# Homework Help: Power in resistive load following rectifier

1. May 2, 2013

### pvshackguy

1. The problem statement, all variables and given/known data

Simple question: is the effective voltage of an idealised full-wave rectification not simply the rms voltage, as it is with the original AC waveform?

Backgound...

In an electrical apprentice program I am being instructed to use average DC voltage to calculate power in a resistive load following a rectifier e.g. full-wave bridge. This is the idealized case; no diode drops and no filter.

Surely this is incorrect(?) I can't see why we shouldn't still use rms.

Our textbooks have some egregious errors, and so it's plausible that this is another. It's been decades since I took some engineering and my texts and notes are not at hand, though, and despite searching articles I can't find clear backup for my instincts here.

2. Relevant equations

3. The attempt at a solution

Last edited: May 2, 2013
2. May 2, 2013

### Staff: Mentor

pvshackguy, people tend to be loose with the term "power". You are correct that the power in a resistive load is unchanged by interposing an ideal full-wave rectifier.

But the questioner may be envisaging something that responds to the average value, such as an electromagnet or DC motor, or electrolysis or battery-charging, etc., even though he then framed the question in terms of a resistive load. It the load has significant inductance (and many industrial loads do) then that acts as an averaging filter. So even though on the face of it, it does seem wrong, there may be some explanation behind it which is being glossed over so as to not complicate things at this introductory stage.

In isolation, the term "effective voltage" means nothing to me. It's situational specific, I'd say.

I understand average voltage. I understand RMS voltage.

3. May 2, 2013

### pvshackguy

Ahhhh... This is the missing factor I was looking for. I'm very grateful.

BTW in our course "effective" is equated with rms. Sorry to have accepted this definition at face value. :-) It's hard to tell what to trust when the principle text contains statements like "The joule is the SI equivalent of the watt." Sad.