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Old 2nd November 2006, 01:42 AM   #1
vax9000 is offline vax9000  United States
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Default half wave rectifier will not saturate transformer core?

Does a half wave rectifier saturate the transformer core? I imagine that there is an average DC current through the secondary winding so it seems to. However, today I read something at http://www.beigebag.com/case_xfrmer_4.htm and I got confused. Under this question,
quote
"I am driving a transformer primary with a unipolar drive waveform. One end of the primary is connected to +30V. The other end is be periodically switched to ground, and then opened . The switch has a 50% duty cycle. That would present an average 15V DC to the primary winding!!!! Why does the transformer not saturate with a net DC voltage across the primary?"
end quote
The author argued why the core will not saturate. I think his argument can well apply to the half wave rectifier situation. So the core will not saturate. Yes or no? Any thought? Thank you.

vax, 9000
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Old 2nd November 2006, 03:33 AM   #2
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You are comparing apples and oranges. The circuit the author describes is very similar to a single ended circuit, where there is a net dc current present, but it is below the saturation level of the iron. The signal (or current) can increase or decrease around this net DC current without ever entering saturation. For this to work, the AC superimposed on the DC should be somewhat symmetrical. Too much in the positive direction, and the transformer will indeed saturate.

Your question, I believe, is regarding using a half wave rectifier on the output of a power transformer. Here, you do have the POTENTIAL to saturate the core, but only if your current level is high. Most applications (such as a fixed bias supply) draw a very tiny current, therefore do not saturate the core.

For a thorough explanation, you should review the B-H curve. The nonlinearity enables you to draw a certain amount of net DC without fully saturating, as each subsequent negative cycle restores balance. Draw too much current, and you have a type of clipping or rounding of the positive wave as the core saturates.

IEEE 519, for all intents and purposes, prohibits the use of half wave rectifiers, as they produce even harmonics into the power system, which do indeed saturate power transformers.

You would not want to use a HW rectifier for larger currents anyway, as you twice as long between charging cycles.
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