when we measure Vce in the lap we use a DC voltmeter thogh V

when we measure Vce in the lap we use a DC voltmeter ,thogh Vce is like the signal but bigger (lets say sine wave),

so why we donot use AC voltmeter to measure the rms value of it ?

Solution

For sine waves we see that RMS is equal to 0.707 times the peak value while Average is 0.637 times the peak, so we can divide one figure by the other to obtain an average-to-RMS conversion factor of 1.109

Measuring RMS values is a bit more expensive than measuring average values, so most multimeters avoid the former. Instead they presume your signal is a sine and measure the average value for the rectified sine or the peak value, after which they apply a conversion factor to find the presumed RMS value.

Most cheap (non-RMS-reading) multimeters are AC-coupled on the AC voltage ranges, so you would read the correct RMS value of the ripple only on an AC range, assuming the ripple is sinusoidal, due to the correction factor which is applied. If you happen to have one that\'s not AC coupled, just stick a large value film cap in series (such that Xc << input resistance of your meter).

Reading the voltage on a DC-volts range will give you the correct average value of the total signal. That also happens to be the RMS value of the offset, assuming a sinusoidal ripple.

The RMS value of the total (refer tothis derivation if you need proof) is just the quadrature sum of the two components:

VRMS= Sqrt(Vdc^2+Vac^2)

In your example, you would read 10.00V on the DC range and 0.707V on the AC range, so the RMS value would be 10.03V.

For a sine wave Vpeak = 1.414 Vrms or Vrms = 0.7071 x Vpeak.
In those two formulas Vpeak is measured from ground (or zero volts) to Vmax. As an AC waveform alternates on either side of ground it will alternate between a maximum of Vmax and then -Vmax. So Vpeak-to-peak = total difference between max +ve value and

max -ve value = 2 x Vpeak