Measuring mains voltage changes while appliance is powered

[ollihd]

Hello all!

I would like to measure the voltage changes in my mains output while an appliance is powered on (for example washing machine). I have a Rigol DS1054Z and was thinking of buying these differential high voltage probes https://www.reichelt.de/Test-probes-and-BNC-adapters/SI-9001/3/index.html?ACTION=3&LA=446&ARTICLE=50637&GROUPID=7230&artnr=SI+9001&SEARCH=differential%2Bprobe

I would like to connect the probes in between mains output (socket) and the appliances power plug. Would I succeed with this setup? Are the probes the right kind? The probes are universal?

Thanks in advance!

[capt bullshot]

The probe is fine to measure the line (mains) voltage. Connect it in parallel to your appliance' supply input. You won't be able to trigger on the line voltage dip resulting from the power on event of the appliance (the joe average oscilloscpe doesn't have a suitable trigger mode). One way to trigger would be: get a current probe (a low cost, line frequency to some kHz type is sufficient, typically clamp on ct style), clamp it over the live wire and trigger on the inrush current of the applicance. You'll be able to see its effect on the line voltage using the voltage probe connected to some other channel.

[ollihd]

Thanks for the reply. One more reason to get the differential probe is that I could use it in other probings also (without the need for an isolation transformer). While not being able to trigger, I could still manually set the scope to see the slight changes in AC over time, right? I would mainly use this for logging.

[capt bullshot]

Yes, these kind of probes are great for eliminating isolation transformes (as long as you stay inside the limits, say: signal isn't too small, common mode isn't too large, signal frequency isn't too high).
You can watch the line voltage over time without triggering by setting the scope to peak detect aquisition mode and using a slow time base (roll mode). You'll see then the peak to peak envelope of the voltage. The probe doesn't provide any means to rectify or rms convert its input signal.

[djnz]

Do you want to measure what happens in the few milliseconds / seconds when the appliance is powered on, or do you want to continuously log changes in voltage periodically over a long duration of time - say a few hours? If it's the latter, you might be better off getting a high-count multimeter with data logging / SCPI, instead of the oscilloscope.

Suppose you are using those linked probes with your oscilloscope. Assuming you want to do this in Finland, AC RMS is 220V so peak amplitude is about 310V. This means you will have to use the probes with x100 attenuation, not x10. So, the oscilloscope will see a signal going from +3.1V to -3.1V. I think your oscilloscope has an 8-bit ADC. I do not trust oscilloscopes / measurements when a part of the signal is outside the screen. So, in the best case, putting all those 8 bits to bin signals between +3.1 and -3.1V, you get an on-oscilloscope resolution of 0.0242 volts which translates to 2.42 volts of resolution for the line voltage (x100 attenuation).

This means that with the linked probes, you won't be able to meaningfully measure any changes to line voltage which are smaller than 2.42V. I don't know if that's good enough for your application.

[capt bullshot]

This means that with the linked probes, you won't be able to meaningfully measure any changes to line voltage which are smaller than 2.42V. I don't know if that's good enough for your application.

You can get higher resolution out of this setup by selecting a more sensitive range and shifting the signal offset (part of the sine wave is off screen).
Check first, don't just say I don't trust without checking. Sometimes off screen signal doesn't work out well, other times it does, so check.

[ollihd]

Do you want to measure what happens in the few milliseconds / seconds when the appliance is powered on, or do you want to continuously log changes in voltage periodically over a long duration of time - say a few hours?

I would say my need is to measure seconds after the appliance is powered on and over a long duration (30min-1h).

You can get higher resolution out of this setup by selecting a more sensitive range and shifting the signal offset (part of the sine wave is off screen).

I will try this for sure. Thanks!


I think I will go with this probe since it's still in my price range. I also see using it in the future for LV-projects without the need for an isolation transformer.

[Hideki]

I would say my need is to measure seconds after the appliance is powered on and over a long duration (30min-1h).

An oscilloscope seems like completely the wrong tool for this sort of thing, unless you want to look at the waveform (seems like you don't?).

If the only important parameter is the AC voltage, I would use a datalogging multimeter.

[ollihd]

I would say my need is to measure seconds after the appliance is powered on and over a long duration (30min-1h).

An oscilloscope seems like completely the wrong tool for this sort of thing, unless you want to look at the waveform (seems like you don't?).

If the only important parameter is the AC voltage, I would use a datalogging multimeter.

Actually the waveform is what I am really interested in. Sorry if I was vague before.

[Jeroen3]

Rent a Fluke Power Analyzer. Much better tool for the job than a DSO.

[ollihd]

Might be a totally stupid question, but this is my first time using a differential probe. The purpose of the auxiliary earth alligator clip on the BNC-side of the probe is for scopes that are not earth referenced by default, right? By this I mean that the scopes outer shell of the BNC would not be earth referenced thus you would have to connect the alligator to earth by some other means?

[ollihd]

Rent a Fluke Power Analyzer. Much better tool for the job than a DSO.

Would do this if it was a one time thing.... I'm doing measurements for months so rent would cost the same as the device, and the device is out of my price range. Will have to do with the differential probe for now. Thanks for the advice though!

[ollihd]

Could someone still enlighten me about the auxiliary earth alligator clip? It's for scopes that are not earth referenced via the bnc shell?

[Vtile]

No definite idea here, I can't remember such a extra grounding connector elsewhere. What do the manual say.

Maybe it is ground fail detector input. 

[ollihd]

What do the manual say.

Manual just says to connect it to ground. Doesn't make any sense since the probe is already connected to ground via its bnc shell and the auxiliary is connected to the bnc shell... 

[capt bullshot]

I think the ground lead is  useful in case you use an isolated (non earthed) input. Leaving the reference point of a differential amplifier float usually isn't a good idea since this might be a source of strange common mode noise. So: no connection required if your 'scopes input is earthed, but recommended to ground if you are using an isolated 'scope or multimeter input.

[sibeen]

Actually the waveform is what I am really interested in. Sorry if I was vague before.

I must admit I'm a bit bemused here. If you were monitoring for a few seconds then the scope would be the right tool as you can easily visually check through the waveforms for any anomaly; but if you are monitoring for an hour? That's 180,000 cycles at 50 Hz. I'd hate to be the person who had to go through all that.

A data logging DM that can also do a min/max and peak etc would seem to be the way to go. Another would be a scope recorder but you're entering a completely different price bracket there.

[ollihd]

I think the ground lead is  useful in case you use an isolated (non earthed) input. Leaving the reference point of a differential amplifier float usually isn't a good idea since this might be a source of strange common mode noise. So: no connection required if your 'scopes input is earthed, but recommended to ground if you are using an isolated 'scope or multimeter input.

Thank you for clearing this up

Actually the waveform is what I am really interested in. Sorry if I was vague before.

I must admit I'm a bit bemused here. If you were monitoring for a few seconds then the scope would be the right tool as you can easily visually check through the waveforms for any anomaly; but if you are monitoring for an hour? That's 180,000 cycles at 50 Hz. I'd hate to be the person who had to go through all that.

A data logging DM that can also do a min/max and peak etc would seem to be the way to go. Another would be a scope recorder but you're entering a completely different price bracket there.

Actualy going at it at 100 Hz, but nonetheless I'm not planning on visually checking the whole capture. I'm planning to record from scope to pc and automate the "checking" process on the pc end.