered after this trace is cut and the USB Is plugged in without any connection to the Vin?
    Ok, I'm having trouble parsing this sentence.

    I think you're asking what should happen, but it's pharased as a question about what's actually happening with your circuitry, which of course we're not able to see. We're working blind here over the internet, from only what you write. So clear language matters....

    So, if your Teensy has a USB cable plugged in, and if there's no other power sources, only the Teensy and stuff like displays that consume but do not source power, then nothing should be able to get any voltage.

    Look, there's only 4 wires in the USB cable. When VUSB is cut, that leaves only ground and the 2 data wires. On your PC side, those 2 data wires are tri-state with 15K resistors to ground when the USB port is inactive (waiting for a USB device). So all 3 wires are at zero volts. PCs detect when a USB device is connected due to a 1.5K resistors (which is inside the chip) connecting to 3.3V. When the PC sees the resulting 3.0 volts (due to the resistor divider of 1.5K and 15K), it begins trying to communicate. But before either data wire has voltage, the whole thing with all 3 wires just sits at ground. There's no voltage. Hopefully this long-winded description helps you understand how impossible it should be for your Teensy to get any voltage, to be able to run in any way, if the VUSB-VIN pads really are cut apart.

    The only plausible explanations are the pads really aren't cut apart, or you've got something else supplying power to the Teensy, or something is connected *very* badly in ways I can't anticipate working blind like this over the internet, from only the words you've written.

  13. #38
    All is good . . . Old age, bad eyes and too much pot��

    Just kidding on the last one!

  14. #39
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    Hi, hope this thread is not too old to re-bump. I had a similar problem where I cut the trace and somehow it still gets powered by the USB. I probed infinite resistance between the two pads I cut (and I also went over it a few more times with the knife just to be sure), but it still gets power from the USB. I've also "successfully" cut the trace before for so I'm not sure what happened here. I'm using a Teensy 3.2, Oshpark edition.

    Click image for larger version. 

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    What is the difference between 1 and 2? I think I cut 1. Was that incorrect? I wasn't sure which pad is what. My searches keep turning up https://www.pjrc.com/teensy/external_power.html which gives instructions for a different Teensy.

    Is there any way to buy the Teensys with the trace cut? We're working on a project where we'll have a lot of them and I don't want to have to personally cut a hundred traces, especially since I couldn't seem to do it properly And just to check- as an alternative to soldering diodes to the pads, I can use that VUSB pin right there and put a diode on my host PCB between VUSB and Vin, and another between battery (or in my case, regulator) and Vin?

  15. #40
    Senior Member+ defragster's Avatar
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    The proper cut is #2 - between the two similar pads.

  16. #41
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    Ah, got it- thanks for the quick response. So I'm guessing it's something like this?

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  17. #42
    Junior Member marmil's Avatar
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    Re: your question "also, what is this?" -- that's a fuse symbol, but it's not a true fuse.
    See this post:
    https://forum.pjrc.com/threads/28551-Teensy-fuse-values

  18. #43
    Senior Member+ defragster's Avatar
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    Yes, between A and B as marked - like the picture in prior post #26 … Cutting-VIN-from-VUSB-Teensy-3-0. Those pads seem to be uniform across the Teensy back to 3.0 as shown on the back of the card. But still a bit disturbing taking a blade to the PCB.

  19. #44
    Senior Member PaulStoffregen's Avatar
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    Quote Originally Posted by c0rnmuffin View Post
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    The "C" pad is half of the giant & never-used 100 uF capacitor, which is shown in light gray on the schematic.

    Ages ago, back in 2012 when Teensy 3.0 was designed, I thought we might someday wish to add a huge 100 uF capacitor for USB host mode. Back then, 100 uF usually came only in tantalum & aluminum electrolytic. Teensy 3.1 & 3.2 continued this design, even though we never actually used that capacitor.

    Finally, Teensy 3.6 added support for USB host and has a 100 uF capacitor. In recent years, 100 uF has become affordable and commonly available in ceramic. On that board, we also have a current limiter chip, which allows the 100 uF cap to used without causing too much trouble for USB device mode. A 100 uF capacitor should not be used without that chip.

    But the Teensy 3.2 board still has an unused place to solder a (larger tantalum type) capacitor. That's what the "C" pad is.

  20. #45
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    ah hah. yes, this was exactly the info I was looking for, and now I see the other pad for the 100 uF. Also glad that my problem had such a simple explanation.

    Thanks everyone!

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