[RFI] Autopsy on a Lutron dimmer

[Dale]

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.  Thanks.

73, Dale
WA9ENA