Ton, The short answer to your question is that there is no magic formula, rule of thumb, etc that will match a load having unspecified characteristics to an over-current protective device, nor to a power source - if the intent is to select a source closely matched to the load.
I suggest that you consider requirements for the load equipment and supply equipment separately. I will do this below. Let us consider the load equipment first. You say that the Rated Current is 1A. If we look at the definition of this term per IEC 60950-1 then we learn that this means "The input current of the equipment as declared by the manufacturer" and (from cl. 1.6.2) that "The steady state input current of the equipment shall not exceed the RATED CURRENT by more than 10 % under NORMAL LOAD". Hence, knowing the Rated Current provides no information on peak current drawn by the load at start-up or peak load; neither does it provide info on the current drawn under fault conditions. The load equipment may require the delivery of peak currents significantly higher than the Rated Current, perhaps 2A or more (e.g. due to infrequent operation of a solenoid, motor, etc). Of course, the peak demand on the external supply for such loads could be smoothed out by use of large value capacitors, but it is likely that the requirement from the adapter would still require a peak current in excess of 1A, and perhaps in excess of 1.5A. You mention having a fuse, but the location of this device is not clear and neither is the reason for having such a device. If you have a fuse then (irrespective of its location) it will need to be chosen so that it not blow under steady state conditions or when carrying anticipate peak load currents (or switch-on currents). Since fuses open more readily when they are hot, the manufacturer's fuse data should be checked to ensure that you will not get nuisance tripping at high ambient temperatures. You could have a fuse in the load equipment to prevent damage to power connectors or pcb tracking etc. in the event of an excessive short-circuit due to a different power adapter than that specified. You could have a fuse in the power adapter to prevent damage to it in the event of a short-circuit in the load equipment, but this would seem unnecessary if the psu has built-in over-current protection. There are lots of reasons for fitting over-current protection in an electrical circuit; sometimes fuses are best, sometimes PTCs are better and sometimes electronic circuits or circuit breakers are better still. It is for the design engineer to consider these alternatives and decide what protection is required where, for what reason. Consequently, it is not possible to say that what you need is "a 2A time delay fuse with a prospective current rating of 20A minimum". In addition to the above, if you look at the characteristics of fuses you will see that the time-current curves for one type of fuse (e.g. sub-miniature surface-mount type) are different from another fuse (e.g. glass-bodied IEC 60127 type). This another reason why a universal answer can be given. As to the adapter, it must of course have an output current rating that is adequate for the intended load. You may also want to have an adapter having outputs that comply with those for a limited power source (per IEC 60950-1 cl 2.5) if the intention is to eliminate the requirement for a fire enclosure (per IEC 60950-1 cl 4.7.2.1, 4th dash). In this case do not choose a connector that is readily obtainable from any hardware store, because then you could design-out the fire enclosure only to have the unit powered from a PSU capable of delivering 100A or so! I trust that the above is of help. Regards, Richard Hughes Safety Answers Limited www.safetyanswers.ltd.uk In a message dated 25/12/2003 16:07:47 GMT Standard Time, [email protected] writes: Hello Group, I have a question for products which an external DC power adapter. 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. 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. However if the rating of the power adapter does not exceed 1.5A, the fuse would probably never blow. Is this acceptable? Is there a rule of thumb for the rating of a fuse and for a power adapter? kind regards, Ton
