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#1




rms value of a rectangular pulse
dear sir
my waveform is rectangular,T on is .25 sec and T 0ff is .75 second so Total Time period is 1 second..there is nothing negative part in it means its a pure positive rectangular pulse having frequency 4 Hz.the value of amplitude is 5 volt............ i am fully confused about the measurement of it....what will happen when i measure it with multimer.......plz help me................. 
#2




A meter probably can't measure it accurately. Even if it is a true RMS meter, it has limitations on bandwidth and crest factor which will throw the reading off.

#3




Mathematically,
RMS of PWM square wave is a√D Where a is high voltage value, and D is duty cycle. Meters will measure this in varying ways. A thermal measurement type meter is the most accurate for this type of waveform.
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#4




sir i have used a multimeter only....at its DC voltage mode........every time when the pulse reaches at its peak the value in multimeter is not repeating...what is the reason behind it....

#5




Dear sir acording to my knowledge we calculate RMS value when frequency is very high and the direction goes +ve and ve continuesly...like an AC current.......but in my case i am talking about a rectangular wave which is always +ve at its ON time and 0 at its off time.........then why i am unable to calculate its exact amplitude value at each time when the pulse is in its on mode....multimeters reading are fluctuating....why is it all happening..

#6




well, your time varying signal isn't centered around 0v, so it's basically the same as a signal that IS centered around 0v, but with a DC component. maybe this DC component is messing with the multimeter because it wasn't designed to handle DC when measuring RMS. maybe try adding a voltage offset so the average voltage becomes 0v.
NOTE: this is only a guess, i don't know that much about measuring the RMS value of square wave signals. also, you're measuring a 1Hz signal, is your multimeter rated to a frequency that low? 
#7




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Leave that to those that know it better and learn by reading what the say...
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#8




What is RMS value?
Exactly that, the Root Mean Square, i.e. the square root of the mean value of the values squared. To determine the RMS value, you take a series of sampled points, square each value, add the values and take the mean value, then take the square root to give the result. Does your meter do this? Must likely not. The only other way is to feed the signal into a resistive heater and measure the power generated in the heater. This is how a hot wire ammeter works.
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#9




Quote:
I set up a function generator to produce your waveform and when I connected my Fluke 189 to measure the True RMS AC+DC, the readings were bouncing all over the place. The Fluke takes about 5 readings per second. What would you expect for a waveform with a period of 1 second? The reading you get is only going to use about 1/5 of a second worth of the waveform. The particular 1/5 of a second that the meter sees will be random, so the reading will not be stable. In post #3, thatoneguy gave you a formula for the RMS value of your waveform; you can find that formula here: http://en.wikipedia.org/wiki/Root_mean_square If you want to actually measure the RMS value, rather than calculate it with a formula, you will probably have to use an oscilloscope that can do waveform math. The attachment shows the result I get with a scope measurement. The formula says the RMS value of your waveform should be 2.5 volts RMS, and that's essentially what the scope says. Last edited by The Electrician; 02012013 at 10:21 PM. 
#10




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pulse, rectangular, rms 
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