All About Circuits Forum rms value of a rectangular pulse
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#1
01-31-2013, 05:36 AM
 lucky007tiwari New Member Join Date: Nov 2012 Location: noida Posts: 6
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
01-31-2013, 05:50 AM
 bountyhunter Senior Member Join Date: Sep 2009 Posts: 2,498

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
01-31-2013, 07:35 AM
 thatoneguy Senior Member Join Date: Feb 2009 Location: Midwest USA Posts: 6,356 Blog Entries: 4

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
01-31-2013, 07:55 AM
 lucky007tiwari New Member Join Date: Nov 2012 Location: noida Posts: 6

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
01-31-2013, 08:06 AM
 lucky007tiwari New Member Join Date: Nov 2012 Location: noida Posts: 6

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
01-31-2013, 10:08 AM
 Salaja Junior Member Join Date: Jan 2013 Posts: 23

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
01-31-2013, 12:11 PM
 atferrari Senior Member Join Date: Jan 2004 Location: Buenos Aires - Argentina Posts: 1,513

Quote:
 Originally Posted by Salaja . NOTE: this is only a guess, i don't know that much about measuring the RMS value of square wave signals.
Why do you answer then? Guessing could be right or wrong. You do not actually help the OP with that.

Leave that to those that know it better and learn by reading what the say...
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#8
01-31-2013, 12:56 PM
 MrChips Super Moderator Join Date: Oct 2009 Posts: 9,019 Blog Entries: 23

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
01-31-2013, 05:46 PM
 The Electrician Senior Member Join Date: Oct 2007 Posts: 1,722

Quote:
 Originally Posted by lucky007tiwari 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..
The reason why your reading is fluctuating is because your waveform has a period of 1 second. Typical meters are designed to work with 50 Hz or 60 Hz waveforms, and they are designed to take several readings per second.

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.
Attached Images
 scope_0.png (25.5 KB, 9 views)

Last edited by The Electrician; 02-01-2013 at 10:21 PM.
#10
01-31-2013, 09:45 PM
 bountyhunter Senior Member Join Date: Sep 2009 Posts: 2,498

Quote:
 Originally Posted by The Electrician The reason why your reading is fluctuating is because your waveform has a period of 1 second. Typical meters are designed to work with 50 Hz or 60 Hz waveforms, and they are designed to take several readings per second.
Exactly. As I said many posts back:

Quote:
 Even if it is a true RMS meter, it has limitations on bandwidth
A 1 Hz signal is too low in frequency to be read accurately

 Tags pulse, rectangular, rms

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