Author Topic: Oscilloscope, measure thyself - Rigol DS2072 AC current waveshape  (Read 1937 times)

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Offline Circlotron

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Just looking at the current waveshape of the mains power going into the scope, and seeing it is a normal SMPS inside without any PFC, (or at least that is what it appeared to be in Dave's teardown video) I'm a bit surprised the waveshape is as good as it is. After this test i grabbed a battery charged for my electric screwdriver and had a look and it was much more like I expected. Actually the leading edges are a bit spiky, but anyhoo.

Anyone got any comment on how the scope's waveform is better than what you would normally expect from a SMPS?
 

Offline Ghydda

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Re: Oscilloscope, measure thyself - Rigol DS2072 AC current waveshape
« Reply #1 on: December 22, 2013, 03:28:52 AM »
Well the pfc-inductor and 450V storage capacitor is easily spotted in the frame grab attached.
I am a little puzzled to why you missed what I so easily spotted.

Anyway, this goes a long way to explain the waveform you've captured - which despite the pfc input stage still looks plenty rich in harmonic content.



Cheers
/Ghydda
If we learn from our mistakes then I reckon I'm getting a great education!
 

Offline Circlotron

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Re: Oscilloscope, measure thyself - Rigol DS2072 AC current waveshape
« Reply #2 on: January 01, 2014, 11:49:11 PM »
Hi Ghydda, thanks for that. Yeah, I must have been daydreaming during the teardown video.  :=\ What surprises me though is that they would bother to do PFC for such a low powered supply. My cheap and nasty wattmeter showed 28 watts going in so I didn't think regulations for PFC applied to stuff that low in power.
 

Offline peter.mitchell

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Re: Oscilloscope, measure thyself - Rigol DS2072 AC current waveshape
« Reply #3 on: January 02, 2014, 01:02:00 AM »
They do it because these are often not used in a home environment, so if used in a industrial/commercial lab, with lots of other smps, sure 28w may not be much, but if you have 50 28w non-pfc devices, you'd be pretty piss-poor. And these are the people who get charged based on aparrent power, not real power too.
 

Offline Circlotron

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Re: Oscilloscope, measure thyself - Rigol DS2072 AC current waveshape
« Reply #4 on: January 02, 2014, 09:36:16 AM »
My worst power factor story is back in '90-'92 I worked for this company that imported and sold a line of standby UPSs. The Taiwanese manufacturer decided to branch into on-line UPSs. Their first offering was a 240V 3kW unit that took incoming AC mains into a 2 diode, 2 SCR controlled rectifier and then STRAIGHT INTO a 3300uF 450V cap. NO INDUCTOR! We measured a power factor of 0.3 with a strong assymetrical (DC) component. Unbelievable.
 


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