Difference between multimeter AC measurement and Vpp at scope

Arjan-Q

Joined Sep 20, 2021
7
Hello all,

I have searched but couldn't find the answer to my question (or I haven't searched correctly...). I hope that you can help me and give me some insight.

I have a 230Vac to 6Vac adaptor. When using a multimeter for checking the adaptor its output I get a reading of ~7Vac (ok, its 1Vac higher than it says on the adaptor but I don't mind). However, when measuring with my scope I see something which I didn't expect. I get an (imperfect, ok) sine wave which measures only 2 Vpp... I expected to see at my scope ~20Vpp (i.e. 2* [7 * sqrt(2)] = 9.9 V). It seems it doesn't matter whether or not I load the adaptor.

What am I missing?

Kind regards,
Arjan

Audioguru again

Joined Oct 21, 2019
3,866
Obviously, you have a divide-by-10 probe on your scope.

KeepItSimpleStupid

Joined Mar 4, 2014
5,090
10/2 = 5 not 10

Tonyr1084

Joined Sep 24, 2015
6,462
The probe is a 10x attenuator. Meaning whatever voltage is at the probe tip it is attenuated (reduced) by a factor of 10. So 20VAC attenuated by 10 (20 ÷ 10) equals 2Vpp. Simple mistake I've made many times. OR the scale is set incorrectly.

Arjan-Q

Joined Sep 20, 2021
7
Obviously, you have a divide-by-10 probe on your scope.
Argh... You are completely right. I actually had no idea my probe was a 10x attenuator. Just checked, it makes sense, I totally missed it.

Thank you
Arjan

Joined Jan 15, 2015
6,221
I get an (imperfect, ok) sine wave which measures only 2 Vpp...
Something else to consider and I appreciate you were using a scope. Many inexpensive multimeters are average responding RMS indicating. Unless the AC measured sine wave is a pure non distorted sine wave the meter will not indicate the actual RMS value. A good true RMS responding RMS indicating meter will give the true RMS value. Just as a side note.

Ron

dcbingaman

Joined Jun 30, 2021
498
You may want to stick to using the x10 probe in the future as well. This reduces the chance of damaging your scope if you try to look at larger AC signals.

Arjan-Q

Joined Sep 20, 2021
7
Something else to consider and I appreciate you were using a scope. Many inexpensive multimeters are average responding RMS indicating. Unless the AC measured sine wave is a pure non distorted sine wave the meter will not indicate the actual RMS value. A good true RMS responding RMS indicating meter will give the true RMS value. Just as a side note.

Ron
That's a valuable addition Ron, thanks for mentioning it.

You may want to stick to using the x10 probe in the future as well. This reduces the chance of damaging your scope if you try to look at larger AC signals.
As a matter of fact, I was thinking of how to proceed moving to higher AC voltages while lowering the chance of damaging my scope. Using the 10x probe is indeed a good way to do so, thanks for pointing it out.

Thank you again for all your input.

Arjan

KeepItSimpleStupid

Joined Mar 4, 2014
5,090
The naked scope with a x1 probe actually has a lesser bandwidth than stated.
With a compensated x10 probe, bandwidth will be within spec and the input impedance changes from 1M||22pf to 10 Meg-ohms.