# RMS vs MEAN Oscilliscope

#### ben sorenson

Joined Feb 28, 2022
180
I have a DC voltage source of 600mV that can output a maximum current of 100uA. The current is passed through a resistor of 5ohms and pulsed through an air core solinoid with a resistance of 75ohms and 5000 turns.

My oscilliscope reads as follows.

Max Voltage 10V
RMS (unstable) 3-5V
Mean: -400mV
Freq:116khz
Time Div (1us)
I'm having trouble understanding the difference beetween "mean" voltage and RMS voltage and also why the "mean" voltage would be a negative number.

#### MrChips

Joined Oct 2, 2009
30,494
If you have a sinusoidal waveform that is symmetrical about 0V then the mean is also 0V. Mean voltage can be negative if the waveform drifts even slightly below 0V.

The RMS is the heating effect of the signal. The RMS value of a sinusoidal waveform is 0.707 x amplitude.

#### tonyStewart

Joined May 8, 2012
127
Mean = Average

But if it is air coupled, the "mean" on an oscilloscope over 1 repeating cycle will be 0V but with a fraction of a cycle at the end of the trace, it won't.

RMS is "root mean squared"

#### ben sorenson

Joined Feb 28, 2022
180
Mean = Average

But if it is air coupled, the "mean" on an oscilloscope over 1 repeating cycle will be 0V but with a fraction of a cycle at the end of the trace, it won't.

RMS is "root mean squared"
Ok, thank you so on the oscilliscope in terms of what the voltage / power is, on a sinusoidal waveform is it more accurate to use the RMS voltage then not the Mean?

#### MrChips

Joined Oct 2, 2009
30,494
Ok, thank you so on the oscilliscope in terms of what the voltage / power is, on a sinusoidal waveform is it more accurate to use the RMS voltage then not the Mean?
Yes.

Power = square of voltage / resistance = V x V / R

RMS = root mean square = the DC voltage that has equivalent power.

Hence we compute the square of the voltage, find the mean, then take the square root.
Hence, square root of the mean of the square.

(Since we square the voltage, RMS is never negative.)