IEEE Consultants' Network of Long Island

RMS versus Average, A Second Look - John Dunn, Consultant, Ambertec, P.E., P.C.

We saw before that for a zero to +E square pulse train, that the RMS value equals the average value divided by the square root of the duty cycle. The narrower the duty cycle gets, the higher the ratio of the RMS to the average becomes.

Here, we look instead at the ratios of RMS to average for sine tips of different conduction angles.

In each case below, the waveform goes from zero to +100 and back again to zero along sinusoidal trajectories. The RMS and average values for several conduction angles are calculated and their ratios are compared.

Current Pumps - John Dunn, Consultant, Ambertec, P.E., P.C.

It's always handy to have a few constant-current injection circuits in one's bag of tricks, so here are just a few that should be at least considered when the occasion calls for one:

 
The junction FET circuit was a little troublesome at first. It wanted to oscillate. That's why the 0.01 µF is there to quiet it down. The response time was a little slow, but for the application at hand, that didn't matter.

Using One 115V Fan On Two Line Voltages - John Dunn, Consultant, Ambertec, P.E., P.C.

A product's cooling fan was rated only for 115 VAC service in a product that needed to switch between 115 VAC and 230 VAC. To accomplish this, I made the product's power transformer serve the fan as an autotransformer when 230V service was called for:
At the lower line voltage, the fan was simply driven right from the line. At the higher line voltage, the fan acted as a load on the transformer's primary coils which served just fine as an autotransformer.