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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