On 4/3/2021 1:09 AM, Jeff Liebermann wrote:
On Sat, 3 Apr 2021 02:14:42 +0000 (UTC), Ralph Barone
wrote:

Since light bulbs don’t put out much light at low voltage and tend
to die at high voltage (I seem to recall bulb life being inversely
proportional to the fourth power of voltage), it was important to try and
find a way to produce a constant voltage from a variable speed generator.

I beg to differ. As I understand it, the bulb life is thermal
problem, not a voltage problem. For example, a halogen light needs to
be run a rather high temperature so that the tungsten particles
evaporated off the filament are re-deposited back onto the filament,
thus giving the halogen bulb a longer life. This doesn't work unless
the filament is really hot. Since the temperature of the filament is
mostly determined by the power (watts) dissipated in the filament,
lamps of equal operating wattage will last about the same number of
hours no matter what the voltage. Or more crudely, a 6v 1A lamp
running at rated voltage, will last approximately the same number of
hours as a 12v 0.5A bulb running at rated voltage, because they both
burn the same number of watts and operate at the same temperature.

Notice I wrote "approximately". The difference is the surface area of
the helical filament. A low voltage filament uses heavier wire than a
high voltage filament. Since brightness and tungsten evaporation
amount are partially determined by the surface area (for the same
wattage), things go non-linear in a hurry. We would probably be using
heavy gauge filament wire for maximum surface area if tungsten wasn't
so expensive.

Where the 4th power rule comes in is where the operating voltage is
lowered in order to reduce the current, which reduces the dissipated
wattage, which dramatically increases the bulb life. It's not by the
4th power but rather by the 3.5th power:
https://www.edn.com/incandescent-lamps-and-service-life/
"Candlepower = (Applied voltage / Rated voltage)3.5 × MSCP
or the mean spherical candlepower at the design voltage.
From there, Lumens = Candlepower × 4 pi "

Reality tends to be more complicated. See the section on Pg 7 "Lamp
Life" in:
https://www.alliedelec.com/m/d/04fe4bc2ac7e247d70e8e7d88fe48b19.pdf

That all sounds correct, but largely moot for our purposes. I doubt
anyone is doing original engineering with the intent of using halogen
bulbs in bike headlights.

In fact, this company http://www.reflectalite.com/ sells LED
replacements for many halogen bulbs.

IME the resultant beam focus is not quite as good, but the light output
is better. I gave one to a friend who still uses a low power battery
light and he says he likes it.


--
- Frank Krygowski