[TenTec] Switching Power Supplies
Sat, 09 Aug 1997 19:53:25 -0500
> Hi Folks,
> My "travelling" station is a 555 Scout with one of those new Astron
> switching power supplies. It's rated at 15A continuous, 18 A
> intermittent. Although switching supplies are notorious for EMI, the
> marketing hype faked me out and I bought one, since the spec sheet
> actually said "High efficiency switching technology specifically
> filtered for use with communications equipment, for all frequencies
> including HF." The word "HF" is boldfaced and underlined.
> Needless to say, the 80, 40, 30, and 20 meter bands are filled with S3
> to S9 broadband pulses every 60 kHz or so that drift as the supply
> heats up. The noise peaks on 40. I've kind of determined that it is a
> combination of radiation, induction, and conduction (on the power
> cable itself), but it is hard to tell (my scope is down at the
> Question: Has anyone done any work to quiet these things down? Could
> anybody using the Ten Tec switcher with their Scout comment as to the
> quietness or noisiness of that particular power supply? Thanks very
> Al W6LX
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Al, I had the same problem with the same power supply. I then purchased
the higher current version (22 amps) of the same power supply. The noise
was very bad. It sounded like a parasitic, and drifted across the
frequency I was listening to. Finally, I wrote the factory that suppled
the poweer supply to TenTec. I found that the owner is a HAM, and a
great guy. He sent me the following message. Now my supply is free from
noise, or at least, I don't hear any. It works great, and is worth the
trouble making the following changes. Note that John talks about the
SS-22, which is the high current version of the same supply you have. It
is the same physical size, but has an internal fan. If you make these
changes, you will find the supply works just great.
Best 73 de Jim, K5ROV
You have just described the classic problem with switch mode regulated
power supplies (SMPS). The regulator system varies the width of the
sample (pulse width modulation) and this width varies directly with the
load. Hence, the signal will drift up and down the range. Since the
switching is almost square (sharp corners), the harmonic content is very
high. The harmonics typically drop off in the higher frequencies above
As you probably already know, the DOD has a complete research section
devoted to electomagnetic compatability.
The SS-22 is verified to meet FCC Part 15 class B for residential
devices but that is not typically low enough noise for an HF SSB rig
that can hear a 0.2 uV signal clearly. I can hear all the color TVs,
computers, VCRs, etc that all use SMPS from the neighbors on all sides.
The noise you are hearing is primarily escaping on the DC output. It
can be reduced (but not eliminated) by adding hash chokes (iron core)
and 0.01 bypass caps (lead to lead and lead to cabinet) to the DC leads
inside the cabinet , both + and -. Keeping the dc leads to the rig to a
minimum length helps. Make certain the electrical system safety ground
is in good condition, also.
You might try using paralleled short lengths of coax from the SMPS to
the rig. Connect the braids to the cabinets on each end. This will
further reduce dc line radiation.
J W Miller has some 3.3 uH / 20 amp hash chokes available. Mouser or
DigiKey are good parts distributors/parts sources. Iron core is
prefered over ferrite because the ferrite will typically saturate at a
few milli-amps or more DC and the inductance / filtering goes down like
Using coax fed antennas will help as the coax is non-radiating /
non-receiving. Using an antenna tuner with a long wire or open feeders
is the worst poaaible combination. This places the receiving antenna
closest to the source.
I use the SS-22 with a TS-50. The rig has 6 inch power leads and the
hash chokes and caps are on the inside. The noise (after mods) is about
the same level as the other SMPS in the house / neighborhood. I use 75
and 160 mtrs. When the bands are open (night time) it is not much of a
problem. Daytime on 160 is a problem but who uaes 160 in the day?
I have not tried to put the SMPS SS-22 inside another steel box to
provide double shielding like a screen room. All leads would be
filtered and bypassed like a screen room. I guess that would have to be
the last resort.
Keep the receiving antenna away from the SMPS by using all coax fed
Keep the dc leads to minimum length. Only a few inches.
Add a common mode filter (double Pi, two chokes and 4 caps) to the + and
_ inside the cabinet. Use hash chokes rated to carry the current.
Make certain the gounds are in good order and the same goes for the
green safety grounds in the power cords. Use shielded dc power leads /
coax. You may wish to also use a shielded AC power cord. These are
available from most computer sources or Mouser / DigiKey.
I'm kind of running out of ideas.
I hope this helps...
James (Jim), Parsons, K5ROV USAF, Ret.
firstname.lastname@example.org QCWA, NWQRP, Fists, ARRL
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