Dimmed lights go out spontaneously

I have a row of 10 40W incandescent bulbs over a dressing mirror in my
bedroom.

They are controlled by a dimmer switch. Pushing the knob will toggle
between off and on, and when the lights are on, turning the knob
counter-clockwise will dim the lights and turning them clockwise will
make them brigher.

If I go to bed early before my wife does, I often turn these lights on
and dim them until they are very low, so she can see her way in the
bedrooom.

Usually they go out completely before she gets there. What is going on?
Is the rheostat cooling down (we don't heat the upstairs) and losing
what littleconductivity it had at that low setting?

The three dimmed 60W incandescents in my dining room cieling fixture do
not behave this way.

 

Many dimmers can only go down so far and then they "drop out", going
off. After that you have to turn them to a somewhat brighter position
before they start conducting again. Your bathroom dimmers may be in that
low range where a line voltage dip would make them "drop out".
As for the dining room not doing this: some dimmers are a little
different from others, and some lamps (light bulbs) are noticeably
different from others in terms of light output at low voltages (RMS
voltage or "effective voltage" that is).

- Don Klipstein ()

 

It isn't the rheostat. The hysteresis is caused by the characteristics
of the triac and its trigger circuit.

Way wrong. The filament has no hysteresis. It does not have to "start"
like a discharge lamp does. I'm not sure what you mean by a
"mechanical" dimmer. Do you mean a variable autotransformer? if so,
these have no hysteresis either and you can operate incandescent lamp
at as low a value as you want and they will never go "out" as long as
the voltage is above 0. You just will have a very hard time seeing the
light at low input voltage.

The trigger characteristics of the Triac will vary will temperature,
so dimmers at different locations may have different low voltage
cutoff points.

 

Both use either Triacs or SCRs. You can use a variable resistor to dim
incandescent lamps, but it needs a large heat sink to get rid of all
the waste heat.

Warm but not constant. The losses in the Triac, which are a function
it its forward voltage drop and the amount of current it is carrying,
will tend to produce a certain temperature RISE above ambient. If the
ambient changes the temperature of the Triac also changes. If the load
(lamp) current changes, the temperature rise will change, which will
change the temperature of the Triac even if the ambient is the same.

BTW - I do have light switches and dimmers mounted on outer walls.