rms

[shaw]

:?:
rms value of ac signal [sine wave] at 0 dc level is 0.707 Vp
if wave is shifted at 3 V dc level the
is rms value remains same?? :idea:

 

[Nigel Goodwin]

:?:
rms value of ac signal [sine wave] at 0 dc level is 0.707 Vp
if wave is shifted at 3 V dc level the
is rms value remains same?? :idea:

No, how can it?, you've added 3V DC to the original ac signal.

RMS is the heating value of the AC wave that equals the heating effect of the same value DC. So 3V DC would warm a resistor exactly the same as 3V RMS.

 

[_3iMaJ]

If you review the definition of root mean squared you'll see that level shifting a sinusoid will infact change it's RMS value. Nigel is correct, I just wanted to show the equation so it made sense. If you're like me you see things better in terms of equations. If you're not, then it may look like a mess

 

[Nigel Goodwin]

If you're like me you see things better in terms of equations. If you're not, then it may look like a mess

Yes - it looks like a mess 8)

Probably the easiest way to visualise the amount of power is to draw it as a graph, and then colour the area of power - for a standard sinewave you need to colour both the positive and negative excursions. With a sinewave sat on a DC level, colour everything between the DC level and the sinewave (assuming the sinewave doesn't drop below zero).

 

[_3iMaJ]

I wrote out the derivation and explained some of my thought process:

 

[Nigel Goodwin]

Bigger mess! :lol: :lol:

 

[shaw]

thanks for reply
i know this fact i want to confire
clamper circuit shift the wave at some dc level whereas input has 0 dc level
then is it not violated the law of cinservation of energy?

what physically rms value of a signal means?
can we say that it is energy of a signal?
please explain :shock:

 

[fsahmed]

MAthematically, RMS is a rather more accurate way of average finding,

REgarding electricity, the RMS value of AC represents the value of DC that would be required to produce the same heating (energy transfer) as the AC whose RMS value u have calculated. A 312V AC would produce the heating effect of only 220V DC.

 

[Nigel Goodwin]

thanks for reply
i know this fact i want to confire
clamper circuit shift the wave at some dc level whereas input has 0 dc level
then is it not violated the law of cinservation of energy?

No, why should it?, you're adding energy from the DC level.

what physically rms value of a signal means?
can we say that it is energy of a signal?
please explain :shock:

I explained it earlier, and fsahmed just has again. RMS is simply the AC value that gives the same heating effect as the same DC voltage. So if you'e got a light buld, 230V AC will give EXACTLY the same brightness as 230V DC - it doesn't matter what waveshape the AC is, that's the whole point of RMS.

 

[_3iMaJ]

MAthematically, RMS is a rather more accurate way of average finding,

REgarding electricity, the RMS value of AC represents the value of DC that would be required to produce the same heating (energy transfer) as the AC whose RMS value u have calculated. A 312V AC would produce the heating effect of only 220V DC.

Its not a more accurate way, its just a slightly different calculation. If I want to find the average of a set of numbers I'm certainly not going to use RMS. It is simply a tool to calculate (at least in terms of circuits) the effective DC voltage of a waveform. It is no more accurate or no less accurate than anything else. It is simply just the mean of the squared values, sometimes called the quadratic mean.

 

[shaw]

:!: AND WHAT ABOUT ENERGY OF A SIGNAL :?:

 

[fsahmed]

Add the RMS value of the AC component to the DC. Thats about it.

 

[Styx]

Add the RMS value of the AC component to the DC. Thats about it.

Or more precicely sum the squares of the RMS's, then square-root the total

 

[shaw]

i still have confusion about energy of a signal
please explain

 

[Styx]

i still have confusion about energy of a signal
please explain

energy is just the sum of the power (or the integral over time of the power)

 

[_3iMaJ]

i still have confusion about energy of a signal
please explain

energy is just the sum of the power (or the integral over time of the power)

Styx is correct, in the terms of units Power (Watts) are in joules/sec. Once you integrate over all frequency or over a period (see Parseval's Theorem) you get a number that represents the energy in the signal. Parseval's Theorem just states that there is a conservation of energy between time domain and frequency domain.

 

[shaw]

i still have confusion about energy of a signal
please explain

energy is just the sum of the power (or the integral over time of the power)

and what in a power of a signal :?:

 

[_3iMaJ]

Thats what the power spectral density explains.

Just look at the integral. Its the integral of the absolute value squared of the transfer function multiplied by the fourier transform of the input function integrated over all frequency.

 

[shaw]

Thats what the power spectral density explains.

Just look at the integral. Its the integral of the absolute value squared of the transfer function multiplied by the fourier transform of the input function integrated over all frequency.

WHAT?

 

[Nigel Goodwin]

Thats what the power spectral density explains.

Just look at the integral. Its the integral of the absolute value squared of the transfer function multiplied by the fourier transform of the input function integrated over all frequency.

WHAT?

I thought the same :lol:

It's probably easiest to visualise if you draw it out?.

Get a piece of graph paper, and draw a horizontal line across the middle (0V x-axis) - bear in mind, this now looks like a scope display!. Now draw a sine wave (or whatever wave you want) along the x-axis you just drew, going both above and below 0V. Next get a coloured pencil (or crayon) and fill it in, between the 0V line and the wave you drew, both above and below the 0V line - this will give you a series of coloured half-sine pulses, alternately positive and negative. This is the total amount of power contained in the signal.

Next, draw another sinewave, but this time DC shift it ABOVE the 0V line, so it doesn't cross 0V anywhere - then, as before, colour between the 0V line and the wave you drew (this time all the colour will be above the line). The coloured part again shows the amount of power in the signal.

In both cases (and all other ones) to measure the amount of power you need to calculate the surface area 'under the curve', which with graph paper can consist of counting the little squares :lol: Doing it electronically is FAR more complicated though, and requires 'true RMS' designs, which are essentially hardware multipliers.