[RFI] Low voltage lighting solid state "transformers"

Jim Brown 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 
this filter. 

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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