Jim, I'll second that pet peeve!
Here's another design for compliance tip for USB ports...An alternative to the use of PTC's for USB port current limiting, is the use of a USB port power controller IC, such as a Micrel MIC2012 http://www.micrel.com/ These are UL Recognized as Low Voltage Solid State Overcurrent Protectors, category QVGS2. Micrel makes many models to suit your specific needs, and I'm sure that other IC manufacturers make comparable parts. And, while we're on the topic of I/O ports, don't forget current limiting of DVI ports, PS2 ports, and non-standard implementations of RS-232 ports. VGA, parallel, and Ethernet ports typically do not provide 5v outputs directly >from the unlimited power supply rail. I have seen DVI ports that did not provide overcurrent protection; the output overload test was quite interesting - they passed the fault testing (the resulting smoke, heat, etc was contained by the fire enclosure and did not ignite the cheescloth), but of course, our recommendation always is to provide overcurrent protection /LPS, since you can't know for sure that all monitors connected to your DVI port will have a fire enclosure. I have more concern about a DVI port than a VGA port; flat screen monitors are often powered by a desktop adapter power supply, which, I assume, may have LPS outputs; therefore, the flat screen display may not need a fire enclosure. VGA monitors have built-in power supplies, so I would naturally assume that a fire enclosure encompasses the VGA monitor. Of course, making assumptions like these can get you into trouble!! NEVER ASSUME THAT THE OTHER GUY WILL MITIGATE THE POTENTIAL HAZARD FOR YOU!! Doug Massey, NCE Manager, Product Safety Engineering Advanced Compliance Solutions, Inc. Ph. (770) 831-8048 FAX (770) 831-8598 From: [email protected] [mailto:[email protected]] On Behalf Of Jim Bacher Sent: Tuesday, December 21, 2004 10:22 AM To: '[email protected]'; [email protected]; [email protected] Subject: RE: USB interface Limited Power source ? A pet peeve of mine is that a lot of devices powered by USB ports are not UL Listed. I am sure that the big computer companies have insured they have power limited the 5 volt supply to the USB port, but what about the clone companies that never certify anything or the bare board companies? Do they know to current limit the USB's 5 volt power supply? As a PC's 5 volt power supply is usually an unlimited power supply (greater than 8 amps), so having a device in a 94V0 enclosure is a good idea, not to mention the PCB's being 94V0. Jim Jim Bacher, Senior Engineer Paxar Americas, Inc. 170 Monarch Lane Miamisburg, Ohio 45342 USA e-mail: [email protected] or [email protected] From: [email protected] [mailto:[email protected]]On Behalf Of [email protected] Subject: Re: USB interface Limited Power source ? Piotr, The requirements for a fire enclosure are described in IEC/EN/UL/etc 60950-1. While these products are small there is no reason why you could design the product to use a UL Recognised thermoplastic having the required flammability (V-1) at the thickness used. Of course, if the product is already designed then you need to speak to your plastics supplier to see if they have a suitable plastic that could be substituted. Note that 60950-1 permits you to test the complete product to Annex A2 as an alternative to using a V-1 material. With such devices it would also be easy to add a very small surface mount fuse at the connector. You are presumably not concerned about the ability of the over-current device to self-reset, as the pen drive electronics is the only load and if it goes short-circuit then the product is dead anyway. You could also cross your fingers and hope that the USB interface is current limited, as per the original question. Let's now see if others agree... Richard Hughes This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/listserv/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/listserv/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
