# AC Voltage and current, amplitude, rms, peak value

1. Aug 28, 2011

### Deathfish

General enquiries (template may not be suitable)

- When the question provides value of AC voltage amplitude, is this the same value of peak voltage?

- Also, in current waveform, when do we use peak current and rms current?

- In ohm's law equations we use rms current, so in the current waveform graph do we need to times square root of 2?

- If provided with current waveform, can we get output dc current from using the same equation for voltage? eg. 2 x Vmax / $\pi$, Vmax / $\pi$

Relevant Equations :

V (odc) = 2 x Vmax / $\pi$, V(odc) = Vmax / $\pi$,
V (peak) = Vrms x $\sqrt{2}$

Last edited: Aug 28, 2011
2. Aug 28, 2011

### wukunlin

yes, voltage amplitude means maximum deviation from DC value, or half the peak-to-peak
there isn't a set rule for this unfortunately, in my electrical engineering course it is much easier to use peak values for sinusoidal or complex expressions. only cases I have come across using RMS is calculation of power dissapation
as long as you are consistent, ie use only rms or only peak values in your calculatoins, ohm's law will hold, just like converting all units to metric units
i'm not sure what these voltage formalae are used for but to obtain DC current you can use the ac waveform and find its average value over one cycle, more often then not they are quite obvious from inspection

3. Aug 28, 2011

### Staff: Mentor

Unless otherwise specified, AC voltage and current values are given as RMS. This is convention.

Sometimes it is necessary to know about peak values when specifying things like dielectric strengths or insulation. Also, if nonlinear components are involved (like diodes) and time constants or triggering levels are involved, you may need to work with the actual voltage or current waveforms in their 'peak' form.