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Thread: Teensy 4.1 PTC fuse getting really hot, is it repairable?

  1. #1
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    Teensy 4.1 PTC fuse getting really hot, is it repairable?

    Hi all,

    I am mentoring a group of students that are using the teensy 4.1 , they have been having problems with the VIN source exceeding well above 5 volts because of faulty circuits which led to damaging the teensy. The teensy doesn't get recognized by laptops anymore and the PTC fuse gets really hot when power is applied to it. The intended way to power the teensy was to power it with a 5V LM7805C Voltage Regulator, with a 12V input and a 5V output going directly to the VIN of the board.

    When inspecting the damage I could feel that the PTC fuse is getting really hot, the rest of the components are not. I am wondering what this means exactly, from existing posts on this forum I understood that the PTC fuse is rather a resistor that stabilizes the current and doesn't actually blow like a fuse. As the fuse is still getting hot I wonder if the fuse itself may be damaged and/or something on the board might still be damaged? Also, does the PTC fuse in a way save other components from getting damaged or is it more like an indication that something is already or still is damaged?

    Does anyone know some troubleshooting steps I could follow in order to find the part(s) that need to be replaced to revive the teensy, that is if it is possible at all?

    Thank you in advance!
    Last edited by nn28; 03-18-2023 at 01:37 AM.

  2. #2
    Senior Member+ defragster's Avatar
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    As noted on T_4.1 spec Card - VIN should be 3.6 to 5.5 V Max.
    Somewhere over 5.5V components will suffer damage.

    Damaged components can go in various ways - sounds like something has gone to shorted and is over drawing current through the PTC and causing it to block the current and get hot.

    This doesn't help ID the problem or nature of the damage - but seems to be showing something was indeed mistreated by high voltage explaining the hot PTC.

  3. #3
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    I am quiet new to the teensy boards, from my experience with Arduino boards when applying high voltage to the VIN you can clearly tell for example if an onboard voltage regulator has exploded either by the magic smoke, the puncture hole inside the regulator or it becoming very hot. The same goes for a damaged chip. But the only thing I could tell is faulty on the teensy is the PTC fuse, which gave me hope that maybe the components are saved by the fuse? In contrast, if something is still over drawing current then that definitely means something is still wrong. Or does the PTC Fuse itself also get damaged?

  4. #4
    Senior Member BriComp's Avatar
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    Can you try powering it via USB?

  5. #5
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    Powering it via USB also results in the PTC fuse getting too hot.

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    An update: I have quite a few teensy's laying around with this problem, for troubleshooting purposes I desoldered the PTC fuse and shorted it to see what other component would get hot. I have only powered it on very briefly but it turned out to be the TPDS014 current limit switch (see picture) for the USB port. Would replacing this component fix the teensy (getting recognized by the PC again) or are there typically more components that will be damaged which would make replacing each of them not worth it?
    Click image for larger version. 

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    Last edited by nn28; 03-18-2023 at 06:27 PM.

  7. #7
    Senior Member PaulStoffregen's Avatar
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    You could just remove the TPD3S014 chip. It's only needed for USB host power. Looks like you're not making use of USB host.

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    Quote Originally Posted by PaulStoffregen View Post
    You could just remove the TPD3S014 chip. It's only needed for USB host power. Looks like you're not making use of USB host.
    My students are using an external power supply indeed but to also use the serial monitor while using this external power supply I gave them USB cables of which the red wire is cut off. Is this still the correct way of doing things? Or is it not necessary anymore because the removal of the USB host?

  9. #9
    Senior Member PaulStoffregen's Avatar
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    Cutting the red wire is a good approach if using external power.

    Removing the TPD3S014 chip doesn't change anything.

    Also, normally TPD3S014 doesn't overheat, and it certainly should not overheat if you haven't even soldered pins to the USB host port. But voltage regulator right next to it is the part known to overheat if something is shorting the 3.3V power to GND. That voltage regulator has a bottom side thermal pad which couples heat to the PCB's internal ground plane layers. Any chance it is really the part overheating and you're just observing the higher temperature when you touch the taller nearby part?

  10. #10
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    I did remove the TPD3S014 chip and could feel another chip getting hot afterward see picture
    Click image for larger version. 

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    Quote Originally Posted by PaulStoffregen View Post
    That voltage regulator has a bottom side thermal pad which couples heat to the PCB's internal ground plane layers. Any chance it is really the part overheating and you're just observing the higher temperature when you touch the taller nearby part?
    I have more teensy's laying around in this state so I could soon double check it.

  11. #11
    Senior Member PaulStoffregen's Avatar
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    U4 is the chip expected to get hot if the 3.3V power is shorted anywhere on the board.



    Try measuring the 3.3V power. If you see 3.3V, then looking at chips other than U4 may be worthwhile. But if you get something much less than 3.3V, the heat is almost certainly coming from U4 and nearby parts probably also warm up.

    If you see only millivolts, odds are strong the short is metal, like stray solder or debris that fell onto the board. Usually those problems can be fixed, if you're able to locate the stray metal and remove it. But if you see something like 0.4 to 0.9 volts, odds are the short is a semiconductor, probably failure inside one of the chips. Typically a semiconductor short isn't repairable.

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