Checking for "dirty" a.c. power with an o-scope - diyAudio
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Old 30th December 2012, 12:50 PM   #1
Huck50 is offline Huck50  Canada
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Default Checking for "dirty" a.c. power with an o-scope

Hi:
Maybe a dumb question,but can I check for how much "dirty" power I have using an oscilloscope? I thought that if the wave-form was not smooth,but jaggedy,that would mean 'dirty" power?.
Next question is, what size o-scope do I need for just checking the a.c. wave forms,I see 2channel,4channel, 50mhz.100mhz scopes. What is the minimum in a scope that I need?
Also,can I check power going into,say a power conditioner and then checking power after the conditioner,which should show some difference in the wave-form?Thanks,Huck50

Last edited by Huck50; 30th December 2012 at 01:01 PM.
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Old 30th December 2012, 01:14 PM   #2
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You will be shocked at how ugly most AC lines are.
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Old 30th December 2012, 01:18 PM   #3
Huck50 is offline Huck50  Canada
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Quote:
Originally Posted by Barleywater View Post
You will be shocked at how ugly most AC lines are.
Thanks,but can I check power going in and coming out of a power conditioner using a scope to see if in fact the power is "cleaner" coming out? Thanks,Huck
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Old 30th December 2012, 01:42 PM   #4
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Certainly.
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Old 30th December 2012, 02:42 PM   #5
Huck50 is offline Huck50  Canada
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Quote:
Originally Posted by Barleywater View Post
Certainly.
Thanks for all the help! Any idea what would be a good,basic scope? I see this one:SDS5032E 30MHz 250MS/s 2-Ch Oscilloscope ($299.00) : Saelig Online Store. Don't need it for anything else. Thanks
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Old 30th December 2012, 03:17 PM   #6
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With appropriate divider network a soundcard would work just as well. Scope dedicated to this single task is complete overkill.
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Old 30th December 2012, 03:18 PM   #7
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You need a scope that does NOT have the chassis on AC ground.

Or else you need to carefully identify the actual ground wire on both the scope and the DUT (Device Under Test) and make 110% CERTAIN that you have the scope ground and the DUT ground connected, not the other way around!

The newer inexpensive "digital" scopes that run off a low voltage power supply may or may not be "floating". You need to know that.Also they tend to have certain input voltage limitations that are lower than traditional scopes.

A better plan is to find a way to run a lower voltage sample of the AC mains into your sound card and inspect it via an FFT program, seeing the harmonics amplitude and frequency up to ~20kHz. For RF you would need a scope, in which case you want a pretty decent High Pass filter to get rid of the 60Hz and 120Hz leaving only the other stuff...

...in both cases, a good idea would be a "sampler" which might consist of a module, enclosed for safety that plugged into the wall, and held one of those inexpensive AC "LED polarity testers" to make 100% certain that your AC is not reversed - then a voltage divider, which would be high impedance (to keep from drawing too much current and getting hot) that reduces the voltage being tested, then the HP filter, then a connection for the scope.

HOWEVER, you do NOT want to use a coaxial connector, like a BNC, because the outside of it COULD be "hot" IF the AC mains is wired backwards. Pin jacks or similarly insulated and mounted banana jacks might be best.

So, possibly the inclusion of a GFI receptacle inside the unit and before any circuitry to make it less possible to pull juice on the errant wired side would be good for safety.

The case for this must be metal and grounded to the "third pin" on the AC wall outlet.

The idea here is to both make certain that the voltages appearing on the scope and external to the test box are non-lethal. AND that the scope ground is not floated to 120vac, where it could be inadvertently touched, along with a "real" ground, causing electrocution.
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Last edited by bear; 30th December 2012 at 03:20 PM.
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Old 30th December 2012, 03:25 PM   #8
Huck50 is offline Huck50  Canada
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Thanks everyone!
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Old 30th December 2012, 03:34 PM   #9
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Hi,
I used an step down transformer to check how clean the AC it is. like 120/24 volts.
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Old 30th December 2012, 03:53 PM   #10
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I have used a single-ended output transformer with the primary connected to the power line and the secondary connected to the scope. Regular power transformers often don't have good enough high frequency response to show the crud on the power line in detail.

Power quality has certainly gotten a lot worse over the last 15 years. I blame it on switching power supply wall-worts, computers, and CFLs - all with cap-input power supplies.

- John Atwood
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