Measure mains voltage with oscilloscope

Sometimes there are needs to measure mains voltages with oscilloscope. You must be very careful when measuring voltages on the mains with any measurement instrument. The mains voltages are absolutely deadly. Measuring mains voltages with multimeter can be dangerous. Trying to do the measurement of mains voltage with oscilloscope is potentially more dangerous, because typically oscilloscopes are are grounded devices that have the ground side of the probe connected to the equipment case. Doing connection to mains voltage slightly wrongly can end up causing short circuit, damage the probe, your oscilloscope or electrocute you.

If you are using a traditional oscilloscope that is mains powered, ALWAYS MAKE SURE YOUR SCOPE IS GROUNDED BECAUSE THE GROUND IS THERE FOR YOUR SAFETY!

Usually my first choice when needing to measure mains voltage with oscilloscope is to use a completely isolated scope-meter. I have UT-81B scope-meter for this purpose. I can use it with multimeter probes just like a normal multimeter, but get oscilloscope view on the it’s screen.

If I need to use my mains powered desktop oscilloscope, then my first choice is to use a CAT rated differential probe which has been designed to measure the voltage levels I want to measure. That’s the best choice.

I do not want to connect my normal oscilloscope probes anywhere near the device that is powered directly from mains outlet. In some cases I might consider doing measurement to mains voltage devices if I can power the device under test through mains isolation transformer.

Here are some videos on measuring mains voltage with oscilloscope:

Measuring Mains Voltage with Oscilloscopes

Correct use of the oscilloscope when probing circuits connected to mains

EEVblog #279? – How NOT To Blow Up Your Oscilloscope!

Check also Using oscilloscope safely with AC mains and measuring mains with oscilloscope and Using oscilloscope safely with AC mains discussions.

8 Comments

  1. Tomi Engdahl says:

    EEVblog 1415 – Reverse Engineering the DP10007 Differential Probe
    https://www.youtube.com/watch?v=GOk1BYKSsOI

    Reverse engineering the Micsig DP10007 high voltage differential probe.
    Turning the PCB into a schematic.

    EEVblog 1413 – Oscilloscope Current Probe TEARDOWN + DEMO
    https://www.youtube.com/watch?v=suf3VWqs5zw

    Teardown and measurements with the Micsig CP2100B 2.5MHz oscilloscope current probe.
    Also a demonstration of mains switch mode power supply current and noise measurement, and also some performance tests.

    Reply
  2. Tomi Engdahl says:

    Everything you need to know when buying/using an Oscilloscope! EB#49
    https://www.youtube.com/watch?v=d58GzhXKKG8

    In this electronics basics episode we will be having a look at the biggest mistake you can do when working with an oscilloscope. But I will not only show you how to do safe mains voltage measurements but also show you all the basics when it comes to oscilloscopes. That means I will show you how to choose one, how to do simple voltage and current measurements and finally how FFT works. I will explain passive probes, triggering, voltage & time division, measuring functions, the cursor function, AC & DC coupling and single mode capturing. Let’s get started!

    Reply
  3. Tomi Engdahl says:

    Everything you need to know when buying/using an Oscilloscope! EB#49
    https://www.youtube.com/watch?v=d58GzhXKKG8

    In this electronics basics episode we will be having a look at the biggest mistake you can do when working with an oscilloscope. But I will not only show you how to do safe mains voltage measurements but also show you all the basics when it comes to oscilloscopes. That means I will show you how to choose one, how to do simple voltage and current measurements and finally how FFT works. I will explain passive probes, triggering, voltage & time division, measuring functions, the cursor function, AC & DC coupling and single mode capturing. Let’s get started!

    0:00 The big mistake when using an oscilloscope
    1:08 Intro
    2:02 How to choose a scope?
    4:44 Passive probes & scaling factor
    5:55 Trigger
    6:41 Voltage division
    7:11 Time division
    7:37 Measure function
    7:47 Cursor function
    8:19 AC & DC coupling
    9:08 Single mode capturing
    9:36 Current measurement
    10:16 Safe mains voltage measurement
    11:37 Differential probe
    11:52 Math & FFT

    Reply
  4. Tomi Engdahl says:

    Fiber Optic Isolated Voltage Probe. ISOVP
    https://saker-mv.com/fiber-isolated-voltage-probe/?gclid=EAIaIQobChMI2-WT15Od9AIVRisYCh2aoAONEAEYASAAEgKVFfD_BwE

    The ISOVP isolated voltage probe allows the safe and accurate measurement of floating voltages in power electronic modules or electronics. Although it is best suited for medium voltage equipment due to the isolation provided, the high common mode and EMI rejection make it a perfect choice for general low voltage measurements. The 25V input voltage range with the supplied x10 attenuator is ideal to measure gate voltage in various semiconductor switching devices and study potential problems and improvements. Also input capacitance has been kept to a minimum as this is what really hurts most high-frequency measurements, causing loading of high and medium impedance signal sources and signal degradation.

    Reply
  5. Tomi Engdahl says:

    How to (Safely) Measure Full Wave Rectified AC Mains On An Oscilloscope
    https://www.youtube.com/watch?v=idb7XRBVImQ

    Video response to EEVblog #279 where Dave talks about pitfalls when connecting your oscilloscope’s ground lead to your circuit. Great video, but he left out and especially sneaky scenario which has caused problems for me (and others I’m sure) in the past. Sneaky because it looks fine on paper, but the problem arises because of the way homes and offices in the US are connected to mains. I’m not sure how many countries are like this, but I know there are at least a few, so worth watching and being aware of regardless of where you live.

    EEVblog#279:

    Video response to EEVblog #279 where Dave talks about pitfalls when connecting your oscilloscope’s ground lead to your circuit. Great video, but he left out and especially sneaky scenario which has caused problems for me (and others I’m sure) in the past. Sneaky because it looks fine on paper, but the problem arises because of the way homes and offices in the US are connected to mains. I’m not sure how many countries are like this, but I know there are at least a few, so worth watching and being aware of regardless of where you live.

    EEVblog#279:
    https://www.youtube.com/watch?v=xaELqAo4kkQ

    Reply
  6. Tomi Engdahl says:

    Analog VS Digital Scopes for Glitch Captue
    https://www.youtube.com/watch?v=3jDRH-6IvZc

    Another Twitter question, a demo of the vast superiority of digital oscilloscopes compared to analog scopes when detecting infrequent glitches. And the importance of Persistence mode in debugging.

    Reply

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