Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!mips!pacbell.com!pacbell!osc!jgk
From: jgk@osc.COM (Joe Keane)
Newsgroups: sci.electronics
Subject: Re: IR Reception - Modulation and Quality
Keywords: harmonics
Message-ID: <4739@osc.COM>
Date: 16 Apr 91 23:35:21 GMT
References: <159639@felix.UUCP> <160142@felix.UUCP> <4729@osc.COM> <1991Apr10.235418.10741@cbnewsj.att.com>
Reply-To: jgk@osc.COM (Joe Keane)
Organization: Versant Object Technology, Menlo Park, CA
Lines: 37

In article <4729@osc.COM> i write:
>Note that, especially with a large filter capacitor, the peak current can be a
>lot higher than the average current if the rectifier only conducts during a
>small portion of a cycle.  The choke should reduce this problem somewhat, but
>you should make sure the choke is rated for the peak current.

In article <1991Apr10.235418.10741@cbnewsj.att.com> asd@cbnewsj.att.com (Adam
S. Denton) writes:
>You DO NOT want to `filter' pure AC.  You'll just be introducing a voltage
>loss.  You DO want to filter the AC component of DC.  Thus, the choke goes
>after the diodes (and initial filter cap), not before.

I stand by my original statement.  It's more important to filter the current
before the capacitor.

The key phrase here is `harmonic current'.  A rectifier and capacitor
connected directly to the mains would produce large harmonic currents.
Somewhere in this path you need some impedance to smooth out the current.
Like i said before, cheap power supplies rely on the resistance and leakage
inductance of the transformer.  With reasonable filtering before the
rectifier, the high harmonics aren't generated in the first place.  This is a
lot easier than trying to filter them out later.

With large rectifiers, series inductors are mandatory.  Large AC-DC converters
have elaborate filtering schemes, with series inductors and multiple tuned
shunts, and these are all on the line side.  The power company would have a
bird if you hooked up a large rectifier and capacitor directly to the power
line.  I don't know what specifically `large' would be though.

Assumedly, the voltage on the filter capacitor is subsequently regulated.  A
regulator can deal with some 120 Hz hum, and if that's all there is, very
little will get through to the output.  That's because it can respond well to
slowly changing voltages, and 120 Hz is fairly slow.  However, high-frequency
transients will shoot right through, or around, by capacitive coupling.
--
Joe Keane, not a real EE
jgk@osc.com (...!uunet!stratus!osc!jgk)