I read in !emc-pstc that Wan Juang Foo <f...@np.edu.sg> wrote (in <of7340f59a.f0a87124-on48256d6f.000df921-48256d6f.0013d...@np.edu.sg>) about 'self blinking LEDs as EMI sources WAS: LED lamps' on Sat, 26 Jul 2003:
> These circuits >were found to (well at any rate, seems to) emit interference that cause >a telephone to pick up (50Hz) hum! Up to that point, your treatment of the phenomenon was quite scientific. > It looks like the mains hum was pick >up and modulated by the 'device' and reradiated or broadcasted... I think that is improbable. For such a small circuit to pick up significant mains hum, the magnetic field would have to be very strong, but in that case, why did not the circuits in the telephone, which are much larger in area, pick up the hum directly? Then again, such a small circuit as you describe for the LEDs would not re-radiate significantly at 50 Hz. And the presence of a flashing LED would seem to play no part in any re-radiation. You don't say that the hum was modulated on/off or in level in time with the flashing of the LED, and if there were no such modulation, that would seem to confirm that the LED circuit was not causing the hum. The situation might be different if the LED circuit included a precision timer, based on a high-frequency clock. This might radiate over short distances, and the radiation *might* be modulated by quite a small voltage induced by a weak 50 Hz magnetic field. In that case, you should be able to find the high-frequency emission with a spectrum analyser and sniffer loop, or a scanning receiver, whether or not the 50 Hz field is present. But the induced hum would be modulated at the flash rate. -- Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk Interested in professional sound reinforcement and distribution? Then go to http://www.isce.org.uk PLEASE do NOT copy news posts to me by E-MAIL! This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: emc_p...@symbol.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
