On 04/06/09 01:19 pm Jeff Wisnia wrote:

I replaced some existing incandescent bulbs by LED bulbs, then noticed
that they do not switch off completely but still glow dimly.

Checking the voltage at the lamp socket with a high-resistance
electronic multimeter, I get a reading of 48v!! A few volts might be
OK -- induction, stray voltages, etc., but 48V!!!

This occurs in two different rooms, but I think the various lighting
circuits are all connected to one breaker.

What could cause this?

Get yourself a "Y" socket adaptor like this:

http://tinyurl.com/d8oha9

With the wall switch OFF, screw the adaptor into the socket of interest
and put an LED bulb in one side and measure the "switch off" voltage in
the open socket.

Betcha it'll be a lot less than 48 volts then.

I don't need the "Y" adapter, because this is a multi-lamp setup anyway:
three lamp fittings on a track. The 48V reading is with identical LED
bulbs in the other two fittings.

Now, screw a regular incandescent bulb into the open socket.

If the LED bulb stops glowing when the incandescent is put in then I
agree with the other posters who said that capacitive coupling, probably
in a "switch leg" piece of cable is letting enough current run through
the LED bulb to make it glow.

Yes, with a regular incandescent bulb in one fitting, the LED bulbs in
the other two fittings do not glow.

You could experiment by screwing a plug adaptor like this:

http://tinyurl.com/dm2rvz

into the open socket so you could easily stick the leads of different
value carbon resistors into its slots to see if you could get the LEDs
to stop glowing with a resistance value high enough so that if it was
permenantly wired across the socket it wouldn't generate much sensible
heat.

I'd start with a one watt 20,000 ohm carbon resistor and see if that
works. If it does, try an even higher resistance, say 100,000 ohms. If
you can get the LEDs to stop glowing with a resistor of 20,000 ohms or
greater, then you could wire resistors of that size directly across the
fixture socket(s).

That's an idea. I'll try it when I can get to the store and buy those items.

Let us all know how you make out.

I will.

Perce