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Thread: Teensy 4.1 hot voltage regulator and 4.7v on 3.3v pin

  1. #1
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    Teensy 4.1 hot voltage regulator and 4.7v on 3.3v pin

    I have a Teensy 4.1 wo ethernet I got from a friend who said it wasn't working correctly. Well, friend of a friend so I don't have much information on what it was used for when it started acting up. The pads are cut to separate vin from vusb so I'm assuming it was powered by some kind of external battery or other supply. I do have a couple other Teensy's myself so I know my setup should work properly.

    It does power on and work with 5v on Vin or usb by jumpering the Vin/Vusb pads. I reset it by holding program button and the blink program seems to be working properly.
    I noticed the voltage regulator (or something around it) was getting pretty hot pretty quick.

    I was able to pull voltages and am reading 4.54v on the 3.3v pin. It was pulling around 410mA from the power supply. I'm reading around 11.1ohms from 3.3v to Gnd. The program button doesn't seem to change the voltage on the 3.3v pin but does reset the Teensy program. It stops it and the red led by the usb lights up, but the program doesn't seem to restart after releasing the program button. Cycling the power restarts the blink program. I think I saw on one post here where that program button should put 3.3v to gnd possibly but not for sure.

    Again, it is powering up and connecting to the pc via usb. I do not think it came with pre-soldered leads so I looked over and tested the ones that have been soldered on. They appear and test ok, but I guess something could still be wrong there. I've read a lot on here with low or no voltage on the 3.3v pin but nothing I've seen reading higher than the 3.3v.

    Just wondering if anyone has any suggestions on where to start troubleshooting. I haven't really left it on for very long as I don't want to completely destroy it since it's getting so hot. I'm attaching two pics here as well.

    Thank you in advance for any help or suggestions.
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  2. #2
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    Just wanted to add I'm getting no short between 5v Vin and 3.3v pin.

  3. #3
    Senior Member BriComp's Avatar
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    Check for a short on the soldered pins. Those to the left of the SD card slot don't look very good. A small slither of solder can cause these sorts of problems.

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    Ok, just got a chance to go through it again. Actually found my multi-meter was not working properly on continuity so had to get a new one. Also got a microscope to get a better look at things. I found a couple very thin (hair like) strands on two pins but they weren't touching anything but did get rid of them. Not sure they were long enough to touch something but could very well have.

    Now I'm seeing 3.3v shorted to Gnd. It doesn't appear to be from the pin solder joints. Could it be the voltage regulator chip? I'm getting continuity between pins of U4 (GDT) 1, 2, 4, 5, 7, 8 and then GND. If pin 1 is top left reading GDT on the chip.
    Not sure if that's just because something else is shorting 3.3 to gnd or what.

  5. #5
    Senior Member PaulS's Avatar
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    Quote Originally Posted by skiwe View Post
    Could it be the voltage regulator chip?
    The NCV8186 regulator is over-current protected therefor it is rather unliky to fail internally.
    I would look really close for shorts on the rest of the board. There could even be metal particles under the processor package, shorting BGA pins.

    Paul

  6. #6
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    I looked through things and nothing jumped out. I tried to get some better pics to see if you all see anything I should check.
    Thanks again for the help and suggestions.
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  7. #7
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    Finally got a chance to work on this a bit more. Wasn't able to find anything so pulled U4 off the board. The short is gone now and I can power up via the 3.3v line and everything seems to be working with no high heat anywhere. I can give it 5v and program as well.

    Does this hurt anything if I just don't utilize the 5v except for when I want to program? I'm not utilizing the usb interface for anything else.

  8. #8
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    I did look through the post about startup sequence potential issues with using th 3.3v pin. Just curios if there could be any issues with the voltage regulator U4 being removed?

  9. #9
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    Aha. I would not be surprised if your U4 LDO died under the same 'reverse current' scenario that I posted on yesterday/today https://forum.pjrc.com/threads/71253...6AMN330TAG-ics

    Once you've had 4.7v on 3.3v pin, expect more damage has been done already to the other Teensy chips...

  10. #10
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    Interesting. I got this board from a friend of friend and have been trying to get any information on what happened to it and how it came to die...

    I have some projects where I power other circuits off the 3.3v line so could be susceptible to this though. I have toasted a 4.0 board in the past but figured it was my shoddy soldering on a test circuit but it had a few things powered off the 3.3v line as well. I've started running a separate 5v to 3.3v voltage regulator to isolate everything else as well. Mainly because of some potential draws exceeding the teensy output, but that may have fixed this glitch without realizing it.

    Sounds like you are now adding this Schottkey diode from 3V3 to VIN on all of your boards. Is that correct?

  11. #11
    Senior Member BriComp's Avatar
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    I've started running a separate 5v to 3.3v voltage regulator to isolate everything else as well.
    When powering down ensure that the secondary 3.3V supply goes down before the Teensy 3.3V otherwise you risk supplying 3.3V to the Teensy pins after the Teensy 3.3V has gone down.
    Applying 3.3V to Teensy when it is not powered will damage the pins and possibly more.

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