Note to Rick and All,
Placing a bypass cap across the switching transistor of a switch mode
power supply is NOT the place to put a cap, as Rick found out. There will
be a snubber cap already included in the circuit, if the circuit topology
supports use of snubbers. Please refer to any of a number of sources on
switching power supply design before attempting to "improve" them for
noise performance.
Some of you on this list may recall that the ARRL lab ran tests on several
commercial switching power supplies intended for amateur radio use. This
was back around the 2000 to 2002 time frame, if I recall. The best (least
noise) power supplies were those from Astron and MFJ. I can't speak for
how MFJ quiets their power suuplies, but I own the SS-35 Astron unit and
took it apart to see how Astron did the job. Answer: lots of ferrites
and bypassing (in the correct places), as well as good board layout
techniques. What they did FAR exceeds FCC requirements for Part 15
devices and their associated power sources, making for an excellent
in-shack supply for running most rigs and accessories.
My point in all of this is that if you have a sleazo cheapo wall wart
switcher that is powering some device in your shack or house, do the
second thing that Rick did: replace it with a linear supply equivalent.
Otherwise, plan to do some "sniffing" with probes and possibly some major
surgery to said el cheapo wall wart in order to quiet it down. One thing
to keep in mind: it will probably be easier and faster to get a linear
equivalent power supply going. Most devices seem to use voltages of 4.5,
5.0, 6.0, 9.0, or 12.0 from their wall warts. Many of us have plenty of
old linear wall warts laying around that are unregulated, but "clean" in
the RF sense. If you have such an old wart gathering dust in the junque
box, you can easily select one that has an unregulated voltage that is at
elast 3 to 5 volts more than your device requires and then (gasp and
horrors!) homebrew a simple series linear regulator to provide just
exactly the voltage you want. Check out the LM78-series of regulators.
They are plentiful and cheap, come in TO-220 packages, and a suitable
circuit can be assembled on a small piece of circuit card, perf board, or
even a plain, old-fashioned 3 or 4 lug terminal strip. Be creative!
73, Dale
WA9ENA
rick darwicki <n6pe@yahoo.com>
Sent by: rfi-bounces@contesting.com
03/05/2008 11:07 AM
To
rfi@contesting.com
cc
Subject
[RFI] Modem PS noise
I finally got a well enough regulated 5V non-switching PS to power up my 2
Wire 2701 Modem.
The S-9+ noise on 160m is completely gone.
I tried a little bypass capacitor on the switching transistor which also
cured the noise, but only because I shorted the transistor and blew the
supply up hi hi
Rick, N6PE
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