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
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
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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
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
With appropriate divider network a soundcard would work just as well. Scope dedicated to this single task is complete overkill.
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.
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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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
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
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...
There is gross sinewave distortion (shape) and HF interposition on otherwise fine sinewaves...
Instead of a transformer for isolation, why not a high impedance voltage divider with a ceramic cap on either end?
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
- John Atwood
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
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...
_-_-
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
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
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
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!
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