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Thread: Max current through VUSB to PCB

  1. #1

    Max current through VUSB to PCB

    I have a project in which I am mounting the Teensy 3.1 onto a custom fabricated PCB. I would like to be able to charge my Lithium Ion battery through the Teensy 3.1 USB port. I have cut the VUSB trace from Vin on the Teensy, and am planning on mounting a header that will plug into the PCB, connecting the PCB to the VUSB pin.

    The current would then flow from VUSB into my charge controller sub-circuit.

    My question is: how much current can I safely draw continuously through the Teensy VUSB pin?

    This Teensy LiPo charger add-on from tindie.com seems to draw only 100 mA through the VUSB pin. Is this the max?

    Here is a schematic showing how I intend to connect the VUSB pin of my Teensy through a diode to my Li-On Battery charger sub-circuit:



    Click image for larger version. 

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  2. #2
    Senior Member+ MichaelMeissner's Avatar
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    In the past, I believe Paul has said there is a fuse that on both the VIN/VUSB lines, that would prevent you from drawing more than 500mA from those pins, even if more power was available in the USB plug. I don't believe the Teensy indicates that it can draw high power with the D+/D- pins, so 'smart' usb chargers may limit the amount of current you can draw to the standard amounts (100mA or 500mA).

    If you look on this schematic (http://www.pjrc.com/teensy/schematic.html), you can see the 500mA fuse between the USB micro port (left hand middle) and the VUSB (left hand top), VIN (right hand top) pins.
    Last edited by MichaelMeissner; 02-24-2015 at 12:08 AM.

  3. #3
    Senior Member PaulStoffregen's Avatar
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    Yup, TL;DR = 500 mA.

    In reality, a PTC "fuse" is just a resistor that drastically changes value when it gets hot, due to too much current flow. It's hardly a high precision device. The PCB traces, connector pins, and USB cable are also made of real material with non-zero resistance. USB cables vary quite a lot, and some very cheap ones use incredibly thin wires. All these minor factors may or may not cause issues if you try to draw about 500 mA or more.

  4. #4
    The Teensy LC schematic shows the VUSB designator twice, and this appears to short circuit the PTC.

    Is this a problem with the documentation or the actual design?
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  5. #5
    Senior Member Constantin's Avatar
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    The lower VUSB dot is not a pin hole on the PCB but rather a very small pad on the underside of the Teensy. It's presumably there for factory programming purposes. The difference of OEM use pads vs. user pins and pads on the schematic is the use of little filled squares (OEM use pads) vs. round circles (User-accessible pin holes and pads)

    In other words, unless you start experimenting with the un-marked pads on the underside of your Teensy, you will never encounter the lower, non-protected VUSB pad. But if you want to go hunting for them, they're between the 2x7 pad column and the USB connector. My guess would be that the pad in question is the one over the '1' pin hole in the image below.
    Last edited by Constantin; 10-11-2015 at 11:38 PM.

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