Wednesday 23 January 2019

my 'always-on' stair lights

Sometimes things work better than they were designed to.

The lights on my stairs work as expected when I turn them on, but continue to glow when I turn them off!

So whats going on?

The lights in question are on my 1st & 2nd floor landings, lighting the stair way between the two. They are controlled by a couple of 2 way switches so that the light can be turned on/off from either the 1st or 2nd floors. As far as I can tell, the wiring is a standard configuration.

2 lights, 2 switches, 2 floors

When the 2nd floor was added to the house back in the 1980s, the two light fittings had tungsten filament bulbs, and all was well. These bulbs were replaced in the 1990s with energy saving Compact Fluorescent Lamps (CFL), still no problem.

But a few days ago I bought a couple of 2W led bulbs.

a 2Watt led is perfect for a hall light

While most domestic leds are around 5Watt and generate a fairly bright light, I was looking for something much less bright, so that the bedrooms are not flooded with light if someone gets up in the middle of the night, and turns the hall lights on.

On the first night of use we couldn't help noticing that these bulbs continued to glow after they were switched off. When off they glow very dimly. In fact when you look at them during the day you might notice a very faint ring of light on the neck, near the base.

notice the faint glow of light around the neck of the bulb

We have a second 2 way circuit between the ground floor and 1st floor. I get the same effect when I fit one of these bulbs, although slightly brighter (this circuit has two switches but only one light). However, there is no problem if I fit these bulbs in a regular room light with a single on/off switch.

I've checked to ensure that these light circuits have not 'borrowed' a neutral from another circuit. Also, the bulbs don't light up when the circuit is turned off, so its not 'free power' from the nearby 4G mast.

A few checks with a multi-meter determined that there was approximately 42 volts a.c. across the bulbs.My meter did not display any current, and when I selected frequency it would flicker between 150Hz - 250Hz. I really need to put a scope on it, but that is a lot easier said than done.


At the moment I think the relatively long cable run between switches is creating a voltage in the wires that are open circuit when the lights are switched off (i.e. there is a live wire laying next to the bulb feed wire). Since both the live and feed wires are open circuit, I assume its capacitive coupling that is creating this voltage.

Does it matter? No, not unless it shortens the life of the bulbs. I may be drawing micro Watts, but any cost is insignificant. Anyway, its nice to have a dim glow in the hall and over the stairs at night. So I'll get a third bulb for the other stair light the next time I'm near TLC.

We have never got out of the habit of leaving a neon night light running in the hallway, despite all our children having grown up and left home!

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