mjohnsto@uunet.uu.net (Mike Johnston) (03/30/91)
I just put one of those sleek looking, floor standing hologen lamps in my living room. Imagine my chagrin when after turning the lamp on I tried to use my cordless phone and discovered a loud hum. Incidentally, this hum varies with the brightness of the lamp. I.E the lower I dim the lamp, the louder the hum in my phone. Help! What's a guy to do? Is there any way I can shield my lamp from this or am I just stuck? Michael R. Johnston mjohnsto@shearson.com || mjohnstonn@mcimail.com System Administrator UUCP: uunet!slcpi!mjohnsto Lehman Brothers Inc. Phone: (212) 640-9116
nils@ooc.uva.nl (Nils Arbeitstein) (04/01/91)
This story sounds familiar to me. Try and shield the transformer, which usually is located within the stand of the halogen light. Don't forget to not just put some metal around it, but also to connect it to ground. That ought to do the trick. nils@ooc.uva.nl
uccxmgm@unx2.ucc.okstate.edu (04/02/91)
The interference to the cordless phone from the halogen lamp is a perfect example of an irritating problem which began to show up about 25 years or so ago when silicone control rectifiers, (SCR's) and triacs, (double SCR's), began to be used as control devices in everything from power tools to light dimmers. These marvelous devices have made it possible to build light dimmers and motor speed controllers which are a fraction of the size and cost of older rheostat based controls. The problem is that they do this voltage control by turning on once each AC cycle in the case of SCR's and once each half-cycle in the case of triacs. Any time current is rapidly switched into a conductor, electromagnetic energy is produced. When the halogen lamp is dimmed, the control lead of the SCR or triac is being fed with an AC signal with sufficient phase shift to turn the SCR on just as the AC sine wave is nearing the zero point. The SCR turns itself off when zero is reached and won't come on again until about the same point in the next cycle. The result is that the bulb is fed with short saw-tooth pulses rather than a sine wave of lower voltage. The upshot of this involved description is that modern solid-state lighting and motor controls can generate stupendous electromagnetic interference in the low-frequency radio spectrum. The best defense is a good AC line filter between the lamp and the power outlet. The problem may be in finding a filter which can handle the wattage of the lamp without cooking. Finally, try this little test. If you are lucky, no one will see you do this and think your crazy, but it demonstrates what I just told you. If the room is very quiet, put your ear near the lamp bulb and very slowly, turn the brightness control up. If the control is a full-range control, the bulb may barely start to glow. At the same time, your interference should start and you should also hear a faint buzzing or ringing sound from the bulb. This is the electromagnetic shock of that saw-toothed wave form. Now turn up the light and you will probably hear the noise vanish or soften. This is because the wave form is more like a sine wave. Don't burn your ear on the bulb while doing your research. Martin McCormick WB5AGZ Oklahoma State University Computer Center Data Communications Group Stillwater, OK
Mark Cheeseman <ycomputr@runx.oz.au> (04/11/91)
In article <telecom11.255.3@eecs.nwu.edu>, slcpi!admin8779.shearson. com!mjohnsto@uunet.uu.net (Mike Johnston) says: > I just put one of those sleek looking, floor standing hologen lamps in > my living room. Imagine my chagrin when after turning the lamp on I > tried to use my cordless phone and discovered a loud hum. > Incidentally, this hum varies with the brightness of the lamp. I.E the > lower I dim the lamp, the louder the hum in my phone. Help! What's a > guy to do? Is there any way I can shield my lamp from this or am I > just stuck? Sounds to me like poor power supply rejection in the base unit of the phone. Obvoiusly, the lamp has a dimmer in it, which would typically use some form of triac switching to achieve the dimming action. However, this results in rather sharp current rise times, which tend to get into everything else connected to the same supply. What is needed is some level of inductance in the supply to the lamp (it really should be built- in, if the manufacturer cared about the possibility of interference), but I can't quote any figures off the top of my head. A few tens of turns around a ferrite core should provide a signifcant improvement though. The other possibility is that the noise is being radiated RF-wise, but I'm assuming that the cordless phone is using FM, and that it has reasonable AM rejection, which should kill of dimmer noise. If this is the case, the solution is really the same - limit the rate of current rise in the lamp, using an inductor. I hope this helps. Mark