Dear "First Responders" of the Forum,
many thanks to you all for extremely efficient and professional responses. I
definitely support Mike's opinion what this forum is all about learning
experience and excelent one at that.
Now I will chew on all of this great amount of information, experiences and
specs and will try to serve my management with 1-2 paragraphs of desert for
them to swallow.
Holy Smokes! (I liked that one)
Thanks
Mark Gandler
_____
From: Mike Mertinooke <mertino...@skyskan.com>
To: ri...@ieee.org
CC: 'Mark Gandler' <markgand...@hotmail.com>
Subject: Re: USB cable catching fire?
List-Post: emc-pstc@listserv.ieee.org
Date: Thu, 25 Jan 2007 10:20:30 -0500
>Hi Rich;
>
>Aha! That explains everything. The limit per
>port is 500 mA, but the limit per hub is 5A.
>
>This has been a learning experience, which
>I suppose is what the mail list is all about.
>
>Cheers!
>Mike
>
>
>ri...@ieee.org wrote:
>> Hi Mike:
>> Thanks for providing the USB 2.0 spec.
>> I was able to find the host overcurrent
>>limit spec:
>> 7.2.1.2.1 Over-current Protection
>>
>>The host and all self-powered hubs must implement over-current
>>protection
>>for safety reasons, and the hub must have a way to detect the
>>over-current
>>condition and report it to the USB software. Should the aggregate
>>current
>>drawn by a gang of downstream facing ports exceed a preset value,
>>the
>>over-current protection circuit removes or reduces power from all
>>affected
>>downstream facing ports. The over-current condition is reported
>>through the
>>hub to Host Controller, as described in Section 11.12.5. The preset
>>value
>>cannot exceed 5.0 A and must be sufficiently above the maximum
>>allowable
>>port current such that transient currents (e.g., during power up or
>>dynamic
>>attach or reconfiguration) do not trip the over-current protector.
>>If an
>>over-current condition occurs on any port, subsequent operation of
>>the USB
>>is not guaranteed, and once the condition is removed, it may be
>>necessary to
>>reinitialize the bus as would be done upon power-up. The
>>over-current
>>limiting mechanism must be resettable without user mechanical
>>intervention.
>>Polymeric PTCs and solid-state switches are examples of methods,
>>which can
>>be used for over-current limiting.
>>
>> So, the host can source up to 5 amps, or
>>25 watts before it limits or shuts down.
>> 5 amps is max for 20 AWG wires in a USB
>>cable, and more than enough to overheat 28 AWG in a USB cable.
>> Best regards,
>>Rich
>>
>>
>>-----Original Message-----
>>From: Mike Mertinooke [mailto:mertino...@skyskan.com] Sent:
>>Wednesday, January 24, 2007 4:09 PM
>>To: ri...@ieee.org
>>Cc: 'Mark Gandler'
>>Subject: Re: USB cable catching fire?
>>
>>
>>Hi Rich;
>>
>>The sections I cited clearly state 500 mA maximum.
>>
>>Just to be sure, I downloaded the current USB 2.0
>>specs, including the low-speed portion of the
>>test/qualification procedure. There are about 9.5 MB
>>of data, much too big for email. So I posted them to
>>a directory in our "public" ftp site.
>>
>>At your browser command line, type:
>>ftp://client:chic...@skyskan.com
>>
>>The folder is "USB_spec." This will be automatically
>>deleted in 24 hours.
>>
>>Best regards,
>>Mike Mertinooke
>>
>>
>>
>>
>>ri...@ieee.org wrote:
>>
>>Hi Mike:
>>
>>
>>
>>
>>
>>Thanks for your comments.
>>
>>
>>
>>While the spec defines a unit load as
>>
>>100 mA, I don't believe there is a spec
>>
>>for maximum current the host can have
>>
>>available -- at least I haven't been
>>
>>able to find it. In other words, the host
>>
>>must supply 500 mA minimum to meet the
>>
>>5 unit load spec.
>>
>>
>>
>>And, as far as I know, the host can
>>
>>parallel USB port power, so a host can
>>
>>have available 500 mA to each USB port.
>>
>>In turn, this means, under fault conditions
>>
>>at one port, all of the available current
>>
>>can go into the fault.
>>
>>
>>
>>I understand that the typical current-
>>
>>limiting device in the host is a 2-amp
>>
>>PTC. I was not aware that the host can
>>
>>control the current limit to each USB
>>
>>port. Can you explain this further?
>>
>>
>>
>>I was not able to find a spec on maximum
>>
>>current for the host. Can you? If so,
>>
>>please send, and I will correct my
>>
>>message.
>>
>>
>>
>>
>>
>>Thanks, and best regards,
>>
>>Rich
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>-----Original Message-----
>>
>>From: Mike Mertinooke [mailto:mertino...@skyskan.com]
>>
>>Sent: Wednesday, January 24, 2007 2:35 PM
>>
>>To: ri...@ieee.org
>>
>>Cc: 'Mark Gandler'
>>
>>Subject: Re: USB cable catching fire?
>>
>>
>>
>>
>>
>>Hi Rich;
>>
>>
>>
>>I take exception to your third paragraph. The USB
>>
>>2.0 spec (para. 7.2.1) defines a "unit load" as 100 mA
>>
>>maximum, and allows software-controlled transition
>>
>>to "high power" state of 5 unit loads (500 mA)
>>
>>MAXIMUM. Not 500 mA minimum, as you stated.
>>
>>
>>
>>If you are designing a USB hub, you need to plan on
>>
>>5 unit loads per output, plus one additional unit load
>>
>>for the hub itself (still in para. 7.2.1).
>>
>>
>>
>>When you referred to 2 amps per connector,
>>
>>could you have been speaking of overstress test
>>
>>loads to be applied to the cable?
>>
>>
>>
>>Mike Mertinooke
>>
>>
>>
>>
>>
>>Richard Nute wrote:
>>
>>
>>
>>
>>
>>
>>
>>Hi Mark:
>>
>>
>>
>>
>>
>>did anyone ever hear about USB cable catches fire, especially USB
>>
>>cable which connects PC and printer? If the answer is yes,
>>
>>
>>
>>would it be
>>
>>
>>
>>most likely as a result of "bad cable design or manufactuirng" or
>>
>>let's say short on a device connected by the cable?
>>
>>
>>
>>The issue of a USB cable sustaining electrical
>>
>>overheating and catching fire has to be related
>>
>>to some fault in the load device connected to
>>
>>the load end of the USB cable.
>>
>>
>>
>>According to the USB spec, the power supplied
>>
>>to a device is 5 volts, 500 mA minimum per
>>
>>output connector. Typically, the current per
>>
>>connector is limited to about 2 amps by a PTC
>>
>>device. The available power is about 10 watts.
>>
>>However, the power may be more, depending on
>>
>>the sourcing equipment.
>>
>>
>>
>>An electrically-caused fire requires power to
>>
>>be dissipated in a resistance. In a USB cable,
>>
>>there are two candidate resistances. One
>>
>>candidate resistance is the wire itself. The
>>
>>other candidate resistance is the connection
>>
>>between the wire and the connector terminals.
>>
>>
>>
>>The wire (carrying the power) is, typically,
>>
>>20 AWG, but could be 28 AWG. For 20 AWG, the
>>
>>resistance is about 0.01 ohms per foot. This
>>
>>gauge is good for 5 amps without overheating.
>>
>>For 28 AWG, the resistance is about 0.06 ohms
>>
>>per foot. This gauge is good for 1.6 amps
>>
>>without overheating. See:
>>
>>
>>
>>http://www.geocities.com/CapeCanaveral/Lab/9643/awg.htm
>>
>>
>>
>>So, 20 AWG wire is not a good candidate for
>>
>>power dissipation and overheating, but 28 AWG
>>
>>is. For a 28 AWG cable with 2 amps, I would
>>
>>expect to see signs of an overheated jacket.
>>
>>
>>
>>This connections at the ends of the USB cable
>>
>>between the wire and the connector terminals
>>
>>are also candidates for overheating.
>>
>>
>>
>>In my experience, 10 watts is not enough power
>>
>>to cause a fire.
>>
>>
>>
>>Typical USB cable jacket and wire insulation
>>
>>are flame retardant. They won't sustain a
>>
>>fire unless in the presence of a flame or a
>>
>>continuing heat source.
>>
>>
>>
>>I believe a USB cable fire being electrically-
>>
>>caused is highly unlikely. You should not
>>
>>rule out the possibility of such a fire being
>>
>>caused by some other energy source.
>>
>>
>>
>>I doubt the problem is a bad cable design or
>>
>>bad cable manufacturing. Rather, I am
>>
>>concerned with use of an extension cable,
>>
>>and with use of a cable with 28 AWG for power.
>>
>>
>>
>>Are there different wire gauges available for USB cable?
>>
>>
>>
>>The reason I
>>
>>
>>
>>am asking, is let's say comapny supplies an extension cable for a
>>
>>wireless adapter and the customer using this extension cable to
>>
>>connect a printer. Is the printer USB cable more robust
>>
>>
>>
>>than regular
>>
>>
>>
>>extension one?
>>
>>
>>
>>I believe USB extension cables are outlawed
>>
>>by the USB spec. See:
>>
>>
>>
>> <http://www.usb.org/developers/whitepapers/cablew~1.pdf>
>><http://www.usb.org/developers/whitepapers/cablew~1.pdf>
>>
>>http://www.usb.org/developers/whitepapers/cablew~1.pdf.
>>
>>
>>
>>According to this web site, USB cables are
>>
>>made with different wire gauges:
>>
>>
>>
>>http://www.interfacebus.com/Design_Connector_USB.html#h
>>
>>
>>
>>and click on
>>
>> <http://www.interfacebus.com/Design_Connector_USB.html#h>
>><http://www.interfacebus.com/Design_Connector_USB.html#h>
>>
>>Converter, Adapter, Cables
>>
>>
>>
>> >From this web site, USB:
>>
>>
>>
>>"Cable is 4 wire. The data wires are 28 AWG
>>
>> <http://www.interfacebus.com/Copper_Wire_AWG_SIze.html>
>><http://www.interfacebus.com/Copper_Wire_AWG_SIze.html> , the power
>>
>>wires are 20 to 28 AWG. Two power lines are un-twisted and 2 data
>>
>>lines, twisted. Longer cables will use 20 AWG for power.
>>
>>
>>
>>The wire are
>>
>>
>>
>>color coded per the drawing above. Cables will have an A
>>
>>
>>
>>plug on one
>>
>>
>>
>>end an a B plug on the opposite end. The maximum cable length is 5
>>
>>meters; however that can be increased by using Hubs."
>>
>>
>>
>>
>>
>>Best regards,
>>
>>Rich
>>
>>---------------------------
>>
>>Richard Nute
>>
>>Product Safety Consultant
>>
>>11830 Wilmington Road
>>
>>San Diego, California 92128
>>
>>USA
>>
>>
>>
>>Tel/FAX: +1-858-592-2620
>>
>>Mobile: +1-858-776-1618
>>
>>e-mail: ri...@ieee.org
>>
>> ri...@alumni.calpoly.edu
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>-
>>
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>>
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>>
>
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