🚀 go-pugleaf

usenet@crd.ge.com (Required for NNTP) writes:

>>In article <1020006@hpdmd48.boi.hp.com> rrr@hpdmd48.boi.hp.com (Rudi Rynders) writes:
>>
>>    Given an incandescent lamp with a dimmer switch. The lamp is on.
...
>>    switch up, no matter how slow, the light always comes on rather
>>    suddenly and much brighter than in the reverse process .

>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.

I beg to differ.  The reason that you don't observe the effect with
a variac is that a variac is a low impedance (constant voltage) source
and the bulb can draw as much current as it needs when the filament is
cold (and has a low resistance).

The triac on the other hand is a high impedance source and with a 
variable resistance, as represented by a lightbulb at different
temperatures, will create a bistable system. I can state with some
confidence that it has nothing to do with the hysteresis of the
triac for two reasons;

1) One observes the same effect if the dimmer is a rheostat.

2) the triac hysteresis is repeated 60 times a second but the bulb
   can stay in one of its two states essentially forever.

	Jim