When do I use Ammeter's AC mode?

[AceOfHearts]

Two questions:

1) I was wondering, when do I use the AC mode on the Ammeter? Perhaps what I mean is, what does this mode show?

Suppose we have V(t)=V.sin(t), a sine wave in other words, which we want to measure; what would the reading be telling us? The peak voltage V? rms? If rms, can someone explain to me what this means.

2) I have a 7805 connected to a motor via another device. The motor is using PWM, which are square waves. The period of these waves are about 100ms. If I want to measure the current output from this 7805 while the motor is running under this PWM signal, should I be using the AC mode, or the DC mode?

Many thanks.

 

[kchriste]

1) When you want to measure AC current.
2) What you "measure" will depend on what you measure it with. If you use a scope it will be easy to read the peak-peak voltage. With a true RMS voltmeter you will get the RMS value. RMS means Root Mean Square which in simple non mathematical language means the RMS voltage will produce the same power into a resistive load as will the same DC voltage. Same goes for RMS current.
2)You would use a DC ammeter in series with the 7805 output. This is because it is pulsating DC and not AC.

 

[Sceadwian]

The AC measurement mode is all but useless on most cheap meters as it's not true RMS and depends on the input signal being a pure sine wave between 50-60hz Measuring a PWM signal going to a motor with a hand held meter like that is a waste of time and will give you virtually no useful information. Even true RMS multimeters have a frequency range which they work in.