Electronics > Beginners
AC Line Waveform Distortion
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bdunham7:

--- Quote from: Gyro on April 29, 2019, 07:45:08 am ---
Also, many Chinese scope probes are Cat I rated at 300V (even if they claim otherwise).

--- End quote ---

Is there evidence of this?  I seriously doubt Rigol has faked their CAT rating, and even if they have, then they aren't CAT rated at all, not CAT I.

I'll respect the rules, if that's what they are, but I think you are doing even beginners a big disservice in telling them to avoid a very simple, common measurement technique instead of telling them how to do it safely. 
IanB:

--- Quote from: bdunham7 on April 29, 2019, 03:03:52 pm ---I'll respect the rules, if that's what they are, but I think you are doing even beginners a big disservice in telling them to avoid a very simple, common measurement technique instead of telling them how to do it safely.

--- End quote ---

I think it's important to point out the risks of trying to measure mains with oscilloscopes. Bench scopes generally do not have isolated and floating inputs. I have created unfortunate sparks and overloads when working with low voltages when I was careless. Doing the same with the mains could have had much more severe consequences.
Gyro:

--- Quote from: bdunham7 on April 29, 2019, 03:03:52 pm ---
--- Quote from: Gyro on April 29, 2019, 07:45:08 am ---
Also, many Chinese scope probes are Cat I rated at 300V (even if they claim otherwise).

--- End quote ---

Is there evidence of this?  I seriously doubt Rigol has faked their CAT rating, and even if they have, then they aren't CAT rated at all, not CAT I.

I'll respect the rules, if that's what they are, but I think you are doing even beginners a big disservice in telling them to avoid a very simple, common measurement technique instead of telling them how to do it safely.

--- End quote ---

Ok, I'll concede that original Rigol probes do most likely meet their CAT rating, although as I indicated, low cost Chinese probes are not all created equal. If the probe is accidentally set to it's X1 position, the safety rating immediately reverts to the CAT I rating of the scope input. Stated CAT rating of scope probes is therefore 'suspect'.

You do still need to find a way of connecting the probe to an outlet (without exposing the ground skirt*). Mains outlets don't naturally lend themselves to direct probing with scope probes, this inevitably leads to dodgy hookups of one sort or another, either objects inserted, or leads with a plug on one end. You also can't assume that the user has suitably tied back and insulated, or removed the dangling ground clip. There are simply too many variables to assure inexperienced user safety.

In the Beginners section we do not assume a level of knowledge. There have certainly been cases of beginners (of all ages) receiving their shiny new scopes and having exhausted the Probe cal output and in the absence of other signal sources, go straight to probing the mains (not suggesting this is the case with the OP - in fact he was playing safe and using a transformer). This is the reason Dave produced his comprehensive and oft quoted 'How not to blow up your scope' video. It is too common an occurrence. As Ian says getting it wrong can have nasty consequences.

Not criticizing, just explaining.

* Edit: Image captured from an ebay listing ( :o) a few years back: https://www.ebay.co.uk/itm/Professional-BNC-60MHz-Oscilloscope-Probe-Clip-P6060-X1-X10-600V-/161805210779?hash=item25ac57949b:g:An8AAOSwgQ9V3JuV
The Electrician:
The OP did not say that he used a transformer; it was radiolistener that said he used a transformer.

Tell us, spiff72, did you use a transformer, or did you probe the mains directly with a standard oscilloscope probe?
spiff72:
Sorry for the lengthy delay in response.  I didn't have notifications enabled until just now, and didn't think to come back and check this thread.

My method to measure in my OP was to remove the ground lead from the probe (not the scope power cord) and then measured directly.  I know that the outlets are wired correctly, and i have a plug and short wire length with the correct indicators for hot, neutral, and ground. The scope was set to use AC as the trigger.

I am aware of the risks, and was comfortable with them.


--- Quote from: bdunham7 on April 29, 2019, 12:31:43 am ---How is the scope connected to the mains?  Direct, voltage divider, transformer?

3rd, 5th and 7th harmonics are common.  3rd would be mostly generated locally, typically an unloaded transformer will generate some 3rd due to hysteresis.  However, even though industrial loads and distribution systems do generate a lot of 3rd, you don't see them much because in a 3-phase system, they cancel out.

5th is generated by induction motors and certain generator configurations.

What you have looks like an unloaded transformer to me, although rectifier/capacitor systems could be contributing as well.  THD is supposed to be under 5% and it looks like you may be approaching that.

B/T/W, that is a nice clean FFT, especially as a freebie add-on to an entry level scope.  What exact settings (window type, etc) did you use?

--- End quote ---

Thanks for that feedback.  I will have to redo the measurement re-check the settings I was using on the FFT - this was over a month ago so I don't remember them.  I also remember doing a fair amount of fiddling with the settings trying to make sense of it.

I might have to go on a search for unloaded supplies.  Would this be only for linear supplies (both the AC/DC and AC stepdown), and would unloaded SMPS be included too?  I am sure that there are a TON of scattered SMPS (wall warts, phone chargers, etc), but probably not as many linear supplies, and even fewer purely stepdown transformers.  The only stepdown transformer I am aware of would be for the furnace/thermostat - and maybe the doorbell.

There are probably enough options to just shut off breakers until it is eliminated (assuming the source is in the house).

EDIT:  I now realize that maybe the question about FFT settings was directed at Radiolistener (I would like to know how he got that plot as well, and what scope he used).
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