[Elecraft] Switching power supplies...yet another comment

[John Ragle]

On 7/17/2010 11:10 AM, Douglas Furton wrote: (JLR edited)
> ...Perhaps switching PSs are better engineered now than in the past...
=====

I can tell the assembled multitude that this assessment is correct. 
Before retirement, I did a lot of weak-signal work in the lab (related 
to NMR imaging), and in the earliest days of this work, ca. 1970 or so, 
I used Apple-2-Plus machines for data collection. Some of you may recall 
that these small microcomputers were powered by open-board switching 
power supplies, and the cabinets contained no shielding whatsoever. They 
were exceedingly potent sources of RFI and had to be kept "isolated" 
from the spectrometer circuitry. At some point, I considered "isolating" 
them by taking them out in a field and burying them. Fortunately, 
technology advances.

My comments about proximity were to the point that modern devices 
(computers or switching power supplies) are very well shielded, as are 
modern XCVRs. The Radio Shack 25 amp switchers that I have contain good 
shielding AND massive ferrites on the inside of the cabinet directly on 
the DC terminals. The AC line cord is similarly ferrite-trapped, 
directly inside the metal cabinet. The computers (a la FCC 
type-acceptance regulations) have RF-tight enclosures with close-fitting 
fingerstock around the openable places and below-cutoff screening on the 
fan ports. My K2 and K3 are exceedingly well shielded...surely an open 
100-watt source a couple of feet from a computer would make itself known.

I think it is really a question of "physician, heal thyself" -- I 
believe that some truly have the problems they report, and that they 
have taken the correct steps to ameliorate them. But...my overall 
opinion is that switching power supplies have taken a bad rap. My linear 
supplies are gathering dust, keeping their shelving from floating up 
into the air. Maybe the way to convince oneself one way or the other is 
to buy an inexpensive Astron or Radio Shack supply and try it. 
Also...with care...batteries make very good "filters."

John Ragle -- W1ZI