Efficiency aside, a linear power supply is about the only way to guarantee
a "clean" RF environment around the power supply and its associated cables
and load. However, I can tell you from direct observation, that some of
the problems would go away if the off-shore manufacturers of switchers had
their feet held to the fire and were FORCED to make certain that every
power supply they ship to the USA (and other countries with EMC
requirements) had ALL of the parts installed that are intended to control
emissions. In other instances, the problem really is bad (or non-existent)
design for emission control. (I would dearly love to see a rebuttal on
this issue from a manufacturer, but I doubt they bother to read good-sense
reflectors like this one or care about a few "inconvenienced" hams.)
I hear many RFI complaints on this reflector over the wall wart issue.
Many of you have come up with innovative ways to replace the warts with
inexpensive linear equivalents, and others have applied good emissions
control techniques to the connecting cables. Cortland uses a "re-purposed"
commercial linear supply, and that is fine. (Keep in mind that commercial
switchers are NOT OK in most cases because they only have to meet FCC Part
15 Class A requirements [when applicable], and so those units tend to be
"screamers".) What I have not seen much is a movement to homebrew linear
power supplies to replace noisy switchers.
Folks, think about this: a simple, single output low voltage linear power
supply is nothing more than a transformer (often a common filament variety
will work), a diode or diode bridge, and at least 1 electrolytic capacitor
(sized to be around 1000 mfd per amp of output). Those few parts make up a
non-regulated linear supply. Your app needs a regulated source? OK, add a
couple more caps and packaged voltage regulator IC, such as the LM78 or
LM79 series. Things get a little more complex if you want output
variability or if you need more than 1 amp of regulated output. Yet, NONE
of this should be beyond the ability of most licensed hams.
I just checked my Jameco catalog, and they offer several linear "desk top"
power supplies (larger than a wall wart, but still pretty small). If you
find a linear supply with the correct output voltage and adequate current
capability for your app, go for it! Don't worry about the connector on the
end of the output cord. Before you scrap that noisy switcher, take your
DMM (or similar device) and determine polarity (in many cases, the polarity
is marked on the housing of the device being powered). If the connector on
the linear supply is not the right type, so what? Just cut it off and
splice on the connector from the switcher, observing polarity, of course.
So, the next time you get hit with a bad news switcher, determine its
output and go from there. You may be able to tame that critter with
applied emission control techniques, but unless you have a good stash of
the correct materials (toroids, beads, by-pass caps, etc.), you may find it
less expensive to toss the switcher and replace it with a linear supply, be
it commercial or homebrew. I suspect that the big driver on efficiency for
commercial interests is the EnergyStar rating, and that is probably not
achievable with linear power sources. However, I have not seen anything
that forbids a person from "rolling their own" power source. Yeah, your
electric bill may increase by a few cents per month, but the RFI levels
will go way down. Not a bad trade-off in my book.
73, Dale
WA9ENA
> [Original Message]
> From: Cortland Richmond <ka5s@earthlink.net>
> To: <rfi@contesting.com>
> Date: 6/21/2011 4:56:10
> Subject: Re: [RFI] Comcast Wall warts
>
> I am preparing to replace several 5V switchers for USB port expanders
with
> a common Lambda 5A 5V open-frame PS. We may not be able to do this for
> long; efficiency regulations actually forbid making (and maybe selling)
> linear-regulated (i.e.: dissipative) power supplies
>
> Cortland
> KA5S
>
> -----Original Message-----
> From: rfi-bounces@contesting.com [mailto:rfi-bounces@contesting.com] On
> Behalf Of KD7JYK DM09
> Sent: Monday, June 20, 2011 7:56 PM
> To: rfi@contesting.com
> Subject: Re: [RFI] Comcast Wall warts
>
> "The wall warts were extremely noisy and generated so much hash I was
unable
> to listen to any AM radio in my entire home."
>
> I replace such noisy supplies with 25 cent linear supplies of the same
> specification bought at a thrift shop.
>
> Kurt
>
> _______________________________________________
> RFI mailing list
> RFI@contesting.com
> http://lists.contesting.com/mailman/listinfo/rfi
_______________________________________________
RFI mailing list
RFI@contesting.com
http://lists.contesting.com/mailman/listinfo/rfi
|