🚀 go-pugleaf

In article <STEINLY.92Apr6175636@topaz.ucsc.edu> steinly@topaz.ucsc.edu (Steinn Sigurdsson) writes:
>In article <1020006@hpdmd48.boi.hp.com> rrr@hpdmd48.boi.hp.com (Rudi Rynders) writes:
>
>
>    Can someone versed in physics please explain this phenomenon?
>    Given an incandescent lamp with a dimmer switch. The lamp is on.
>    One can then slowly turn the dimmer knob and reduce the light
>    intensity until the filament is just barely glowing.
>    If , however the lamp is off, and one slowly turns the dimmer
>    switch up, no matter how slow, the light always comes on rather
>    suddenly and much brighter than in the reverse process .
>    Why is that so ?
>
>No one tackled this one?
>I'd say because the filament resistance is a function of temperature,
>that should suffice...
>

This is not a physics problem, it is an electrical engineering problem.
If you were the incandescent lamp with a Variac, it would turn off and on
at the same point, smoothly and without hysteresis. The reason it behaves
as described by Rudi is that residential dimmers do not use variable
transformers, but do use a semiconductor switch called a Triac to apply
the 60Hz power to the lamp for variable portions of the cycle. That is, at
half power, full voltage is applied for half of each 60Hz cycle. The
triggering circuit for this Triac has considerable hysteresis, which
accounts for the behavior documented above.

Vic Roberts
GE Research and Development Center, Schenectady, NY
roberts@crd.ge.com