Measuring Power Lines Noise

[ed]

Hi,

I'm sorry if this is not the right area. I'm new to this, but I got my hands on a TDS210 and my first project is to build a power up cycle current limiter for a set of power amplifiers. I need help in figuring out how to measure power line noise levels.

The project: This project is related to my home theater. The amplifiers, three Rotel 1090, have huge transformers and the power up cycle of these beasts toast up the voltage regulator, a Monster Power AVS2000. I have a couple replaced already (at no cost by MonsterCable). The idea to solve this problem is very simple. Put just put a huge resistor 300W 20 ohms in series so to limit the power up cycle to some 3-6 amps and use solid state relays to bypass the resistor after 2-3 seconds when the current demands are more reasonable. In actuality I'm using 2 relays, a time delay solid state relay of 10 amps that triggers a high current solid state relay of 75 amps. The later is the one that bypasses the resistor. The idea is that the second relay at 75 amps will provide the amplifier with better dynamics than a regular time delay ssr rated at 10 amps. All of this times three (one circuit per amplifier). That is the part I think I have figured out.

The problem: I would like to be able to measure the "noise" of the power line. Besides the voltage stabilizer (AVS-2000) I'm also using line filters (HTPS-2600). I want to be able to measure how clean the power is at several parts of the circuit and see if my modifications are introducing any noise and also see how well the equipment is filtering out the garbage that comes in the power line... may be this is more of a curiosity, but I'm eager to see this stuff measured.

The short version: so I got my hands on this TDS210 oscilloscope from Tecktronics. The probes that come with it are general purpuse passive 10X and I fugure how to use the scope to measure low voltage AC stuff. However when I hook the probe to the wall outlet they don't seem to be measuring anything. I'm new to this... do I need different probes? Am I missing something?

Once I get a reading off the regular power line (assuming we figure what I am doing wrong) how do I identify and measure the "noise"? I presume I'll need a data capture interface. which seems to be going for $350. But if in addition to that I need new probes then things start to get ugly.

So, as you probably noticed I do not have much of an education on this electronics stuff. I'm trying to find/fight my way around it.

Can you help me measure power line noise levels?

Thanks!

[ed]

Well I must have been doing something wrong yesterday night because now it is working--i.e. I can read the phase, neutral and ground and they all seem reasonable readings.

Ground has very little noise.

The neutral has quite a bit.

There is some noise at the peak of phase but I can not look at it properly because at this Y scale magnification there is not enough of an offset to bring in the peak in the screen range.

This is the neutral pic in one of the regular house circuits. I also have a circuit dedicated to audio (gauge 8 straight from the distribution panel) which is significantly better but I have to disconnect everything and measure it at each step as I add components.

If there is interest I'll post follow ups. Otherwise I'll wait to hit a snag or something. I really wanted to be able to measure the power up current demand of the amp, but I think I would need a current probe and those are way too expensive to satisfy a curiosity.

[Ldanielrosa]

The voltage you're reading on the neutral is proportional to the current, so there's your current probe. The only problem is that everything that's downstream of that particular breaker will contribute to your reading. LIke the image you posted- it looks like there's electronic equipment charging a capacitor (the lumps), and a motor or lamp dimmer making the spikes (maybe fluorescent lamp).

If you want a better idea, read across a low value resistor.

[ed]

Thanks! I'll repeat the tests over the weekend when I can turn all lights off and perhaps most of the breakers off as well and then try to figure where it is coming from.

What would I use as a trigger during the power up cycle if I'm trying to measure peak current? I'll have 2 channels (across the resistor) and one extra trigger input.

BTW: I do have a dimmer and I was planning on adding more it is this one http://www.lutron.com/maestro/?s=17000&t=17200 and it is supposed to have "Superior RFI suppression", but may be it is still causing those peaks. I'll do some tests and post the results. BTW the cool thing about this dimmer is that one of the models can be controlled with an universal remote which comes in handy when you are watching a movie, etc. Oh well... we will see.

:)

[Guest Yevgenip]

Check out my post "More Scientific Americn Projects" - There is a project about noise.