Hi Ton: > The product is supplied with a DC power adapter > which is current limited, provides overload and > short circuit protection and is specified to accept > an overload or short circuit condition continuously. This description indicates the DC power adapter is adequately protected against any load condition. > Assuming that the rated current for a product is > 1.0A, and assuming that the fuse rating is > determined at 1.5A, then a rating of the power > adapter of anything over 1.0A would seem to be > sufficient for operation and protection. Yes, although the rating of the power adapter is not related to the rating of the product fuse. > However if the rating of the power adapter does not > exceed 1.5A, the fuse would probably never blow. There is no way to determine this from the data you provided. As a rule-of-thumb, low-power power adapters will output about twice rated current into a fault before the adapter folds back or shuts down. If this rule- of-thumb applies to your 1-amp power adapter, then the adapter output into a fault will be about 2 amps continuous. Indeed, according to the Littlefuse data sheet for a 3AG 312-series 1.5 amp fuse, a 1.5-amp fuse will never operate at 2 amps (at room temperature). See http://www.littlefuse.com/PDFs/Products/312-318.tc.pdf However, the fuse must operate in less than 1 minute at 3 amps (200% of rated current). If your adapter is rated 1.5 amps, then it would output 3 amps into a fault, and the fuse would operate. > Is this acceptable? Is there a rule of thumb for > the rating of a fuse and for a power adapter? There is NO RELATIONSHIP between rated current of a product (whether an adapter or the product itself) and rated current of a fuse. The rated current of a fuse is based on FAULT current, not based on rated current of an adapter or of the product itself. Think of it this way: The fuse rating sets the maximum CONTINUOUS FAULT CURRENT. At this current, the product should be safe, i.e., meet all of the safety requirements applicable to the product. For currents exceeding the fuse rating, the fuse sets the MAXIMUM DURATION of the fault current. The product must meet all safety requirements until the fuse operates. Best wishes for the New Year, Rich This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ieee-pses.org/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [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
