[RFI] Anyone know if this 12v ps is RFI-clean?

Sat Nov 6 11:51:34 PDT 2010

Roger,

Very good comment.  I think the key word in your message is "lab".  While
not specifically intended for communications apps, lab applications can be
very demanding as well.  Good point and worth mentioning.  I have only one
caveat to add, Tom:  low noise and ripple on the output are hallmarks of a
good quality power supply  - agreed.  However, those are no indication of
what is radiated from the cabinet or pushed back upon the AC power line,
where the noise can re-radiate anyway.  Specs for those parameters are very
hard to comeby for all except military power supplies (specifically,
MIL-STD-461.)  There is some hope if a supply is certified to the IEC
requirements and bears the "CE" mark.  However, in my mind, nothing takes
the place of actual specs from the manufacturer or published lab tests (as
ARRL did back in 2002).  

True lab power supplies would (or should) also exceed FCC Part 15 standard
requirements.  I also agree with the "pound per amp" for the large output
linear supplies.  Eons ago, I worked with a few of those bricks that
delivered nearly 100 amps - but by being designed and built in the 60's and
70's, they truly inspired my awe when it came to sheer size and weight.  

In the case of the current unit in question, I still have my doubts about
its suitability to be anywhere near comm gear.  As K9YC noted in his recent
posting, the primary problems for emissions are in the HF region.  Remember
this about switchers (of any type):  the FUNDAMENTAL switching (operating)
frequency for modern power supplies can be anywhere between roughly 50 kHz
at the low end and nearly 3 MHz (sometimes higher!) at the high end.  Most
utility supplies seem to operate in the region of about 100 kHz to 450 kHz.
Potential buyers must understand that the LF and MF bands (including the AM
broadcast band and 160m) are likely to be in either the operating range of
the switcher of within the lower order harmonics.  Lab supplies and those
made for comm applications include extra filtering, by-passing, and care in
design over utility supplies.  That's where the performance comes from -
more parts, better design, more cost.  (As one very prominent EMC
consultant is fond of saying, "The way to spell "EMC" is "$".)

One more thing:  A utility 12 V power supply (such as the one in question)
may very well deliver just that:  12 VDC.  It might have a voltage
adjustment control on it somewhere, but it's almost a given that some work
would have to be done to set it for a nominal 13.6 VDC output.  (A common
design criteria is to have an output adjustment range of +/- 10%.  That
means a supply designed for 12.0 VDC output would have a max adjustment
range of +/- 1.2 VDC, making 13.2 volts the most you could get without
having to change a part or two.)

73, Dale
WA9ENA

> [Original Message]
> From: Roger (K8RI) <k8ri at rogerhalstead.com>
> To: <rfi at contesting.com>
> Date: 11/6/2010 12:59:24
> Subject: Re: [RFI] Anyone know if this 12v ps is RFI-clean?
>
>
>
> On 11/6/2010 12:31 PM, Dale Svetanoff wrote:
> > Doc,
> >
> > I've said it before and I'll say it again:  unless a power supply is
> > designated specifically for communications applications, assume that it
is
> > NOT "clean enough" for communications usage.
> I purchased a 40A lab supply at a very good price, (one Doc mentioned 
> earlier) with digital read out it took a bit of testing but it turned 
> out to be well regulated, and clean. Which BTW is also a switching 
> supply.    Those big "conventional" supplies run just about a pound per
amp.
> The specifications for the PS will tell the story. There are many good 
> switching supplies out there, and there are a lot more that are not 
> suitable, but when you get ripple and noise figures in the 0.1% they are 
> likely pretty good.
>
> 73
>
> Roger (K8RI)
> >   The unit in question is
> > almost surely a switcher, and if intended for commercial applications,
it
> > will only meet FCC Part 15 Class A emission specs, which are several dB
> > worse (higher) than the Class B requirements for computers and digital
> > equipment used in the home.  (Note that the seller is including
shipping in
> > the $50 price - that tells me that the unit is light weight, virtually
> > guaranteeing it to be a switcher.)
> >
> > For truly "clean" power, get a "hernia-maker" - meaning a linear power
> > supply, not a switcher.  You only have to worry about 60 or 120 Hz hum
and
> > ripple with those.  If you don't want big iron like that, then stick
with
> > the select group of switchers that are designed for communications
> > applications.  Those units will exceed performance of FCC Class B
equipment
> > by controlling both the radiated and conducted emissions.  Yes, you will
> > pay more, but you get what you pay for.
> >
> > If you watch closely, there are surplus equipment dealers (and people at
> > hamfests) who sell off big linear power supplies very cheaply.  They
were
> > the norm 20 or 30 years ago, and you can probably get one for under $50.
> > You'll want to find it locally, however, as these babies usually weigh
40
> > pounds or more when you start talking 50 amps (or greater) output
current.
> > Unless malfunctioning, they usually deliver clean power to the loads.
> >
> > 73, Dale
> > WA9ENA
> >
> >
> >> [Original Message]
> >> From: doc at kd4e.com<doc at kd4e.com>
> >> To: RFI List<rfi at contesting.com>
> >> Date: 11/5/2010 9:35:04
> >> Subject: [RFI] Anyone know if this 12v ps is RFI-clean?
> >>
> >> Anyone know if this 12v ps is RFI-clean?
> >>
> >> "12 Volt 47 Amp Power supplies"
> >>
> >> See picture here:
> >> http://swap.qth.com/view_ad.php?counter=866425
> >>
> >> It looks like some sort of modified surplus server PS.
> >>
> >> I have asked the seller a couple of times but no clear
> >> answer yet.  Does it look familiar to anyone?
> >>
> >> -- 
> >>
> >> Thanks!&  73, doc, KD4E
> >> http://KD4E.com
> >> Have an http://ultrafidian.com day
> >> Defend free speech or lose your freedom.
> >> I don't google I SEARCH! http://ixquick.com
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