Mains Vpp

[Davidjp]

I checked the 240vac mains at my place with a scope, and in the auto measure it says ** vpp 704**, vrms 240 and a few other readings, but where does the 700 vpp come from, I expected 350vpp or thereabouts. Any ideas?

 

[Minder]

240vac RMS , peak = 240x1.414 = 339.
Peak to Peak = 339x2.
M.

 

[(*steve*)]

Measuring the mains with your oscilloscope is potentially dangerous.

Please be careful

 

[AnalogKid]

As above, most of it is the difference between peak and peak-to-peak. Some of it might be noise.

If the scope has a vertical channel bandwidth limit function, set it to its lowest corner frequency.

Get a 0.1 uF, 250 Vac, X, X2, or Y rated capacitor and connect it directly across the measurement point (scope probe tip to ground lead) with the shortest possible leads.

No warranties expressed or implied - try not to kill yourself.

ak

 

[Davidjp]

240vac RMS , peak = 240x1.414 = 339.
Peak to Peak = 339x2.
M.

Yes it's something like that,I've checked against the grid and used cursor measurement as well as scopes auto measure, having trouble picturing it. Is the Vrms only calculated on 1/2 the cycle ? Must be! Thanks..

 

[Davidjp]

Measuring the mains with your oscilloscope is potentially dangerous.

Please be careful

Thanks, don't worry I'm very careful.

 

[Davidjp]

As above, most of it is the difference between peak and peak-to-peak. Some of it might be noise.

If the scope has a vertical channel bandwidth limit function, set it to its lowest corner frequency.

Get a 0.1 uF, 250 Vac, X, X2, or Y rated capacitor and connect it directly across the measurement point (scope probe tip to ground lead) with the shortest possible leads.

No warranties expressed or implied - try not to kill yourself.

ak

Don't worry I'm very careful, I thought the peak was both pos. and neg. part of cycle,must be just 1/2 (180°). Was thinking wrong. thanks to all who replied,

David.

 

[Minder]

, having trouble picturing it. Is the Vrms only calculated on 1/2 the cycle ? Must be! Thanks..

Yes, if you measure with a simple VOM it will indicate RMS value between zero cross and peak of one half cycle.
M.

 

[AnalogKid]

Vpeak is the max voltage anywhere in 1/2 cycle, measured with respect to the waveform centerpoint. Vpeak-to-peak is measured between the max positive value in a positive half-cycle and the max negative value in a negative half-cycle. Vrms is calculated using a full cycle.

ak

 

[Davidjp]

Vpeak is the max voltage anywhere in 1/2 cycle, measured with respect to the waveform centerpoint. Vpeak-to-peak is measured between the max positive value in a positive half-cycle and the max negative value in a negative half-cycle. Vrms is calculated using a full cycle.

ak

I've got it sussed now thanks to all repliers. David.

 

[(*steve*)]

RMS values involve squaring, averaging, and taking the square root. Mathematically, doing that will always give you a positive number.

This calculation gives you the DC voltage which would result in the same heating effect in a resistor. If you consider that reversed voltage results in heating just as positive voltages do, the result is intuitively reasonable.