RMS of A Square Pulse Train - John Dunn, Consultant, Ambertec, P.E., P.C.

You can find the root-mean-square (RMS) value of a unipolar square pulse train as sketched below using an average responding voltmeter, typically, a digital multimeter and a scope.

The RMS value equals the average value divided by the square root of the waveform's duty cycle:

 
While there are true-RMS meters that can conveniently yield this waveform's RMS value, such meters have crest factor limitations which become of concern for low values of duty-cycle. For this waveform, this method has no such limitation.

 

Comments

[Power Supply Designer]

John,

Thanks for your practical application of RMS fundamentals. This will be a helpful tool to remember. I was not aware of the crest factor limitations of true RMS reading meters.

May I emphasize something that you briefly stated? There are two measurements required for this RMS calculation. The first is a measurement with an average reading multimeter, and the second is a measurement of duty cycle or Ton and Toff with the oscilloscope. With these two measurements, the RMS value of the waveform can be calculated according the equation relating RMS value to average value and duty cycle.

This technique also presumes that the period of the waveform is substantially shorter than the averaging time constant of the multimeter.

Thanks for your articles! I am learning a lot from them.

Robert W. Stowe
www.TruePowerResearch.com