On 06/11/2010 09:29, David wrote: > At least one eagle eyed person spotted that the inductor (which should > be a ferrite bead) in my USB to serial breakout had no value. Now I > have read the datasheet, it says: > > "The value of the Ferrite Bead depends on the total current drawn by the > application." > > Is there a rule of thumb for picking a correct value? The maximum power > draw will be 500mA. There are plenty to choose from, but after a little > Googling I'm still unsure what value would be correct to act as an EMI > filter on the power line. > > David Depends on the frequencies you wish to supress. I assume you know a bead=20 is a different beast to a "standard" inductor etc. Beads are designed to=20 have low Q and higher resistive losses, so no resonance/peaking like an=20 inductor. As far as selecting one goes the specs can be misleading - 2=20 rated at say 200 ohms at 100MHz may actually be quite different, so it's=20 good to check any charts the manufacturer provides. The above statement=20 "the value depens on current drawn etc..." in your datasheet is only to=20 do with what current rating you need your bead to be, but does not help=20 at all as regards other characteristics, so could also be a bit=20 misleading making you think anything rated at over 500mA will do. Anyway, here is a link that explains things better than I can.. (goes=20 into reasonable detail) http://www.incompliancemag.com/index.php?option=3Dcom_content&view=3Darticl= e&id=3D393&catid=3D26:design&Itemid=3D130=20 --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .