Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!pacific.mps.ohio-state.edu!linac!att!cbnewsj!asd
From: asd@cbnewsj.att.com (Adam S. Denton)
Newsgroups: sci.electronics
Subject: Re: IR Reception - Modulation and Quality
Message-ID: <1991Apr10.235418.10741@cbnewsj.att.com>
Date: 10 Apr 91 23:54:18 GMT
References: <159639@felix.UUCP> <160142@felix.UUCP> <4729@osc.COM>
Organization: AT&T
Lines: 42

In article <4729@osc.COM> jgk@osc.COM (Joe Keane) writes:
>In article <160142@felix.UUCP> dennisg@felix.UUCP (Dennis Griesser) writes:
>>[Thanks again for your response, John.  Anybody else care to step up to bat?]
>
>I think the right solution is to add a series choke (inductor).

Yup!
>inductance of the transformer, which often isn't very large.  It doesn't
>matter whether you put the choke in the primary or secondary, it just means a

Its best location is after the filter capacitor and diode.  Then,
install another filter cap after the choke -- making a `pi' filter.
You will find this very effective.   Use the highest inductance you can
use given the constraints of
    1) physical space
    2) DC resistance of the inductor windings vs. load current and
       tolerable voltage drop (and core saturation if the supply is hefty)
I often just take apart little audio xformers garnered from old surplus
and re-wind them with 24-guage enamel wire -- usu. 150-200 turns.
Has about 2.5 ohms of resistance.  I make both filter caps equal in value.
(Old car radios are a good source for these inductors too.)

The inductance you need depends on the frequencies you want to block.
If you're blocking 120Hz, the more, the merrier.  If it's high-frequency
noise, you can do with a little less.  Compare the inductive reactance with
the capacitive reactance of the post capacitor.  Watch out for
series-resonance!  (Usually not a problem because the Q is so low, since
the load looks like a low-value resistor.)

>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.

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.

Also...the peak current is mostly a function of the initial filter cap.

Adam Denton
asd@mtqua.att.com