measuring mains with oscilloscope

[tggzzz]

I logged a week on an oscilloscope the mains voltage at my house. Captured large voltage dips that likely fried my refrigerator compressor and my old AC compressor. I showed the logs to SCE who paid for those failures after finding a loose connection on the pole I get power from. I then logged another week to make sure it didn't happen again.

You can't do that plugging in a lamp.

Just so, but there are simple easy cheap and safer ways to achieve that than connecting a scope to the mains, e.g.:

• use an ac voltmeter that can capture and upload the values to a PC
• connect a standard 240V/12V (etc) transformer to the mains, and connect the scope to the 12V output

With a little thought and imagination, it is usually possible to avoid the potential dangers of connecting a scope directly to the mains.

There aren't a lot of dangers when you're using a safe scope, or safe probing.

Indeed, but that won't be the case with OPs that are asking questions like in this thread!

I personally used a custom mains cable and an isolated(signal to ground isolation as well as channel to channel and battery powered) scope.

That might not be OK if there is exposed metal anywhere and the probe's shield is connected "unfortunately".

None of my meters were able to catch the occasional dips. The one room they caught anything was the one power was being lost entirely causing the UPS's to kick in. I could have used a transformer but that's potentially more hairy than a properly designed scope or diff probe which are both easier to manage if you're thinking of people who may not be as aware of dangers.

Indeed, but people like the OP are highly unlikely to have that equipment.

In the absence of that equipment, a 240/12V transformer (not auto-transformer!) is probably most difficult (not impossible) for newbies to use dangerously.

[StillTrying]

I don't get this fascination with the mains, but still, since I found how linear a LED to photo diode can be, I've wanted to get around to making an isolated mains waveform shape viewer.

https://www.eevblog.com/forum/beginners/floating-probe!-for-$2-50/msg1849505/#msg1849505

I've scribbled a few attempts to convert the pk-pk mains voltage into the 6mA to 12mA through a LED. If you don't mind the pk-pk isolated output being only around ~50mV to ~150mV, the isolated receiver could be just a photo diode and resistor. It wouldn't be much use for an accurate pk-pk voltage measurement or fast spikes but there's more than enough BW to see the shape of a 50Hz.

[maginnovision]

I logged a week on an oscilloscope the mains voltage at my house. Captured large voltage dips that likely fried my refrigerator compressor and my old AC compressor. I showed the logs to SCE who paid for those failures after finding a loose connection on the pole I get power from. I then logged another week to make sure it didn't happen again.

You can't do that plugging in a lamp.

Just so, but there are simple easy cheap and safer ways to achieve that than connecting a scope to the mains, e.g.:

• use an ac voltmeter that can capture and upload the values to a PC
• connect a standard 240V/12V (etc) transformer to the mains, and connect the scope to the 12V output

With a little thought and imagination, it is usually possible to avoid the potential dangers of connecting a scope directly to the mains.

There aren't a lot of dangers when you're using a safe scope, or safe probing.

Indeed, but that won't be the case with OPs that are asking questions like in this thread!

It will be if they have a diff probe which can easily be used to safely probe things like mains.

I personally used a custom mains cable and an isolated(signal to ground isolation as well as channel to channel and battery powered) scope.

That might not be OK if there is exposed metal anywhere and the probe's shield is connected "unfortunately".

That's true but if your isolated scope was designed in such a way that you could be exposed to the voltages at the probe... I don't know what to tell you. My fluke and my keysight have nothing exposed, even the BNC for the probes are plastic, the probes are all plastic.

None of my meters were able to catch the occasional dips. The one room they caught anything was the one power was being lost entirely causing the UPS's to kick in. I could have used a transformer but that's potentially more hairy than a properly designed scope or diff probe which are both easier to manage if you're thinking of people who may not be as aware of dangers.

Indeed, but people like the OP are highly unlikely to have that equipment.

In the absence of that equipment, a 240/12V transformer (not auto-transformer!) is probably most difficult (not impossible) for newbies to use dangerously.

I think I misunderstood this part, you were thinking something like a plug in transformer and I was thinking a loose transformer. In this case you're right it'd be difficult to misuse. A diff probe however is almost(but not quite) as easy and also provides you a useful tool in that it has many other uses. You can check a power supply and anything else it powers in any way you'd like without shorting it.

You can't do that plugging in a lamp.

I could point a camera at a lamp much more easily than I could record a week's worth of waveforms on my 'scope.

Maybe? I only have LED bulbs. They aren't gonna notice much other than a near power outage. In this case I'd be saving an old bulb as some sort of special tool and I'd rather have a diff probe. There is also the issue of trust with the utilities. I doubt I could even get someone to come out for a dimming bulb assuming my camera would catch it to send them a video. I'd also need a lot of video of that since to get them to pay for failures you really do need to prove it's been going on for some time.

[vk6zgo]

I'd like to know what information people hope to gain by poking at mains with an oscilloscope as opposed to, eg., plugging in a lamp.

For Pete's sake, everybody, buy a bloody differential probe, & those of us with enough knowledge to do it safely in other ways can just quietly die out, without running into hundreds of iterations of this thread  everytime we log on throughout our declining years!

You don't even need that for a quick test. Just pop the ground clips off your probes and set the 'scope to "AC" mode.

(Use two probes and "difference" as necessary).

Indeed, but you probably just generated another twenty or so postings about how inadequate this  method is, & how you can only do any testing with a differential probe ---- yada, yada, yada.

Back in the day, when differential probes cost as much as a small car, if I asked the Boss for such a device, he would have reacted in the mode so succinctly described in the modern abbreviation "ROFLMAO", & I would have been sent off "with a flea in my ear" to find a workaround.

Now that they have become relatively cheap, differential probes have been elevated to something of a cult status,

[tggzzz]

I logged a week on an oscilloscope the mains voltage at my house. Captured large voltage dips that likely fried my refrigerator compressor and my old AC compressor. I showed the logs to SCE who paid for those failures after finding a loose connection on the pole I get power from. I then logged another week to make sure it didn't happen again.

You can't do that plugging in a lamp.

Just so, but there are simple easy cheap and safer ways to achieve that than connecting a scope to the mains, e.g.:

• use an ac voltmeter that can capture and upload the values to a PC
• connect a standard 240V/12V (etc) transformer to the mains, and connect the scope to the 12V output

With a little thought and imagination, it is usually possible to avoid the potential dangers of connecting a scope directly to the mains.

There aren't a lot of dangers when you're using a safe scope, or safe probing.

Indeed, but that won't be the case with OPs that are asking questions like in this thread!

It will be if they have a diff probe which can easily be used to safely probe things like mains.

... and if everybody had good eyesight there would be no need for spectacles!

I personally used a custom mains cable and an isolated(signal to ground isolation as well as channel to channel and battery powered) scope.

That might not be OK if there is exposed metal anywhere and the probe's shield is connected "unfortunately".

That's true but if your isolated scope was designed in such a way that you could be exposed to the voltages at the probe... I don't know what to tell you. My fluke and my keysight have nothing exposed, even the BNC for the probes are plastic, the probes are all plastic.

Lucky you. Unlucky OP.

None of my meters were able to catch the occasional dips. The one room they caught anything was the one power was being lost entirely causing the UPS's to kick in. I could have used a transformer but that's potentially more hairy than a properly designed scope or diff probe which are both easier to manage if you're thinking of people who may not be as aware of dangers.

Indeed, but people like the OP are highly unlikely to have that equipment.

In the absence of that equipment, a 240/12V transformer (not auto-transformer!) is probably most difficult (not impossible) for newbies to use dangerously.

I think I misunderstood this part, you were thinking something like a plug in transformer and I was thinking a loose transformer. In this case you're right it'd be difficult to misuse. A diff probe however is almost(but not quite) as easy and also provides you a useful tool in that it has many other uses. You can check a power supply and anything else it powers in any way you'd like without shorting it.

Even a naked 240V/12V transformer would be more difficult to misuse, since most people have an intuition that 240V=dangerous and 12V=safe. They ought to instinctively put the probes on the 12V side; if not then the species has been improved.

[maginnovision]

Well with a loose transformer the main thing I think of is someone accidentally touching the wrong leads not actually probing them. It's the same reason floating a scope isn't the recommendation most of the time, accidents happen. Purely hypothetical though because I doubt such a person would have one lying around. A plug in step down transformer can be had for things like powered PC speakers though.

[tggzzz]

Well with a loose transformer the main thing I think of is someone accidentally touching the wrong leads not actually probing them. It's the same reason floating a scope isn't the recommendation most of the time, accidents happen. Purely hypothetical though because I doubt such a person would have one lying around. A plug in step down transformer can be had for things like powered PC speakers though.

Agreed.

That was behind the "(not impossible)" in my statement "In the absence of that equipment, a 240/12V transformer (not auto-transformer!) is probably most difficult (not impossible) for newbies to use dangerously."

Mind you, 240V/12V transformers are nowhere near as common as they used to be!