I did read several questions about safely probing high AC voltages with an oscilloscope. I had been taught to isolate the scope, which I see now is dangerous. So I think what I should do is then use an isolation transformer between the DUT and the 120VAC wall outlet. My question is then if I want to measure say just the AC voltage waveform can I just connect my ground clip to the isolated neutral and my probe tip to the isolated line? Assuming both probe and scope input can take the voltage?

Or do I need another transformer between isolated neutral and line and the scope?


2 Answers 2


Use a quality, high impedance, differential probe if your scope doesn't have the capabilities of differential inputs. This is an external device that provides isolation, typically up to 500 or 1000 VDC, and yet delivers a low level (.01 or .001) representation of the input signal.

If you scope does have the capability of differential inputs, Select "A + B", and use those two input channels to measure your signal. Note that you do NOT use common and the probe tip, but only the two probe tips.

Dealing with HV as I do, I much prefer just using the differential isolator for safety sake... It prevents even accidentally touching the (typically) grounded ring at the end of the probe to a live circuit, letting out the magic smoke.

  • \$\begingroup\$ What about measuring a small voltage like 3.3V like a VCC for a micro that's non-isloated? I'm a long time engineer, just new to high voltage work and want to be safe. \$\endgroup\$
    – confused
    Commented Dec 8, 2015 at 16:28
  • 1
    \$\begingroup\$ A Decent differential isolator can handle small signals well. \$\endgroup\$
    – R Drast
    Commented Dec 8, 2015 at 16:48

I use a scope with four isolated channels-- ideal for work on such circuits. It can also be powered by internal lithium battery packs, which can reduce the capacitive coupling through the scope power supply.

The one I have is a Tek TPS2024, but I think there's a newer model out now.


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