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Sure way to limit current

Discussion in 'Electronic Design' started by Jean Berniolles, Dec 12, 2007.

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  1. Hello,

    I've just bought an Nokia Internet Tablet. It has an USB port that can
    be toggled to USB host mode and I would like to use that to plug an
    mini USB keyboard. There is a big problem: if the periferical draw
    more than 106 mA of current, the USB chip burns.

    Hopefully, most USB keyboards use 35-50 mA, but I wouldn't trust
    these manufacturers.

    Anyone knows an 100% sure way to limit the curent to, lets say 65 mA,
    something that could be rearmed, and small enought to be mounted
    inside the keyboard itself ?

    Thanks

    Jean Berniolles
     
  2. Fred Bloggs

    Fred Bloggs Guest

    http://en.wikipedia.org/wiki/Polyswitch
     

  3. How do you know the USB chip will burn out? AFAIK all usb ports have current
    overload protection(usually shutdown if it draws to much).
     
  4. mpm

    mpm Guest

    Come to think of it, I guess I've never seen a USB "Fuse".

    I can see it now: Marketing Hype to "protect your sensitive
    electronics".
    It would be a little male-to-female inline USB doohickey, with nothing
    inside but a circuit board and a single thin trace on the power
    rail......

    -mpm
     
  5. John Larkin

    John Larkin Guest

    Do you know that it burns out? Or is that just the max current spec?
    No USB port should be damaged by current overload.
    Polyfuses typically have a 2:1 or even 3:1 specified trip/hold ratio.
    Add another factor of 2:1 for temperature and - they don't tell you
    this - another 2:1 for the pcb layout, namely how well the end-caps
    are heat sunk. So they're not anywhere near precise enough to hit the
    op's targets.

    John
     
  6. Jim Thompson

    Jim Thompson Guest

    [snip]

    My IOGear KVM switch just sits there and toggles back and forth from
    current limit. (And their "support" doesn't have a clue.)

    Cured by adding powered Kensington powered PocketHUB SE 2.0 between
    KVM and keyboard.

    ...Jim Thompson
     
  7. John Larkin

    John Larkin Guest

    Likely the kvm+keyboard was overloading your pc's usb current limit.

    They sell USB reading lights and personal fans!

    So, why don't any of my cameras recharge from the usb port?

    John
     
  8. Jim Thompson

    Jim Thompson Guest

    Naaah! The KVM has its own wall-wart.
    I think my Sony can, but I've never tried it.

    ...Jim Thompson
     
  9. mpm

    mpm Guest

    I'm no USB expert, but I'm pretty sure only HUBS can take on
    downstream power management.
    (Well, that and controllers or course.)

    Since a KVM is neither, it must draw at least some power from the USB.
    Whether or not that was enough to cause enumeration failure, I can't
    say, but it sure smells like it.

    What is interesting however, is that apparently the Operating System
    kept trying to enumerate the KVM.
    Something it should have given up on eventually...?

    -mpm
     
  10. Jim Thompson

    Jim Thompson Guest

    Looking at the documentation this KVM can draw power from the host USB
    OR from the wall-wart.
    Adding a powered PocketHUB SE 2.0 between the KVM and the keyboard
    cured the cycling that I thought looked like a current limiter kicking
    in and out. So KVM, powered by wall-wart STILL couldn't support the
    keyboard by itself.

    ...Jim Thompson
     
  11. Jim Thompson

    Jim Thompson Guest

    I should have pointed out that IOGear knows that they have a
    problem... when asked what keyboards their KVM supports the official
    response is that it only supports their own :-(

    If it wasn't perfect for my needs I'd shove it up their ass ;-)

    ...Jim Thompson
     
  12. Fred Bloggs

    Fred Bloggs Guest

    I see no such ratios specified on any datasheet. What they do specify is
    time to trip at fault currents in that range.
    I'm not much interested in unrealistic requirements set by ignorant
    OP's. Almost all the manufacturers have a line of fast acting 6V
    resettable fuses for USB protection.
     
  13. John Larkin

    John Larkin Guest

    We should start a database of the things you're not interested in.

    Personally, I'm interested in all sorts of stuff, which is why I did
    the polyfuse/pcb layout experiments.

    John
     
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