[RFI] Dealing with RFI from light dimmers

[Jim Brown]

On 12/26/2019 7:48 PM, Frank O'Donnell wrote:
> I was interested in the recent posts regarding light dimmers, as this is 
> a significant issue for me. We have three dimmers in our house, and 
> three more in a guest house directly behind. As of yet, the brand name 
> on all of these is unknown. Banning dimmers from these houses is not an 
> option.
> 
> Currently I'm operating on 630m and listening across the MF/LF spectrum, 
> where RFI from these devices seems much stronger than at HF. The 
> strongest RFI is getting into a receiver in our main house from dimmers 
> in the guest house, which is physically closer to the receiver's antenna 
> but on a different power service drop. So I'm wondering if the RFI is 
> traveling through AC wiring, or if it might be getting from the dimmers 
> to the antenna. The shack gear's power is all filtered, through 
> Tripp-Lite power strips and wrapping AC cords through FT240 toroids.
> 
> I noticed that one or two people here mentioned having good luck with 
> Lutron dimmers, but others cited issues with these.
> 
> I see the ARRL RFI Book states, "The best cure is to replace the control 
> with a better one that has a built-in RFI filter. (Beware of dimmers in 
> plastic cases!)" It also discusses placing an AC line filter at the 
> dimmer, as well as one or two common-mode chokes made by wrapping AC 
> wiring around an F(T)-240 or F(T)-140 ferrite core.
> 
> Lutron has this application note on RFI and dimmers:
> 
> http://www.lutron.com/TechnicalDocumentLibrary/360484.pdf
> 
> It mentions Lamp Debuzzing Coils (LDCs) as "the most effective way to 
> reduce RFI."

Some of the advice above is inadequate, some of it is dated (because 
technology has moved on, and different lights respond differently to 
dimming.

> 
> So I'm wondering about the best things to try as next steps. Are some 
> Lutron models more effective than others? If the RFI is originating in 
> our guest house on a different service drop, is it likely that adding 
> filters to the AC supply to the dimmers there would help? Has anyone had 
> any luck with LDCs?

My advice -- 1) replace your noisy dimmers with top-of-the-line Lutron. 
Make sure you're buying dimmers designed for use with old fashioned 
incandescent lighting. 2) Replace LED and fluorescent bulbs with 
incandescent bulbs. 3) Apply common mode chokes to both wires connected 
to each dimmer, as close as practical to the dimmer.

The difficulty with chokes is that it can be difficult to fit an 
effective choke into the available space. A good starting point would be 
my applications note on chokes for RX antennas.

http://k9yc.com/RXChokesTransformers.pdf

These chokes are one pair out of CAT5 cable wound on small #75 toroids; 
the voltage rating of CAT5 is insufficient for use on the AC line, but 
these chokes could be wound with pairs of small diameter teflon wire 
with a suitable voltage rating. There's a lot of it on ebay. The cores 
cost a buck or so each in quantity. Depending on the current draw on the 
dimmers, wire as small as #22 - #24 could be practical. Note that the 
current rating of teflon insulated wire is about 75% greater than for 
lower quality insulation. I'd be OK with 5A on a pair of #24 teflon 
wires. 21 turns on two of the #75A cores would be a good starting point 
for 630M-80M, and would be somewhat effective on 40M.

The key here is that the noise is radiated as a common mode signal on 
the power wiring on both sides of the dimmer.

73, Jim K9YC