[Mondial]

But you can't ignore the AC component in the power calculation  because it is very significant. The current waveform though the resistor looks nothing like "pure DC". Look at the diode current waveform (b) below.

[Image: http://www.industrial-electronics.com/im..._1-6-3.jpg]


The current through the 300 ohm series resistor looks like the waveform in (b) above. Remember, it is directly in series with the rectifier cathode before the first filter cap. The waveform illustrates the peak pulse of current which flows through the cathode of the rectifier tube each half cycle. It consists of an average DC component plus significant AC components which cannot be ignored in the power calculation.