[RFI] 24VDC PS - Recommended Quiet Model?

Dale Svetanoff svetanoff at earthlink.net
Fri Oct 5 16:03:10 EDT 2012


I have addressed this issue before in prior posts.  For your benefit, here
are my comments about your requirements, and then about the eBay "delights"
you listed.  Finally, I'll make some suggestions.  Here goes:

1,  Please define"RF quiet".  That will NOT happen with a switcher. 
Period.  Do you mean "quiet for HF" or quiet for "VHF/UHF" applications? 
How close is the intended location of the power supply to your antennas? 
(Distance between power supply and rig hardly matters unless the rig has a
built-in antenna, such as an H-T.)

2.  No problem with you wanting to plan for future power growth, but do you
really need to plan for 4.5 amps?  That can have significant impact on
cost, relative to a 1.0 or 2.0 amp power supply.

3.  Now, about those eBay specials - - 
	A.  The first one, intended for "home appliances":  Open frame switcher,
NO statement made about meeting ANY RFI or safety requirements, nor is
output ripple (usually expressed in "mV pk-to-pk") stated.  Suggestion:
This cheap and sleazy item is exactly the sort of trash you do not want. 
This has the high likelihood of being a "real screamer" insofar as RFI
goes.  Also, its stated switching frequency of 25 kHz is quite low (by
today's standards) for a switcher - most run at 100 kHz or higher.  This
unit could produce very significant hash in the AM broadcast band and be a
bummer if you like to listen to VLF and LF spectrum stations.
	B. The same story applies to the second power supply.  Again, NO relevant
specs listed for RFI or safety, and no statement of output ripple.  Deposit
this one in the same trash can as the first one.

4.  Here's the BIG problem:  NONE of these power supplies, as well as most
commercial grade switching power supplies, HAVE TO MEET the requirements of
FCC Part 15, Class B operation.  If they are "legal" for use in the USA,
then they must meet (in most cases) the requirements of FCC Part 15 Class
A, which is commercial usage, NOT office or residential usage.  Thus, they
are "screamers" in most cases.  Worse yet, FCC regs for Class B services
STILL do not make for "RF quiet" operation.  They provide NO radiated
emission control below 26 MHz, and for the spectrum above that, the Part B
regs are at least 20 to 40 dB deficient relative to MIL-STD-461E or F, the
accepted standard for emission controls (both radiated and conducted) in
military applications.  Note that even then, power supplies that meet the
mil requirements are not "RF quiet", but are a lot quieter than normal
commercial grade stuff.  

5.  All of that being said, I will tell you that my main station 13.6 VDC
supply is a switcher - Astron SS-30M, to be exact.  It is loaded with
ferrites and by-pass caps and causes me ZERO RFI in my station's
operations.  Is that true for everyone?  NO!  While the Astron is very
close to my rigs, it is 50 or more feet from any HF antenna, so no problem
even on 160m when I use my Carolina Windom on that band.  On the other
hand, I live about 180 miles from the Chicago area and like to listen to
certain AM broadcast stations there.  I have an RCA/GE Superadio III on my
kitchen counter that I use to do that listening, mostly when having
breakfast or lunch.  The radio has a large built-in ferrite loop antenna,
so any noise source near the radio will compromise AM band performance.  (I
am lucky enough that there are few "noise makers" in my neighborhood to
cause RFI on the AM band.)  I recently checked two power supplies that are
marked as compliant to FCC Part B requirements, and both wiped out
reception on that radio when placed within 3 to 5 feet of it.  One was a
power supply for my Dell laptop and the other was a charger for the
lithium-ion battery used in my digital camera (Olympus).  

6.  I am not claiming either power supply to be faulty.  I fully expect
that they both meet their respective emission specs.  The problem - there
are NO emission specs in the 0.5 to 1.7 MHz band for switchers.  Read that
and weep.

7  To get what you want, you have 2 choices: find a linear regulated power
supply or be prepared to do a lot of work to clean up a screaming switcher.
The Astron proves that it can be done.  However, how prepared are you to
tackle the fix?

8.  There is hope, but you will probably have to expand your budget.  In my
case, I was able to find (at a commercial "junk" sale) a very nice Sorensen
variable DC output power supply, fully regulated linear, that adjusts over
the range of 2.0 to 35 volts and output current of up to 3.5 amps.  I think
I paid around $25 for it - a true bargain.  I just checked two catalogs I
happen to have from two different vendors: Jameco Electronics and Marlin P
Jones Associates.  Both offer linear regulated power supplies in the
current range you want.  Being linear, they will not generate hash unless
there is something wrong with the regulator circuitry.  (Power regulators
have been known to self-oscillate under certain conditions that usually
accompany component failure.) 

Here is a brief summary of what I found:  (all are regulated)

1. Jameco: 24 VDC out @ 1.0 A, linear tabletop supply, $17.95 with UL
certification for safety, stk #174845

2. Jameco:  24 VDC output @ 0.5 A, linear wall supply, $14.95 with UL
certification for safety, stk #174861

3.  M P Jones:  24 VDC output @ 7.2 A, linear open frame supply, $89.95
with UL, TUV, and CE safety, stk #18424-PS

4.  M P Jones:  24 VDC output @ 4.8 A, linear open frame supply, $97.78
with UL. TUV, and CE safety, stk #6623-PS

5.  M P Jones:  24 VDC output @ 1.2 A, linear open frame supply, $40.42
with UL, TUV, and CE safety, stk #12784-PS

Note: Item #3 is industrial surplus.  Items 4 and 5 are new units, not
surplus.  I believe items 1 and 2 to be new, as well.  I have no interest
of any kind in either vendor, but am a long-time happy customer of both. 

Beware of what's on eBay.  If it says "switcher" anywhere in the
description, assume that is a screamer unless emission controls and/or
output ripple content is clearly stated.  Open frame switchers are
particulary nasty because they are intended to be embedded into some other
final product, so if they meet any specs at all for RFI, it would most
likely be any better than FCC Part 15 Class A, which will not make your
day.  Happy hunting.

Dale Svetanoff, WA9ENA
Sr EMC Engineer
E-N-A Systems, LLC
Specializing in shielding applications, system grounding, and lightning

> [Original Message]
> From: qrv at kd4e.com <qrv at kd4e.com>
> To: RFI List <rfi at contesting.com>
> Date: 10/5/2012 10:24:20
> Subject: [RFI] 24VDC PS - Recommended Quiet Model?
> I have need of a 120vac to 24vdc power supply.
> It, of course, needs to be RF-quiet.
> Anyone familiar enough with these surplus supplies
> who can spot one that is more likely than the others
> to run clean, please?
> Immediate need is for 1a but I like things over-rated
> so they run cool and I'd like headroom in case the
> demand increases -- so am looking for at least 4.5a.
> I'd prefer that it be regulated.
> I'd like to keep the cost under $20., including
> shipping.
> Spotted these on ebay:
> Experiences?  Thoughts, please?
> Thanks!
> David
> -- 
> Thanks! & 73, KD4E.com
> David Colburn nevils-station.com
> I don't google I SEARCH! duckduckgo.com
> Network: groups.yahoo.com/group/qrv
> Restored to design-spec at Heaven's gate 1Cor15:22
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