[RFI] Low voltage lighting solid state "transformers"
jim at audiosystemsgroup.com
Sun Dec 17 11:23:10 EST 2006
On Sat, 16 Dec 2006 19:59:57 -0700, Steven Farmer wrote:
>Can anyone suggest a brand or manufacturer of _quiet_ 120 to 12 volt
>switching power supplies for low voltage lighting?
I have a half dozen or so of these little monsters in the home I bought
in CA. I've done some things to suppress them, with varying degrees of
success. The problem is physical space to mount either the suppression
component or a real transformer.
To put my comments re: the effectiveness of these filters in
perspective, I live in a low population density part of a redwood forest
north of Santa Cruz. I have one neighbor about 250 ft away, two at about
400 ft, and a half dozen that are probably 750 - 1,400 ft away. My noise
level is usually fairly low, and at least 15 dB lower than what I had in
a Chicago residential neighborhood. I can often copy a CW signal that
doesn't move the S-meter on my TS850 with a 250 Hz filter. But when the
XYL goes in the bathroom and turns on the LV lighting, I'm in trouble.
Before I did the suppression, they gave me noise levels of 10 dB over
So far, I've come up with three solutions. The easiest is a commercial
power line filter that fits in the backbox. I found a 3A Delta unit in
their DK series at a local surplus house. They worked well enough that I
bought all they had. I still hear a bit of noise from them on 160 on a
vertical 25 ft away, but not on a dipole 100 ft overhead. Next time I
open the backbox, I'll add 0.22 uF in parallel with the Line side of the
The first thing I tried works well electrically, but it doesn't fit in
the backbox. It's 14 turns of #12 stranded THHN twisted pair wound
around a #31 Fair-Rite 2.4" o.d. toroid (it might be called FT-240-31 in
the ham world), with 0.47 uF on the Line side of the choke. This choke
resonates around 10 MHz, so it is quite effective from about 20 meters
down, but its effectiveness is reduced at higher frequencies. I hear NO
noise on that vertical from the lighting with that filter in line. The
resonant frequency could be raised a bit while still maintaining the low
frequency performance by using smaller gauge wire. Nearly all power
cords are #18. One surprise to me -- zip cord was far less effective in
At one location, I have three of these little power supplies in line
with the switch. I have the choke/capacitor in series with the first one
(closest to the switch) shoved up into the ceiling. In each of the other
two fixtures, I have smaller chokes using #16 twisted pair around
smaller toroids (as I recall, 7 turns around 1.4" toroids). They fit in
the backbox. This works fairly well below 20 meters -- I don't hear
these fixtures on a 160/80/40 dipole that runs 100 ft overhead -- but I
do hear them on a Beverage that runs about 50 ft away, and they are
significantly increasing the noise level on 20M dipoles that are 100 -
200 ft away. My next move will probably be to replace the small chokes
with the same commercial filters I used in the other fixtures.
When thinking about wire size, remember that these fixtures are
typically 40W fixtures, and you're on the 120V side, so some fairly
small wire will do the job. All it needs is insulation rated for use on
the AC line.
I'll be interested to see what options emerge from this thread for the
"transformer." So far I haven't found any. W8JI says he's used filament
transformers to power lighting like this. All the 12V transformers I've
found are far too large to fit in a backbox.
Jim Brown K9YC
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