I don't have many light-dimmers in my house right now, but I'm doing some renovation, adding recessed lights in some areas that I have temporary open access to the floor joist areas and circuit runs, and I'm noticing some radio-unfriendly behavior in the first dimmer I've installed.

I've been looking on digikey for "power line filter modules" and yes they have hundreds of them. They are all, I believe, designed to be installed in appliances and devices of all sorts, inside their respective housing, enclosure or cabinet, and wired on the power-input side of the device. This is presumably to prevent wide-spectrum electrical noise from traveling back from the device to the branch circuit and hence causing the entire circuit to act as a broadcast antenna.

Most of these filters range from $15 to $30, come in a variety of inductance and current ratings, are all housed in a metal can with mounting tabs, have connection options such as spade tabs, screw terminals or posts, or wire leads.

So my first observation is that none of these is what I would consider to be mountable as infrastructure devices that you might locate in an inaccessible place, such as you would a junction box, even if the device is electrically "safe" from a durability / longevity point of view. So if I wanted to use one of these on the input side of a dimmer control for a dimmer controlling say 4 lights, and potentially also put one on the output side (?) I think I'd want to mount them inside a junction box, which is not difficult. Even if these devices have MTBF ratings in the hundreds of thousands of hours, I do wonder about the really long-term installation of these devices in areas that would be somewhat difficult to reach years from now (behind walls, under floors, etc).

So besides where on a circuit these filters should or could go to achieve maximal EMI/RFI filtering effect, I was wondering if anyone else has used these particular devices for this purpose.

Or am I overlooking purpose-built infrastructure-mountable or load-center panel-box mountable products that are easily obtainable, reasonably priced and equally effective?

  • 1
    Dimmers should not product RFI. I suspect you have a cheapo model that emits unwanted interference. You might try a better quality unit.
    – jwh20
    Mar 7, 2020 at 15:20
  • Although this is a shopping question and off topic it could be reworded on the cause of RFI and methods of control with dimmer switched circuits.
    – Ed Beal
    Mar 7, 2020 at 15:52
  • I have surge capacitors, which also function as facility (whole house) EMI/RFI filters mounted on the load center, along with (separate, different) surge arrestors.
    – Ecnerwal
    Mar 7, 2020 at 16:37
  • +1 for noticing packaging differences between electronic components and mains equipment! Many electronics folks get blindsided by this. Mar 8, 2020 at 0:23
  • Look for "full-on bypass" dimmer switches. When the dimmer is all the way up, there will be no noise.
    – www
    Oct 6, 2021 at 2:53

4 Answers 4


High levels of Radio/High Frequency noise are produced by many LED bulbs. Those tiny switched mode power supplies do often miss appropriate filters. The working frequencies are often above the hearing limit, i.e. above 20kHz, sometimes above 100kHz. The switching mode in combination with non-linear elements like fast rectification diodes produce frequencies and (cross) modulations with relative high levels even above 100MHz. The wires from the dimmer to the LED lamp could act like an emitter antenna.

In this case, the standard phase-shift dimmers with under 10 electronic elements are less likely to produce high noise levels in the MHz range, because the base frequency is only 50 or 60 Hz and the current is much lower compared to the current of the secondary side of the switched mode power supply. In addition, there is often a capacitor or RC-unit/snubber bridging the triac and reducing the noise level.

Temporarily replacing the dimmer with a switch and / or the LED lamp with a candescent one can disclose the culprit.


The problem with those modules is they're designed to go in a product

The power-line RFI filter modules you're looking at aren't useful for what you're trying to do. Why? Because they're designed to go in a finished product, and thus are UL Component Recognized (with the RU, or Rather Useless to us, mark), vs being UL Listed as something that can be installed standalone into mains wiring.

(If you can find someone willing to sell you one, there are UL listed facility EMI/RFI filters available; however, most of them are either combination EMI filter/SPD units, and those that are standalone EMI filters are rather hard to find/obtain. Furthermore, heavy-duty facility EMI filters have relatively high line-to-ground leakage currents, which makes them not get along well with GFCIs and many AFCIs.)

However, there is something you can use to fix this

However, if your dimmers are emitting RF noise, you still do have a couple options. First off, upgrading to better quality, specification grade dimmer switches (such as the better stuff Lutron, Leviton, Eaton/Cooper/Arrow-Hart, or Legrand/Pass & Seymour make) will often get you dimmers that go the extra mile on built-in RFI/EMI filtering. If that's insufficient, one can try a Lutron Lamp Debuzzing Coil in series with the dimmer wiring, as per Lutron Application Note 519.


A whole house surge protector may be an option some have RFI filters but your problem is harmonics . All electronic controls create harmonics it is a fact of wave shaping (how electronic control /adjust the power) . Some controls have zero crossing these produce less noise but when dimming all electronic controls create harmonics. Higher end controls do have some filtering built in and really cheap ones run the noise up to the limit. So your options are usually, better switches, or a whole house protector that has RFI filters built in.


In your question you hint at trying to use these filters in an "inaccessible place" or "behind walls, under floors, etc".

From a safety perspective these devices that attach to mains wiring will be required to be inside a junction box or in an approved enclosure. In addition the electrical code requires that all junction boxes remain accessible and serviceable. This means that you cannot freely install these outside of junction boxes and they cannot be hidden away in walls, behind permanent barriers or in areas that you cannot get to.

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