power supply question

isapiens · Post by **isapiens** » 18 Feb 2009, 10:25

So i feel embarrassed asking this question because i am an electrical engineer but unfortunately they don't teach you stuff like this in college.

When my roommate plugs in his laptop power supply in the outlet on his side of the room my PC speakers start buzzing. I assume thats a faulty power supply... what are the physics behind this and can i do anything about it. The funny thing its only one speaker, the other one is fine although they are connected to each other.

cheers

Post by **DNR** » 18 Feb 2009, 10:36

hmm suspecting EMI or even RFI

If the wall outlet was connected to the line to say a 1k watt microwave, or 24cu ft refridgerator, then I would say you have your suspect in ac line noise.
But a laptop? I can't see the laptop creating interferance on the AC line - its not strong enough. the only thing I can think is the wall outlet has too many items connected to it, and _your_ computer is sucking for more juice.

Make sure the ground works in the outlet. You can use a volt meter to test the outlet, if the power flucuates too much you are overloading the circuit the wall outlet is on.

For nice computer systems, always get a uninteruptible power supply, the caps filter your ac power supply.

wait for floodie to respond

DNR

Post by **floodhound2** » 18 Feb 2009, 11:12

This is is a common problem with electronic devices. You should have knowledge of this if you are an Electronics Engineer. I can go into great detail, but wonder if it would be understood.

Basically you could solve the problem by using a isolation transformer. Maybe a ferrite coil is missing on the laptop power plug cable?

This interference could also be received / noticed on an AM radio near the laptop. Try it out and see if you get the buzzing.

DNR
The laptop power has nothing to do with it in my opinion. I believe that the laptop is radiating a frequency that is ridding on the 60 Hz, or the other computer is not filtering correctly. Its a case of cheap or poor power supply design.

Post by **bad_brain** » 18 Feb 2009, 14:34

such interferences are usually caused by ground loops, at least this was almost always the reason in the hifi systems I fixed in my electronics days.
it appears when 2 devices are connected to the same supply but in different rooms (or connected to one another) and both have a ground connection....no ground connection has a resistance of 0 Ohm as it should in theory, and so each device has a different resistance. and because they are now connected in a loop there can't be different potentials in a closed circuit which means the potentials of both ground connections are equalized, and this causes a current flow between those devices...which is the reason for the noise.
(umm...correct me if I am wrong floody, but be mild because I don't really know the exact english terms

).

to fix it floody's tip is a really good one (I use ferrite coils on all my audio video cables), or you can try to simply turn the speaker power plug in the wall outlet so the pins are connected to the other hole (sounds silly, but it sometimes works). another reliable way is to connect all devices to the same outlet so the ground for all devices is on on spot, but this is of course not really practical when the devices are in separate rooms.

Post by **floodhound2** » 18 Feb 2009, 15:51

Well it may not be the ground at all. You see when a device is running at high frequency's it can broadcast some of that frequency outward i.e. back in to the AC 60Hz. It may become a radio signal if a small wire is near the source (even a trace on the PCB is a antenna).

Basically you would have [60HZ AC] with a [15KHz AC] running on top of each other. This is the "Buzzing" say 15Khz frequency that his speakers are picking up from the laptop.

Ferrite beads can deaden the high-Freq by converting it to heat. Most cheap power supply's cut cost by removing key filtering circuitry.