AC Circuits: Understand Vpp, Vp & Vrms

[Weightofananvil]

Hi, I am studying AC circuits involving series-parallel ac circuits, thevenin equivalents, superposition etc.
In labs on our function generators you are always setting a Vp or Vpp depending on model. I'm a little confused though when in calculations you should use peak and when you should use Vrms.

I know most things will work either way, but there has to be a standard?
Its all just a little confusing and I don't see what to use when.

Thanks.

 

[CWatters]

Peak voltages are useful for working with instantaneous voltages, for example the instantaneous voltage of a sine wave at time t is given by V(t)=Vp * sin(wt).

Vrms is typically used when you need to work out average power in an ac circuit. You can't just multiply the current by the average voltage because for a sine wave the average is zero.

 

[berkeman]

To add a little to CWatters' reply, the AC Mains voltages that we talk about (110V, 120V, 220V, 240V, etc.) are RMS voltages, since they are associated with the power grid. so a 120Vrms AC Mains voltages is actually 120V * 2√2 = 339Vpp.

 

[jim hardy]

When looking at a waveform on an oscilloscope it is natural to look at the top and bottom of the wave, that distance is P-P. If you look from zero to top or bottom it's Peak.

RMS is a math process that calculates the "effective heating value" of a non-DC voltage or current. It gives the DC voltage that'd make the same heat in a load as the AC whose heating value is being calculated.
When you see it it almost invariably refers to a sine wave AC voltage or current. Peak to RMS for sinewave is 0.707, √2/2 .

http://www.hobbyprojects.com/oscilloscope_tutorial/voltage_measurements.html

Standard ? Every user will use whichever notation is most convenient for the task at hand. Hopefully he'll make it clear which he's using.