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Sure way to limit current

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

Reply to
Jean Berniolles
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Reply to
Fred Bloggs

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).

Reply to
Jon Slaughter

On Dec 12, 7:22=EF=BF=BDam, Jean Berniolles wrote= :

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

Reply to
mpm

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

Reply to
John Larkin

[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

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
         America: Land of the Free, Because of the Brave
Reply to
Jim Thompson

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

Reply to
John Larkin

Naaah! The KVM has its own wall-wart.

I think my Sony can, but I've never tried it.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
         America: Land of the Free, Because of the Brave
Reply to
Jim Thompson

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

Reply to
mpm

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

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
         America: Land of the Free, Because of the Brave
Reply to
Jim Thompson

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

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
         America: Land of the Free, Because of the Brave
Reply to
Jim Thompson

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.

Reply to
Fred Bloggs

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

Reply to
John Larkin

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