John B wrote:
Your point is taken, that "soft start" does not *necessarily* consist
of
zero-crossing start. Your point is also taken that the low
resistance of a
light bulb changes to a much higher resistance, once the filament
heats up.
For a brief time, a hugh inrush of current is possible due to the
initial
low resistance of the light. That is indeed more likely the culprit
in lamp
mortality, than mere lack of starting at zero-crossing.
I recall from years ago that triac implies start at zero crossing.
Lutron
website says all of its dimmers have triacs as the essential
elements. So
any Lutron dimmer would seemingly assure startup at zero crossing.

Your recollection may be inaccurate. Triac light dimmers actually work
by assuring AGAINST zero crossing startup. Except when full-on or off,
the dimmer operates by delaying the current with respect to the voltage
zero crossing, 120 times per second. When at 50% duty cycle (about 1/2
power), the current starts 1/4 cycle behind the voltage, i.e. at the
PEAK of the voltage sinewave, and stops when the voltage passes through
zero, so only the last half of each positive and negative half-cycle is
used.

Some heater controls modulate power by energizing the element for a
certain number of full cycles, deenergizing for some number of full
cycles, and so on. Here, zero crossing can be employed. This scheme is
not so good for lighting because the "blink rate" is well below 60 Hz
and would be quite noticable.

The question to be answered is how much of a difference does it make
using the last half of the sine "bump" vs. the first half, in terms of
energy imparted to the filament per cycle, taking into account the mass
and heat capacity of the filament, etc. etc.

I suspect that during rapid warmup, one segment of the filament gets
hotter than the rest, which makes its resistance go up higher, which
means it receives more power than than its neighboring segments which
still have lower resistance. Since it receives more power, it heats up
even faster and resistance increases even more. The hotter segment also
expands more rapidly and suffers greater mechanical stress and fatigue
as a result, becoming a likely point of failure.

Bringing the current up slowly, over the course of a second or so,
allows time for all segments to heat at closer to the same rate, as
well as time for heat to diffuse from one segment to another, allowing
more uniform heating and greatly reducing the chance of a local hotspot
forming.

Some of these ideas can be demonstrated by experimenting with bulbs in
series, which is effectively a single filament divided into segments.
Say you have a car battery (12V) and 5 to 10 identical automotive bulbs
in series. When you complete the circuit, you'll see that one gets
initially brighter than the others, then it may even get dimmer as the
others "catch up" to finally achieve uniform brightness. That's because
even though they are "identical," there are slight differences in the
bulbs and the one with the highest initial resistance and/or the
quickest heating filament will absorb most of the power till the others
heat up. I noticed this effect when I was about 6 years old, though I
had no idea what was causing it; I just knew I could rearrange the
order of the bulbs in series till they lit up from left to right and it
was pretty cool.

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