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I've been reading about how to connect an oscilloscope to the mains power so I could see its waveform, but I'm confused with the answers.

I read I could just use some transformer and see the waveform on the new amplitude - and I'd have to ground the transformer too, but the voltage was less, so it would be safer, if I understood correctly the idea of using a transformer in this case. I have one here that gets 230 V to about 4 or 5 V, but suppose I didn't have one.

  • Could I just connect the positive of the probe to the live terminal on the outlet and the negative (ground) to the outlet ground and that's it?
  • What if I connect the probe ground terminal to the neutral outlet wire? What about that? No problem since earth ground and neutral are connected anyways? (Even though there might be a small potential difference between the 2, but I guess that would be fine.)
  • What about which probe I'd use? Above I'm talking about an oscilloscope normal passive probe (probing terminal and ground terminal,) but does it have to be special? Like, must it be a 10x attentuation one? Can't it just have no attentuation at all? I don't have any differential probes nor will I buy one (amazingly expensive and I'm just a hobbyist student, so not much money.)
  • Could I just make a direct connection in this case? Would it be dangerous somehow? I'm also not floating the oscilloscope. It's a very old one. If any cable gets loose inside the scope, I could be doomed and I still like being around.
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    \$\begingroup\$ In theory you can measure mains voltage with an oscilloscope. But to do it, you need the right equipment with right ratings, and you don't say anything about what equipment you have and what ratings they have. You must make sure what is the maximum voltage allowed on scope input, which determines what kind of probe attenuation you need, and also the probe must be rated to handle the voltage. \$\endgroup\$
    – Justme
    Aug 5, 2021 at 5:06
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    \$\begingroup\$ It's also highly likely that your scope's ground is connected to the ground of its AC supply. So if you hook it up to mains don't do it backwards. \$\endgroup\$
    – IceGlasses
    Aug 5, 2021 at 6:18
  • \$\begingroup\$ @IceGlasses sorry, I didn't understand the last part. What do you mean don't do it backwards? \$\endgroup\$
    – Edw590
    Aug 5, 2021 at 23:00
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    \$\begingroup\$ @DADi590 Don't hook the scope's ground to the AC "hot", as that would probably more or less short it. \$\endgroup\$
    – IceGlasses
    Aug 6, 2021 at 0:19
  • \$\begingroup\$ Ah ok . Thanks! \$\endgroup\$
    – Edw590
    Aug 6, 2021 at 0:34

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Most oscilloscope inputs go to a few hundred volts.

'230 V' mains has a peak of around 325 V, though it would be permitted to sometimes go up to 265 Vrms = 375 V peak in many places. That leaves no or little margin. Do you feel lucky connecting it directly?

On top of that, mains often spikes to 1500 V. Anything sold for direct connection to the mains has to be designed to withstand these spikes.

So yes, the probe has to be special. It has to be rated to take a 1500 V impulse without breaking to short circuit.

It's very unwise to connect the scope ground to the wiring neutral. There could be a few volts on it if there's a high current flowing somewhere in the house or its neighbours. There could be 230 Vrms on it if there's a wiring error somewhere in the house. Better to measure live and neutral with two probes and use Y1-Y2 on a dual scope.

A transformer is a good solution, though some could distort the output waveform if they don't have the headroom from saturation. If you have two identical ones, then primaries in series, secondaries in parallel, will mean that they share voltage properly and run at half their design flux.

A solution I use is to build a 100:1 voltage divider, and to build the input resistor from either a resistor properly rated for 1500 V, or a series string of equal value resistors such that their voltage handling is >1500 V. Build two for measuring live and neutral at the same time.

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  • \$\begingroup\$ As a start I had no idea about 1500 V spikes. Interesting to know. Might not put a multimeter not rated for that on an outlet again haha. Thank you! (for that and for the rest) \$\endgroup\$
    – Edw590
    Aug 5, 2021 at 23:14

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