[Elecraft] Local Noise Problem

[Jim Brown]

On 4/2/2011 6:21 AM, Jim McCook wrote:
> There is a very serious problem with the local noise source from low
> voltage lighting systems.  These are sold as pendants, single lights, or
> multiple lights on a support, such as a bar.  The retailer supplies them
> with a choice of two types of transformers, either a regular magnetic
> type (non interfering), or an "electronic" supply, which is a very noisy
> switching supply.  The latter is most popular because it's small enough
> to be contained within the smallest fixture.

SNIP

>   This is a
> noise source that could seriously affect our hobby when new construction
> begins en masse again.
>

Absolutely on target, Jim. I inherited a half dozen of these little 
monsters in the home I bought in California five years ago. There are 
two major components of the probem -- first, that these types of devices 
are exempt from Part 15, thanks to ARRL being asleep at the switch when 
the exemption was made (perhaps so focused on BPL that they saw nothing 
else), and to lobbying effort for "small government" that has 
essentially gutted the FCC.  Second, a real 12V transformer big enough 
to handle the load of these lights is at least 2-4X larger than the the 
limited space available in the electrical back-box for the fixture. So 
from a practical point of view, these switching power supplies are the 
only option. Those sold by the electrical supply houses here in Santa 
Cruz county, CA, are unbranded and carry no FCC labels.

One piece of good news -- most, if not all, of the noise is radiated as 
a common mode signal on the AC power wiring, so  a good AC line filter 
can make a big dent in the RFI.  I was able to fit a filter in the 
backbox with one of these power supplies. It's also possible to suppress 
the noise with a multi-turn toroid choke plus a line-voltage rated 
capacitor across the line. There are two pieces on my website about this 
-- one on RFI and Ham radio, and the other specificcally about these 
filters -- how to use them and where to buy them.

http://audiosystemsgroup.com/publish.htm

73, Jim Brown K9YC