Oscilloscope mystery

[herrdahlin]

I'm measuring voltages with my new 'scope. On VAC i get a result 2,8 - 3,0 times larger than on my DMM. On VDC it's spot on.
What's wrong?

 

[dhaen]

You are probably measuring P-P (peak to peak)of the waveform on the scope. The DMM will normally read approximately the RMS value with a sine wave.

Check the scope calibration using the cal test point on the front.

 

[lumanauw]

Yes, 2.828V (peak to peak) on scope will be read as 1V (RMS reading) on multimeter.

 

[herrdahlin]

Thank you, this is obviously what's happning. Measuring by the probe calibration outlet gives spot-on result.
How do I read the result on the scope to match the result from the DMM?

Now I've just to figure out how to use this information in practice - ie in building my new psu. But that's another story.

 

[dhaen]

Well if measuring a sine wave, as mains is, then divide the peak to peak by 2.828 to get the RMS value.
Other wave shapes will have different ratio's. This article will give you the basics.

 

[MarkRehorst]

The result on the scope does match the meter. They are measuring different things. On the scope waveform you can see the peak to peak (instantaneous) voltage. The meter reads rms (average) voltage of that same peak to peak waveform. Since you are looking at a sine wave you have to divide the peak to peak voltage displayed on the scope by 2*sqrt(2) to get the rms value that is displayed on the meter.

If you look at any waveform other than a sine wave, your meter will not read correct rms voltage unless it is labeled "true rms". Also, any pk-pk waveform other than a sine cannot be converted to an rms value by dividing by 2*sqrt(2).

And yes, 240VAC line voltage is an rms value. That means the pk to pk voltage is 2*sqrt(2)*240=678.8V

I_F

 

[herrdahlin]

Again, thanks all of you for helping me jump this hurdle.