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Thread: Diagnosing cause of short circuit in defective custom Teensy 3.2

  1. #1
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    Diagnosing cause of short circuit in defective custom Teensy 3.2

    I've gotten one of my devices returned back to me as being defective. I opened it and noticed the voltage regulator that brings the USB +5v down to 3.3v was overheating. It turns out that there was a short circuit across the voltage regulator. (see pic below)

    Does anyone have any idea of what a short circuit there implies? Is it possibly a defective regulator? Does this indicate of particular type of failure? I'm attaching a snippet of of the schematic and the board. I would love to hear any experience with this kind of failure. Thank you in advance for taking the time to help.

    Regards,
    Rob
    Click image for larger version. 

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  2. #2
    Senior Member+ defragster's Avatar
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    Had the device been given some power on testing before shipping. Run in the hands of the end user for any time? A large number of others made and working okay?

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    Quote Originally Posted by defragster View Post
    Had the device been given some power on testing before shipping. Run in the hands of the end user for any time? A large number of others made and working okay?
    Ya, the device was powered up and programmed--maybe it saw 10 seconds of running time. And ya, I have sold a couple hundred others without issue.

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    Quote Originally Posted by yeahtuna View Post
    It turns out that there was a short circuit across the voltage regulator.
    Sorry for Question. what means "short circuit across the voltage regulator"?
    does voltage on pin 1 (input) measures 0V, or does pin3 (output) measures 0V.
    in first case, why is polyfused not limiting?
    in second case, short may easily be well after regulator.

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    Hi Rob, I agree with WMXZ, it's important to figure out where your short is happening. Since the regulator's 3.3v output is powering the chips and pins on the board that interact with the outside world, the odds are that the short is beyond the regulator.

    It would be helpful to know the voltages you're measuring when board is powered up and the regulator is getting hot. I would be curious about the voltage across C3, the voltage at pin 1 and pin 3 of the regulator.

    It would also be helpful to know more context about what the board is connected to. Test the board with and without external peripherals connected. What is the board interfacing with? Most (though not all) pins on the Teensy 3.2 are 5V tolerant but is it possible you've had a voltage greater than 5V or less than 0V in contact with the board?

    Is that an older Teensy 3.2 in the photo? It's funny, the ones I have use much smaller regulators, not the SOT-89 shaped one in your photo.

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    Quote Originally Posted by lucky_bloop View Post
    Is that an older Teensy 3.2 in the photo? It's funny, the ones I have use much smaller regulators, not the SOT-89 shaped one in your photo.
    it is a custom one not a PJRC T3.2

  7. #7
    Senior Member PaulStoffregen's Avatar
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    One way to search for a short involves a current limited power supply and a voltmeter with a sensitive mV scale.

    Set the power supply to a low voltage (in case you find and open the short) and very low current, then connect it. Slowly turn up the current, until it's putting as much current through the short as is safe for the wiring.

    Then start measuring mV at various points along the wires. If the current is flowing through the wire, you'll see at least a couple mV. If there's no current in that segment, you'll get very close to zero volts. With a little hunting around the board, you should get a pretty idea of where all the current is (and isn't) really flowing. Usually that can lead you to the short or at least narrow it down to a small area.

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    It would be helpful to know the voltages you're measuring when board is powered up and the regulator is getting hot. I would be curious about the voltage across C3, the voltage at pin 1 and pin 3 of the regulator.
    I've already replaced the voltage regulator in the unit, so I can't measure the voltages at this point.

    I know the source of the short circuit was the voltage regular, however. I tested the resistance between +5v and ground at various places and the lowest resistance (about 16 ohm) was between the +5v input of the regulator and the ground pad. After replacing the regulator, the short circuit was gone, so it's no doubt that the regulator had issues, but why?

    And unfortunately, after replacing the regulator the device is still not working. I need to do some more testing of the MKl04 and MK20D.

    It would also be helpful to know more context about what the board is connected to. Test the board with and without external peripherals connected. What is the board interfacing with? Most (though not all) pins on the Teensy 3.2 are 5V tolerant but is it possible you've had a voltage greater than 5V or less than 0V in contact with the board?
    The device has MIDI In and MIDI Out DINs and the MIDI input is optically isolated. The device also has four TRS input which interface with resistance based foot pedals (expression pedals). Could plugging MIDI cables into the wrong DINs cause issues?

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    Senior Member PaulStoffregen's Avatar
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    Quote Originally Posted by yeahtuna View Post
    Could plugging MIDI cables into the wrong DINs cause issues?
    No, at least not if your board follows the normal MIDI circuit. The resistors limit the current.

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    I've tested the MKL04. Seems to be working properly. When powered up, pin 8 has 3.3v. When I press the program button, it drops to almost 0v.

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