Sawtooth waveform RMS - Multisim

Discussion in 'General Electronics Chat' started by xxxyyyba, Sep 18, 2014.

  1. xxxyyyba

    Thread Starter Member

    Aug 7, 2012
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    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
     
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    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.
     
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  3. crutschow

    Expert

    Mar 14, 2008
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    Yes, the zero voltage point affects the RMS voltage. Put the 0V point at the negative peak and you should get the answer you calculated.
     
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  4. xxxyyyba

    Thread Starter Member

    Aug 7, 2012
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    Sorry but I'm not sure I understood what to do for correct measurement :(
     
  5. crutschow

    Expert

    Mar 14, 2008
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    The Voltage Offset parameter must be set to -2.5V for a 5V sawtooth.
     
  6. xxxyyyba

    Thread Starter Member

    Aug 7, 2012
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    I tried it already but I still get incorrect result :confused:
     
  7. xxxyyyba

    Thread Starter Member

    Aug 7, 2012
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  8. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    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.
     
  9. xxxyyyba

    Thread Starter Member

    Aug 7, 2012
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    Ok, thanks anyway :)
     
  10. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    Try voltage offset of 5.

    Here is my sim:

    sim.jpg
     
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  11. xxxyyyba

    Thread Starter Member

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

    kolo2.jpg
     
  12. Aerb

    New Member

    Jul 9, 2014
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    hey dude, channel a of the oscillator is set to measure ac component. Click on dc and your problem will be solved
     
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  13. Aerb

    New Member

    Jul 9, 2014
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    it's not about voltage offset
     
  14. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    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.
     
  15. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    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
     
  16. Aerb

    New Member

    Jul 9, 2014
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    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.
     
  17. crutschow

    Expert

    Mar 14, 2008
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    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:
     
  18. vk6zgo

    Active Member

    Jul 21, 2012
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    Sorry,but that isn't a sawtooth,it's a triangular wave.
    OK,you can call it a special case of a sawtooth,but it is sufficiently dissimilar as to be regarded as a different waveform.
    A normal sawtooth has a faster return to zero than its rise.
     
  19. Aerb

    New Member

    Jul 9, 2014
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    Ok, now you really blew this out of portion. But instead clarifying myself again, well, i'm outta here.
     
  20. MrCarlos

    Active Member

    Jan 2, 2010
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    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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