[RFI] New switching wallwarts - RFI potential?

Paul Christensen w9ac at arrl.net
Tue Jan 12 08:22:26 PST 2010


> "...The mechanism that causes all of that noise is the rectifier diodes. 
> If you use diodes that have a long minority carrier lifetime, no problem, 
> but with the faster diodes that are being used nowadays..."

That's a new one on me.  I had not heard of RFI/EMI generated by PN junction 
switching in linear-mode power supplies.  In reading through Dallas 
Lankford's description, I was initially certain he was discussing a Radio 
Shack switch-mode supply until I saw the simplicity of the circuit in the 
photos.

What types of short minority carrier lifetime diodes are the culprits and 
why are they now being used in linear power supplies in place of the 
garden-variety 1N400x types?

Paul, W9AC


----- Original Message ----- 
From: "Martin Ewing" <martin.s.ewing at gmail.com>
Cc: "RFI List" <rfi at contesting.com>
Sent: Tuesday, January 12, 2010 11:08 AM
Subject: Re: [RFI] New switching wallwarts - RFI potential?


Interesting about LF EMC, but no info on the new switching supplies.

I have done no serious tests, but so far have not seen any issues with
the new generation mini-switching wall-warts -- and they are really
proliferating around here.

73 Martin AA6E

On Tue, Jan 12, 2010 at 10:53 AM, Tim Duffy K3LR <k3lr at k3lr.com> wrote:
> Hi Pete!
>
> Here is a great paper on wallwarts.
>
> http://www.kongsfjord.no/dl/Antennas/Low%20Noise%20Active%20Antennas%20AC-DC
> %20Power%20Supplies%20II.pdf
>
>
> 73,
> Tim K3LR
>
> -----Original Message-----
> From: rfi-bounces at contesting.com [mailto:rfi-bounces at contesting.com] On
> Behalf Of Pete Smith N4ZR
> Sent: Tuesday, January 12, 2010 9:36 AM
> To: RFI List
> Subject: [RFI] New switching wallwarts - RFI potential?
>
> Recently, I have begun receiving a new type of wall-wart with various
> pieces of consumer equipment and, most recently, with my QS1R SDR. Ones
> I've seen have generally been for 5 volts at 1 amp, are much lighter
> than the old ones, and I even got one that said Switching Adapter on the
> side.
>
> It also claimed, of course, to be FCC approved and UL listed, but no CE.
>
> These adapters are very attractive - in addition to the high current
> rating they are narrow, so that 6 of them would fit on a 6-hole surge
> suppressor, even without wall-wart-spaced outlets. but I'm a little
> concerned - I don't think I'm hearing mine, but my shack is a long way
> from my antennas.
>
> Has anyone had occasion to test these critters?
>
> --
> 73, Pete N4ZR
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-- 
Dr. Martin S. Ewing, AA6E
Member IEEE, URSI, AAS, ARRL
Branford, Connecticut
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