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I am having an odd problem with a light fixture. The fixture has three sockets and is wired to a dimmer. I replaced the bulbs with Philips LED bulbs that were dimmable. However, when I turn the dimmer switch on, to any level, the bulbs remain off. The part that I don't understand is that if I replace one of the bulbs with the original incandescent bulb, all 3 will light up. I can then remove the incandescent and they will remain lit only to refuse to start back up if I let them sit for a bit.

Any idea what could be going on? I am sure the bulbs are compatible.

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  • All you need is tiny <8W chandelier or Xmas bulb asa a load to make your old Triac gate voltage reach threshold to trigger threshold. The dimmer leakage is more than LED in OFF condition
    – Tony Stewart EE75
    Commented Jul 19, 2018 at 11:56

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Your old dimmer has got to go.

Dimmers are powered switches which need power themselves to do their thing. Other examples are lighted switches, motion sensors, smart switches etc. The correct way to power a "powered switch" is to bring always-hot and neutral to the switch, and it can draw power from that in all conditions.

However plain switches don't need neutral; they only need always-hot and switched-hot. So a lot of switch junction boxes don't have neutral. Adding it would require pulling a new cable through the walls. So dimmer manufacturers couldn't rely on neutral being present when dimmers are retrofitted.

As it happens, incandescent bulbs have very low resistance when they are off. So, dimmer manufacturers discovered a cheat - power the dimmer by leaking a small amount of current through the bulb. The incandescent happily lets the small current pass, and doesn't light up because it's too little current.

Enter LED. They have active electronic power supplies. Those power supplies Do Not Like the tiny amount of current. At some additional cost, the power supplies can be enhanced to allow that current to pass. Either your Philips LEDs do not do that, or the old dimmer is flowing too much current for the LEDs to deal with.

One incandescent solves the problem by providing the near-zero-resistance path the old dimmer requires. This can also be solved by a well-placed resistor of appropriate size, and several companies make UL-listed resistor packs for that purpose. (Anything used in mains wiring must be UL listed for that purpose as mains equipment; bare resistor components bought from Mouser or Digi-Key aren't legal even if UL listed as components.)

Start by reading the instructions and labeling on your Philips LEDs. Code requires you obey (because the equipment is not tested/listed for any other use). It may tell you only certain dimmers can be used, in which case do that.

Otherwise aim to replace the old dimmer with a modern one rated for LED, and ideally, if your switch location has a neutral wire, use a dimmer that uses a neutral wire.

Also make sure you're not dealing with Philips HUE or other "smart bulbs" where the dimming is actually supposed to be done by a smart-device network talking to a microcontroller inside the bulb. Those bulbs aren't even meant to work with traditional dimmers: remove the old dimmer entirely and fit either a plain switch or a system-compatible smart dimmer.

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  • Not dealing with any Hue lamps, these are just dumb LED bulbs. I took a look at the dimmer. It's a Lutron MA-LFQ35 and it does not appear to have any neutral wire running to it. The manual I found for the dimmer indicates that it is for Halogen/Incandescent lights.
    – StylePotato
    Commented Jul 19, 2018 at 14:58
  • depends how old it is. The industry was certainly aware of CFL lights as of 1997, so if a 1998 dimmer is not listed for use with CFLs, that means the maker made an active decision to not support things with electronic power supplies. This can be taken as a positive statement prohibiting use with LEDs. With a dimmer older than that, all bets are off.
    – Harper - Reinstate Monica
    Commented Jul 19, 2018 at 18:02

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