Checking for "dirty" a.c. power with an o-scope

[bear]

using a transformer of the audio type or power type (worse) will certainly eliminate all the out of band near RF and RF energy... in some cases that stuff getting into the PS can be a source of real problems... so imo one wants to look at it both from the "audio band" and out of band point of view - if you can.

There is gross sinewave distortion (shape) and HF interposition on otherwise fine sinewaves...

[Sheldon]

Instead of a transformer for isolation, why not a high impedance voltage divider with a ceramic cap on either end?

[JohnAtwood]

This could work, but there may be issues if the power line neutral and the scope "ground" are at different AC potentials. A transformer provides common-mode isolation. Also, you would have to make sure that the voltage divider was compensated to account for the input capacitance of the scope, the same way a 10:1 scope probe is compensated.

- John Atwood

[Sheldon]

Originally Posted by JohnAtwood

Also, you would have to make sure that the voltage divider was compensated to account for the input capacitance of the scope, the same way a 10:1 scope probe is compensated.

In this case the OP is just looking to get an idea of the original waveform and the filtered form, so absolute accuracy is not too important to get a sense of the difference.

Sheldon

[Huck50]

Originally Posted by Sheldon

In this case the OP is just looking to get an idea of the original waveform and the filtered form, so absolute accuracy is not too important to get a sense of the difference.

Sheldon

Yeah....that's all I want. Just want to compare the before and after filtration.Thanks,Huck50

[bear]

Two probes.
One on the input side, one on the output, after filtering.
Ground on the ground of the filter unit.
(you still have the "polarity" of the incoming and outgoing AC signal to determine, but you won't get sparks...)
One input vertical amp on "inverting".
Put the function switch on "Add"
The result when the vertical gains are matched so that you get a null
(smallest trace) will be the difference. More or less. Tweaking the gain will change the shape slightly...

_-_-

[Huck50]

Originally Posted by bear

Two probes.
One on the input side, one on the output, after filtering.
Ground on the ground of the filter unit.
(you still have the "polarity" of the incoming and outgoing AC signal to determine, but you won't get sparks...)
One input vertical amp on "inverting".
Put the function switch on "Add"
The result when the vertical gains are matched so that you get a null
(smallest trace) will be the difference. More or less. Tweaking the gain will change the shape slightly...

_-_-

Thanks! There are so many scopes(used) available for sale,but I am not sure which ones I should be looking at?.Not sure if I need a 2channel,digital,how much bandwidth,etc??.
Maybe I just forget it,as it seems too complicated my my small brain?! Thanks,Huck50

[Huck50]

I got thinking last night and was wondering if it's possible to check the a.c. wave-form that is feeding the scope? Maybe I can plug the scope into the piece of gear in question (power conditioner) and check it's own wave-form shape coming out feeding my gear? Thanks,Huck50

[Huck50]

Wondering if this single channel scope would do what I want: Newegg.com - Tekpower CQ5010C 10 MHz Single Channel Oscilloscope. Where would that wave-form on that display be coming from?? Thanks Huck50

[tubelab.com]

I have used a single-ended output transformer with the primary connected to the power line and the secondary connected to the scope.

I agree. It would be to easy to blow up something expensive, fry your self, or generate corrupt measurements when attempting to connect anything directly to the power lines. Any direct connection to the lines would involve leakage currents that will corrupt the measurements....at the very least.

I have been keeping watch on my power line distortion since I moved into my house 35 years ago. Back then the line voltage THD was about 2%. Now it is routinely over 10% with obvious flat topping of the sine wave viewed on the scope. Many "conditioners", UPS's, and "line filters" do absolutely nothing about this.

How do you measure this? As John said, find a vacuum tube output transformer that is rated for 5 watts or more. It doesn't matter if it is SE or P-P, but the frequency response at low power (1 watt) should go from 20 Hz to at least 50 KHz. Just about any decent OPT will meet this requirement. Any of John's UBT's, the smaller Edcors and Hammonds, and many others. Avoid large OPT's unless you know that the higher end goes to 50 KHz.

Connected the rated load impedance across the secondary, IE an 8 ohm load on the 8 ohm secondary. Plug the primary into the wall socket. Use the entire primary on CT or UL tapped OPT's, and look for a higher primary impedance on 240 volt mains (5K ohms or higher).

Connect your scope, sound card, or other measurement equipment directly across the 8 ohm load just as if you were testing an amp. I have a scope (just about any scope with 1 MHz bandwidth or more will work), an HP8903A audio analyzer, and a PC based FFT box connected to mine.

You can go through your house turning things on and off to see what the major crud contributors are. THe PC, Sony TV and the microwave oven are the bad guys in my house. THe power company replaced the 35 year old pole transformer about 5 years ago. That took me from 5% to 12% in a single day. The new one is a plastic cased Chinese made unit that runs in saturation on many days. The waveform displays the same saturation I see when I play bass guitar through an SE tube amp!