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[RFI] Autopsy on a Lutron dimmer

To: rfi@contesting.com
Subject: [RFI] Autopsy on a Lutron dimmer
From: Dale <svetanoff@earthlink.net>
Reply-to: Dale <svetanoff@earthlink.net>
Date: Sun, 1 Dec 2013 23:55:22 -0600 (GMT-06:00)
List-post: <rfi@contesting.com">mailto:rfi@contesting.com>
RFI Fans and Victims,

Some weeks back, Jim Brown, K9YC, asked for input on what brand of dimmers 
seemed to create the least amount of RFI.  The consensus, supported (in part) 
by me, was that Lutrons seemed to be pretty quiet.  All of my in-service 
dimmers are Lutron brand.

My house was built in 2000, and all of the dimmers, except for one that I added 
after moving in, were installed that year by the electrical contractor.  
Recently, two of those original dimmers have failed due to a mechanical 
problem, NOT electrical issues.  Today, I took one of the deceased units 
partially apart to see what failed and maybe learn why they have low RFI.  Both 
units are the Lutron DIVA series, model DV-600P, intended for up to 600 watts 
of incandescent or halogen lamp load.  They are made in the Decora style (wide 
paddle on-off switch) with a very narrow slider on the side and an internal 
neon lamp to assist with finding the unit in a darkened room.  I found the 
exact replacement units at Home Depot for about $21 each.

The main tool required to open up a Lutron DV-600P is a #8 Torx bit.  Two small 
screws hold the back cover in place.  Remove that and some of the guts are in 
view.  (Alas, most electronic components are not directly visible without doing 
some unsoldering.  I have yet to proceed that far with disassembly.)  However, 
what I regard as 2 of the primary parts that contribute to RFI reduction are 
easily viewed with just cover removal.

First, let me explain what did fail.  On both of my units, the large 
paddle-style on-off switch actuator began to stick and it appeared that 
something was broken in the switch mechanism.  Well, the entire front of the 
unit is one big plastic assembly and the paddle rocks to and fro on pivots.  If 
one of the rocker arms that ride the pivots breaks, the unit is done.  That is 
what happened with my units.  I might add that the rocker paddle has a simple 
actuator tab on it.  I was surprised to see that the actual on-off switching is 
handled by a very nice Omron SPDT miniature snap action switch, not some sleazy 
stamped and formed contacts as I had suspected.  Dimming is controlled with an 
Alpha slide pot - looks to be of good quality.  I was also surprised to see 
that my original units were assembled in that industrial powerhouse known as 
St. Kitts - the replacements are assembled in China.

Now, for the good stuff on RFI.  The single largest electrical component is the 
inductor that appears to be in series with the AC power line and load.  It 
consists of 18 turns/layer of magnet wire, double layer, on a cylindrical core 
(either ferrite or possibly powdered iron) that is about 1/4" OD and about 1" 
long.  The double layer of the winding gives the finished coil an OD of about 
7/16".  This coil, which has a clear insulating sleeve over it (easily removed) 
is on the back side of the pc card and fully visible.  The other significant 
anti-RFI component appears to be a 0.01 mfd cap that I believe is in series 
with the inductor.  That cap is on the component side of the circuit card, but 
is located near the bottom edge, so it can be seen without further disassembly.

I have no doubt that other brands of dimmers use similar components IF they 
offer RFI reduction.  Beware - there are units on the market that seem to lack 
RFI-reduction components.  

So, there you have a partial analysis of the Lutron DV-600P dimmer.  If anyone 
else opens up a similar unit or a different model, please let the list know.  

73, Dale

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