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Thread: Teensy LC: Bug in schematic

  1. #1
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    Teensy LC: Bug in schematic

    There seems to be a bug in the schematic of the Teensy LC on this page:
    https://www.pjrc.com/store/teensylc.html
    The VUSB pad on the back side is not connected directly to the USB connector. It is connected to the VUSB pin which is behind the fuse.

    Question: How much current can an external circuit safely sink from the VUSB or 5V (Vin) pins, before the fuse blows?

    Michael

    Click image for larger version. 

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  2. #2
    Senior Member+ defragster's Avatar
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    Not a schematic reader ... but there are two VUSB tags showing. The tag labelled near 'WRONG'/'CORRECT' has no connection except to the Device connector - appears to be labelling the line for orientation and location perhaps?

    The second tag - shown Uppermost Left corner accurately labels that wire that has passed through the F1 - Fuse, and continues to power the T_LC across the trace to cut that continues to VIN.

    Also noted there for F1 is 350mA which suggests that fuse is rated for 350mA. And it is not a fuse that will blow, but rather a 'soft' fuse that only passes some design current before getting hot and raising resistance to limit/stop current flow.

  3. #3
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    Quote Originally Posted by defragster View Post
    Not a schematic reader ... but there are two VUSB tags showing.
    Yes, there are two VUSB tags in the schematic and there are also two VUSB pads on the board. One is a surface pad on the back side, and the other is a through hole. They are connected to each other and the fuse is not between them.

    Are you sure that it's not a fuse that blows, but a PTC fuse instead?

    Michael

  4. #4
    Senior Member PaulStoffregen's Avatar
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    Can confirm, F1 is a PTC "self resetting fuse", which is really only a resistor that increases resistance with high temperature when large current flows.

  5. #5
    Senior Member+ defragster's Avatar
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    Yeah - PTC ... wasn't sure that was the proper TLA ... so 'soft' fuse ... i.e. not a fuse that 'blows' as noted.

    Correct - looking now - there is a second VUSB 'pad' that replicates the connector pad to expose it in the event the user mounts an alternate means of connecting to the USB HOST - because the connector gets 'lost' or chooses wiring to a panel mount variation. Perhaps also allowing placement of diodes when trace is cut and VIN can optionally have power?

  6. #6
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    Quote Originally Posted by defragster View Post
    Correct - looking now - there is a second VUSB 'pad' that replicates the connector pad to expose it in the event the user mounts an alternate means of connecting to the USB HOST - because the connector gets 'lost' or chooses wiring to a panel mount variation. Perhaps also allowing placement of diodes when trace is cut and VIN can optionally have power?
    No, the VUSB pad does not replicate the connector pad. The fuse is between the connector and the VUSB pad. The other three surface pads (GND, D+ and D-) replicate the connector pads, but VUSB doesn't.

  7. #7
    Senior Member+ defragster's Avatar
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    Quote Originally Posted by mkoch View Post
    No, the VUSB pad does not replicate the connector pad. The fuse is between the connector and the VUSB pad. The other three surface pads (GND, D+ and D-) replicate the connector pads, but VUSB doesn't.
    @Paul should give a look then.

    Indeed, the CARD image shows a wire trace going directly towards VIN - so functionally it replicates the VUSB pad - but would seem to bypass the fuse if installed above board inline with incoming 5V off the connector feeding that pad.

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