Hello Bostjan, There are a number of considerations regarding power from standard USB ports. There are USB ports designed to supply power over an additional set of contacts. I would recommend using one of these connector types where you intend to deliver more power than normally supplied by a USB port. The powered USB ports are designed for up to 30W at 5V. http://en.wikipedia.org/wiki/Powered_USB
The standard USB connector will typically be designed for a maximum continuous current of 1.5A. Drawing more current through a standard connector may raise temperatures beyond what IEC 60950-1 would allow. The temperatures of the plastics of the connector body may exceed their RTI, or the temperatures of the circuit board supporting the connector might exceed the board's maximum rating. Although the USB standard doesn't limit the size of conductors in USB cables, the specification recommends making cable conductors as small as possible to enhance the flexibility of the cable. If you have a standard USB port, you have no control of what the user connects beyond instructional safeguards. The designers of many USB devices presume a limited current from USB ports. Their designs assume that under fault conditions, the USB port will limit the current enough that a short circuit in their device will not result in overheating or other issues. This could be a problem if such a device experiences a fault when connected to the USB port you propose. Ted Eckert Compliance Engineer Microsoft Corporation ted.eck...@microsoft.com The opinions expressed are my own and do not necessarily reflect those of my employer. -----Original Message----- From: Boštjan Glavič [mailto:bostjan.gla...@siq.si] Sent: Monday, May 27, 2013 5:30 AM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: USB output limitation Dear experts, What would be the limit of USB output? I know IEC 60950-1 requires only that it complies with limited power source, but there might be some other requirements to limit power from USB? In particular, I am talking about power supply with USB connector. Can it go up to 5Vd.c./3A? Best regards, Bostjan Glavic SIQ - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <emc-p...@ieee.org> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <emcp...@radiusnorth.net> Mike Cantwell <mcantw...@ieee.org> For policy questions, send mail to: Jim Bacher: <j.bac...@ieee.org> David Heald: <dhe...@gmail.com> - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <emc-p...@ieee.org> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <emcp...@radiusnorth.net> Mike Cantwell <mcantw...@ieee.org> For policy questions, send mail to: Jim Bacher: <j.bac...@ieee.org> David Heald: <dhe...@gmail.com>
