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mpegjohn · 10th Oct 2018, 1:01 pm

I have been trying to get my head around this little problem for some time, and decided I need help from those that know.

i have a simple full wave centre tapped diode rectifier circuit.
The transformer secondaries are spec'd at 12V RMS each winding. Measured at 13.87V RMS.

I have a 1K load resistor, no smoothing cap.

If I use my scope to look at the output across the load, it looks like a full wave rectified waveform, as you would expect. It has a peak voltage of 19.4V. The DC voltage, according to the scope, and my Fluke is 12.43v. Close to 0.63 X 19.4 =12.2V
However the RMS voltage readings I get are a bit strange.
According to me the RMS voltage should be 70% Vpk = 13.7V.
But on my fluke I get 6.49V AC RMS, and this agrees with my copes calculations.

What am I doing wrong with regards to my RMS values?

John.

10th Oct 2018, 1:01 pm	#1
mpegjohn Hexode Join Date: Aug 2013 Location: Chichester, West Sussex, UK. Posts: 270	Full wave rectified RMS voltage I have been trying to get my head around this little problem for some time, and decided I need help from those that know. i have a simple full wave centre tapped diode rectifier circuit. The transformer secondaries are spec'd at 12V RMS each winding. Measured at 13.87V RMS. I have a 1K load resistor, no smoothing cap. If I use my scope to look at the output across the load, it looks like a full wave rectified waveform, as you would expect. It has a peak voltage of 19.4V. The DC voltage, according to the scope, and my Fluke is 12.43v. Close to 0.63 X 19.4 =12.2V However the RMS voltage readings I get are a bit strange. According to me the RMS voltage should be 70% Vpk = 13.7V. But on my fluke I get 6.49V AC RMS, and this agrees with my copes calculations. What am I doing wrong with regards to my RMS values? John.