Flashing LED bulbs on a 3 way circuit.

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Jraef

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San Francisco Bay Area, CA, USA
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Electrical Engineer
I have installed an LED bulb into a hallway fixture controlled by two 3 way switches, both illuminated, meaning the switches glow orange from an internal neon lamp when both are off. this was working fine with the CFL I removed, but as per usual, the CFL took too long to get to full brightness and the user complained. So I put in the LED bulb because although the delay is sometimes perceptible I've had success with people being happier with the light output being bright immediately.

But I got a call back because sometimes, not always, the LED flashes AFTER its been turned off! Not just once, it continues flashing about once every 20 seconds or so. The first time I looked at it, the flashing went on for about a minute then quit, but then again later, it kept on for 20 minutes until I took it out and replaced it with the CFL again. I put it somewhere else and it works fine, I put it in another 3 way hall fixture, without illuminated switches, it works fine. I put it back into the original spot, it flashes again.

I don't do a lot of resi work any more, but I am the one who wired this up (traded Sheetrock work with a friend) so I know I wired it right and like I said it has worked fine for years with an incandescent and a CFL. Anyone else noticed this phenomenon with LED lamps?
 

K8MHZ

Senior Member
Location
Michigan. It's a beautiful peninsula, I've looked
Occupation
Electrician
The neon light is using the switch leg as a return path. When there is no power on the switch leg, the bulb completes the path for the neon light. When power is applied, the voltage is the same across the neon light and it goes out.

The small amount of current needed to light the neon bulb is going through your LED. For an incandescent, the result of that small current is imperceptible. But it appears to be enough to charge up a capacitor in the LED driver which discharges into the LEDs.

Here is a diagram so you can trace it out to see how it works.

View attachment 11598
 

al hildenbrand

Senior Member
Location
Minnesota
Occupation
Electrical Contractor, Electrical Consultant, Electrical Engineer
. . . hallway fixture controlled by two 3 way switches, both illuminated, meaning the switches glow orange from an internal neon lamp when both are off. . . .
I'm going to guess that the illuminated threeways don't have a neutral connection, and are the old style that rely upon the voltage drop across the bulb to cause the illumination. The "neon" lamps are placed in series with the Hallway lamp and the neon lamp current passes through the Hallway lamp.

I'd submit that the electronics of the LED driver are finding just enough power to initially light, say from a charging capacitor, before the run current drains the capacitor. LED driver circuitry varies wildly. Try a different manufacturer's LED lamp, install neutral required illuminated threeways, etc.
 

GoldDigger

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Location
Placerville, CA, USA
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Retired PV System Designer
A very low current resistive load (maybe 100k ohms) in parallel with the LED should allow the neons to light without letting the voltage across the LED build up to the point that the LED will flash.
The switches contain a current limiting resistor in series with the neon. Since there are two neons, a shunt resistor on the LED socket of 1/2 that value should keep the socket voltage low enough, and the neons will be about 1/2 original brightness.
Or, as recommended, just try different LEDs. :)
 
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