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Thread: USB power limits

  1. #1
    Variable Bitrate
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    Nov 2007
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    USB power limits

    I'm trying to figure out if I should add a powered USB hub to my system, and I'd appreciate anyone's thoughts on this...

    My CarPC is a laptop with two serial ports.
    Port 1 goes to an unpowered 7-port USB hub, which connects to:
    * Bluetooth radio
    * OBD2 cable
    * USB-to-serial cable
    * Xenarc Touch-screen
    * USB extension cable into the center console, mostly not used, sometimes used for a flash drive.

    Port 2 goes to an unpowered 4-port hub, which connects to:
    * W3bm4st3r auto-on module (there's a relay that it drives)
    * WiFi radio
    * GPS radio

    At some point I should have a powered USB hub in my car, just for the sake of power draw, and I'm a little concerned that might be at or beyond that point now. Most of this stuff draws very little power, but I'm a little concerned about the Bluetooth and Wi-Fi antennas since they can warm up pretty good when in use.

    Anyone care to speculate on whether I should get power to those hubs?

    Thanks!

  2. #2
    Fusion Brain Creator 2k1Toaster's Avatar
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    Bluetooth, WiFi, and GPS are all big power users. I dont know about the auto-on, but a relay can take 400mA easily depending on the size (or it could take 50mA again depends).

    I think you have actually surpassed the cutoff for getting a powered hub.

    Since it is a laptop, you cant even distribute the usb to not overload any root since the laptop only has 1 root (driver, not physical ports).

    I say get a powered hub. Or at the least, some of those car chargers for mp3 players/cell phones that have a USB port on the other end so you can splice power off of that, and data through the unpowered hub as normal. And keep in mind the hub itself does use a little power.
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  3. #3
    Maximum Bitrate bes51659's Avatar
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    Why is most hubs 4 ports?

    It's because every device is allowed to comsume 100mA and request 500mA from its hub (if passive). So a hub (in itself a device) request 500mA, uses 100mA itself and remainder goes to its four ports.

    A 7 port is like two 4 port hubs in chain, so there are no passive 7 port hubs. If you want to be within spec, and I think you do, considering all sorts of problems you might have with USB in the car anyway, you'd better get a powersupply.

    One more thing if you are wiring USB all over the car and have active hubs: You can cut the USB cable open, leaving just the data wires. That will ensure no ground loops and data traffic is less subseptive to noise.

    I run into an active hub that returned 5V from its supply to the PC, which is a no-no. Cutting the power will for sure prevent that.

    5V DCDC is not cheap, and you need to be able to turn them off or you will drain your battery. There is a clever device that will be turned on and off depending on presens of 5V on its USB input.

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