Isolation transformer for an oscilloscope

I’m looking for an isolation transformer that’s appropriate for probing 120V AC mains with an oscilloscope. While internet searches turn up a lot of talk about the need for an isolation transformer, I’m not having any luck finding an example of what might be appropriate. The Digi-Key forums seem like a good place to ask. :smile:

My goal isn’t to do anything fancy with it. I’d like to be able to safely check 120V AC mains for basic waveform inspection. (I am aware of the hazards of 120V AC mains power, and take appropriate precautions.) A transformer ratio in the order of 10:1 seems appropriate, but I’m open to ideas.

Does anyone have any suggestions?



I am seeing a few posts saying that “floating” a scope is very dangerous.
One recommendation I saw was to use a probe that is built for that.

Take a careful look at the documentation for your 'scope and its probes, and also their labeling; you’ll likely find a rating to the effect of “300V CAT II” or something similar. These communicate the maximum peak working voltage and type of installation that the device is considered capable of safely measuring, per IEC/UL. Category II essentially refers to things that plug into an outlet, whereas CATIII refers to permanent installations, e.g. measurements within a breaker panel if I recall correctly.

So it’s not complete lunacy to probe the outlet using the 300V or 600V CATII probes that many 'scopes ship with.

Ya gotta read the fine print, and know what you’re doing: many probes come with a cute little attenuator switch that varies the voltage passed to the 'scope by a factor of 10, or something like that. Not a good thing to have in the wrong position. There’s also typically a software-selected input impedance on modern 'scopes, with different input limits; mine’s good for <5V on the 50Ω setting, and <400V on the 1MΩ setting.

And one also needs to appreciate that the ground lead on the typical 'scope probe has a direct connection to the Earth ground prong on the 'scope’s power cord. Connect that to the wrong side of the AC line, and you create a dead short through the 'scopes internal ground network. Not good for one’s scope, that.

Ultimately, “safety” here is more a matter of using the tools correctly than the tools themselves. The exact same gear can be used successfully or to unfortunate end, depending on configuration.

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Never plug a piece of test gear into an isolation transformer because it will allow the case of the test equipment to float above ground with potential lethal results!

You plug the equipment under test into the isolation transformer and keep the safety earth ground working on your test gear.

I have a B&K TR110 I bought for TV/CRT repair work back in the 1980s. Nowadays, with no CRT’s in sight, I mostly use it for varying the mains AC input when testing gear.