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Thread: Powering USB Host Boards from Teensy

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  1. #1
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    Powering USB Host Boards from Teensy

    I just bought a couple of USB MIDI Host boards (http://www.hobbytronics.co.uk/usb-ho...-host-board-v2) With the goal of using the Teensy as the merger/router.

    My goal would be to power the Teensy and all the USB host boards from a single USB cable plugged into a USB wall charger (the ~2.5A variant).
    If I power the Teensy from it's microUSB port I assume I can't use the Vin pin to provide 2+ Amps to the host boards ? I would need to split the power from the USB cable before going into the Teensy ?

    Thanks !

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    That would be the better choice given the pin sizes and track widths giving you at least a voltage drop if not outright heating. Note the underside of the PCB has small pads you could use for USB data if going with a panel jack with flying leads, rather than trying to splice or cut a USB cable into the normal plug.

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    Thanks will go that route!

  4. #4
    Senior Member PaulStoffregen's Avatar
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    Quote Originally Posted by nlecaude View Post
    If I power the Teensy from it's microUSB port I assume I can't use the Vin pin to provide 2+ Amps to the host boards ?
    Probably not, but the answer really depends on the USB port suppling the power.

    Most USB ports on PCs and Macs are designed to provide up to 500 mA. But unpowered hubs provide only 100 mA. The idea is they get 500 mA from the upstream computer and give 100 mA to each downstream USB device.

    Another thing to consider with USB power is the inrush current when you hot-plug. The device has capacitors across its power lines, which rapidly charge. The USB spec requires USB devices to have at more 10 uF, and it says hosts must have at least 120 uF. The idea is to prevent the sudden connection of a discharged 10 uF capacitor from dropping the host's voltage too much. There are a variety of things you can do with active circuitry. The USB spec just says the circuitry is supposed to restrict the loading seen by host ports to no more than equivalent resistive & capacitive load for those maximums.

    Meeting all these goals with the power coming from a USB port is pretty much impossible. Compromises have to be made. On Teensy 3.6 I used a current limiting chip and 100 uF. I did several tests with different capacitors on various USB ports and powered hubs. It's a balancing act between being able to power up an unknown device versus losing your own power coming from an unknown host. Maybe that circuit can work for you, or maybe you'll find some other solution. But when the power comes from any arbitrary host and you try to power any other devices, there just aren't any perfect answers.

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