I have had experience with laptop powersupplies introducing harmonics into our sound systems in lecture halls and classrooms. The best solution that I have found was plugging a laptop into another outlet through a long extension cord. If the other outlet is on a seperate ground the problem disappears. An isolation transformer can be connected to the power supply of the laptop but these can be rather expensive. I understand that a cheap transformer can be made (haven't done it) by taking two transformers that convert from 120 V to 12V. And wiring them as shown. I apolagize for the drawing. But Pretty much wire the two transformers secondary windings together observing polarity. + --------/ \-------------/\---------- + 120V Hi \ /+ \/ 120 V Source / \12V low /\ - -------\ /-------------\/--------- - - >From: Dave VanHorn >Reply-To: "Microcontroller discussion list - Public." >To: "Microcontroller discussion list - Public." , >"Microcontroller discussion list - Public." >Subject: Re: [EE] Grounding in a Laptop >Date: Mon, 08 Nov 2004 10:07:22 -0500 > >At 09:54 AM 11/8/2004, Mike Hord wrote: >>And for everyone who pointed out the "ground loop" nature of the problem, >>that is what I strongly suspected (but I never had any confirmation tying >>these symptoms to the name "ground loop"). I should actually pull out >>the outlets and check, but I think the outlets MAY not have any wires >>connected to the ground terminal at all (i.e., at some point, someone just >>replaced two-prong outlets with 3 prong outlets). That would certainly >>account for a difference between the two... > > >This is an old and nasty problem for audio, and RF. >Unfortunately, safety regulations prohibit you from doing the things that >would make these problems go away by having only a single grounding point. > >Isolation between devices, like transformers or optical coupling helps a >lot, but they have their own problems. > >_______________________________________________ >http://www.piclist.com PIC/SX FAQ & list archive >View/change your membership options at >http://mailman.mit.edu/mailman/listinfo/piclist _________________________________________________________________ Express yourself instantly with MSN Messenger! Download today - it's FREE! http://messenger.msn.click-url.com/go/onm00200471ave/direct/01/ _______________________________________________ http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist