[RFI] Dealing with RFI from light dimmers

[Jim Brown]

One correction -- those chokes must be wound as a triplet -- phase, 
neutral, and ground. Buy three colors or wire, tape them together in 
parallel, then wind the chokes. While twisting them seems the logical 
solution, if the twist distance is some multiple of the turn diameter, 
twisting can greatly reduce the choking impedance as some turns cancel 
others. If, however, you decide to use a twisted triplet, the twist 
ratio should be VERY tight. To twist, put one end of the three 
conductors in a suitable bench vise, the other end in a drill motor, and 
twist them a lot. Then set the drill down at least over night with 
tension on the wire to give the twist some memory. I've done this a lot 
with THHN, but never with teflon.

73, Jim K9YC

On 12/27/2019 1:39 AM, Jim Brown wrote:
> 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
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