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sciguy125 09-19-2007 03:09 PM

power line noise
Alright, first off, I'd like to let everyone know that I'm an electrical engineer, not an electrician. I'm here looking for a different perspective that might help me understand my problem.

For the last year, I've been trying to find the source of some electrical noise in my experiments. After some time, I figured out that the noise was coming from the power line. After another year, I figured out that I can mitigate the problem by using a grounded outlet. (My house is from the 50's and the only outlets that are grounded are in the garage.) However, I'm not sure if grounding was the problem, or if it just patches the problem.

In a nutshell, any part of the circuit connected to the power line (oscilloscope, transformer, power supply, etc.) injects noise into it and causes problems. If the equipment is grounded, the problem goes away. I've got some scope pictures here:
Of particular concern is the last two images. You can see some distortion in the waveform from the power line. Is that normal?

After inspecting my house, I've also discovered that my system isn't properly grounded. When everything was installed, they used the water pipes for grounding. However, the installation of a water softener (among other things) has probably disconnected the pipes from earth. I might fix that this weekend.

Right now, I'm trying to figure out if the noise I'm picking up is in the line itself (maybe that distortion that I see), or if something is radiating and grounding everything only mitigates the problem.

Andy in ATL 09-19-2007 03:50 PM

I wish I could help you, but that's beyond me. I recommend you post at Mike or this sites sister Electrician talk (link below). There are lots of engineers on Mike's website and the guys over there would love to see this.

Stubbie 09-19-2007 05:15 PM

Have you tried the battery powered am radio trick to see if the noise is inside your house or coming in on the utility?

If you plug your circuit into another branch circuit do you still have the same results?

Do you have a circuit breaker panel or fuses?

Some of the biggest offenders of putting noise on the power line are aquarium heaters and small transformers like doorbell transformers. But I figure you have probably disconnected any cord and plug stuff to see if that is effecting the circuit. But little transformers are hardwired in and may not be easily located.

Have you talked to the power company? Maybe they can check for noise coming from a faulty old transformer or some of their other noise creating stuff they have.

When you fool with that water line you might want to check it for current that can harm you.

Just some ideas.

That oscilloscope stuff is greek to me but it is darn interesting.:)


elkangorito 09-19-2007 06:26 PM

An interesting situation. The last couple of your CRO pics may give a clue.

You mention that the sine wave has some peculiarities. These could be related to harmonics produced by the usual culprits, ie switchmode power supplies, compact fluorescent lights, electronic ballasts etc. But the level of distortion does not seem to be significant. The most common harmonics found in consumer power is the 3rd harmonic (single phase related) & the fifth harmonic (3 phase related), although harmonics of other orders can be present.
In order to rule this out, you could try to filter out the 2 main harmonics & see what happens.
BTW, harmonics in power systems is a growing problem for energy suppliers all over the world.

But having said all this, you say that 'earthing' solved the problem? It is well documented that if a neutral is 'floating' above earth, a considerable voltage can be present (with respect to earth)...sometimes up to 50% of the line voltage. This extraneous voltage can be representative of 'noise'. One benefit of earthing the neutral correctly is that this situation is negated. This needs to be done as per code & by an electrician.

I wouldn't worry about harmonics too much as they need to be quite significant before they become a problem (under general conditions).
I would pursue the issue of 'earths' & 'neutrals'. Stubbie is probably your best man for that advice.
Good luck! :thumbup:

slakker 09-19-2007 08:36 PM

I have nothing useful to add... but MY GOD MAN, YOU SKINNED A FURBY!!! :eek: :thumbup: :jester: :laughing:

vigilaw 12-02-2007 01:39 PM

In Response To Trying To Eliminate A.C. Line Noise
One of the postings, referred to "a.c. line noise, and trying everything in a old house, which is not properly grounded". Noise still....could not be eliminated.

You have overlooked a couple of things! If you cannot trace where the "line noise is coming from, you may try this". This is what we had to do, when we would continously have to reset and adjust wall electrical clocks, while at Sperry Defense, Pueblo, CO. Many years ago.

When we hooked a "A.C. Line Analyzer" to the plant's main incoming power, we found the problem was not within the plant or company building. All the while...the problem was the incoming main step-down transformer, outside the building.

Here you are, trying to resolve a problem within your "sphere of influence (your house), all the while, it could simply be your "incoming power is rotten"!

Transformers can produce "noise, and spikes on the sinosidoual wave of the incoming 60 cycle (Htz) line". Therefore it is not "true-blue clean power" which may be entering you household.

Tony Vigil I & E Instrumentation Engr.

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