Sawtooth waveform RMS - Multisim

Thread Starter

xxxyyyba

Joined Aug 7, 2012
289
It's well known that RMS voltage of Sawtooth wave is Urms=Up/sqrt(3), where Vp is peak voltage.
Why Multisim gives me wrong RMS of Sawtooth wave? Here is my circuit and settings for sawtooth wave:

multi.jpg

Voltmeter should measure 5V/sqrt(3)=2.886V
 

t_n_k

Joined Mar 6, 2009
5,455
Depends upon whether the waveform is DC offset or not. The waveform you have created has zero average offset which is the reason for the discrepancy.
 

t_n_k

Joined Mar 6, 2009
5,455
I'm not a Multisim user.
Probably because the AC measurement ignores the DC component.
Plus the scope measurement also looks to be AC coupled so we can't really tell what the generator is producing.
 

Thread Starter

xxxyyyba

Joined Aug 7, 2012
289
I get correct result using function generator (I tried different values for voltage offset and every time I get same result):

kolo2.jpg
 

t_n_k

Joined Mar 6, 2009
5,455
In addition:
Not sure why offsetting by -2.5V constrains the lowest signal point on the signal to zero V which is what the sawtooth definition requires for your original statement regarding the "well known" RMS value to be true.
 

t_n_k

Joined Mar 6, 2009
5,455
it's not about voltage offset
Depends if you are talking about a sawtooth which has or doesn't have a zero DC (or mean) value.

The RMS value of the classic sawtooth is indeed

\(V_{rms}=\frac{U_p}{\sqrt{3}}\)

The Mean value of the same waveform is Up/2.

If the classic sawtooth is level adjusted such that its mean value is zero then the RMS value is altered to

\(V_{rms}=U_p\sqrt{\( \frac{1}{3}-\frac{1}{4} \)}=0.2887U_p\)
 

Aerb

Joined Jul 9, 2014
12
Depends if you are talking about a sawtooth which has or doesn't have a zero DC (or mean) value.

The RMS value of the classic sawtooth is indeed

\(V_{rms}=\frac{U_p}{\sqrt{3}}\)

The Mean value of the same waveform is Up/2.

If the classic sawtooth is level adjusted such that its mean value is zero then the RMS value is altered to

\(V_{rms}=U_p\sqrt{\( \frac{1}{3}-\frac{1}{4} \)}=0.2887U_p\)
Oh i love the way people approaching to situations here.

Did you really get what is wrong in xxxyyyba's problem? Did you get what the solution is? I know about voltage offsetting, sawtooth waves, rms and mean values and the calculations above are not connected to problem here. The problem is just about the usage of the oscillator. When the problem is simple, please give simple solutions.
 

crutschow

Joined Mar 14, 2008
25,690
Oh i love the way people approaching to situations here.

Did you really get what is wrong in xxxyyyba's problem? Did you get what the solution is? I know about voltage offsetting, sawtooth waves, rms and mean values and the calculations above are not connected to problem here. The problem is just about the usage of the oscillator. When the problem is simple, please give simple solutions.
And I'm really happy for you that you know all those things. But it's not good form to criticize the way other's answer questions. We all answer them the best way we know how.

But we await your simple answers from your vast store of knowledge. :rolleyes:
 

Aerb

Joined Jul 9, 2014
12
And I'm really happy for you that you know all those things. But it's not good form to criticize the way other's answer questions. We all answer them the best way we know how.

But we await your simple answers from your vast store of knowledge. :rolleyes:
Ok, now you really blew this out of portion. But instead clarifying myself again, well, i'm outta here.
 

MrCarlos

Joined Jan 2, 2010
400
Hello everyone

Have written and asked a lot with respect to: What value does ammeter or voltmeter Measures (RMS, Average or Peak)?.

Just as an example visit this link and see what I think is a tremendous discussion: http://electronics.stackexchange.com/questions/18368/what-value-does-ammeter-or-voltmeter-measures-rms-average-or-peak

We can say that all AC voltmeter show a reading of the RMS voltage of a pure sine wave. This excludes the called True RMS Voltmeter.
If we try to measure another type of signal which is not purely sinusoidal reading will be erroneous. according to what we expect.
And calculate the reading that should give us our AC voltmeter is not as easy as it sounds.

Now, the original question asked by xxxyyyab was: Why Multisim gives me wrong RMS of Sawtooth wave?
The question is somewhat difficult to answer.
Since the simulator for electronic circuits leave much to be desired, some more than others, until the complex to measure with a AC voltmeter a signal that is not purely sinusoidal.
most likely We can find the answer in this video:
http://www.youtube.com/watch?v=ue0wtlrmCJE

xxxyyyab, have a test, please, with a sine wave generator.
You will see that the AC voltmeter reading is closer to the result of formulas written above.
 
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