An electrical and lights relater question

[Brett Dikeman]

At 8:17 AM -0700 4/28/02, Pantelis Giamarellos wrote:

>So the question goes. Does this sound like a working and theoriticaly sound
>system?

No.

It -is- very similar to the way LEDs are driven on traffic lights,
signs, etc...really bright applications.  LEDs are rated in terms of
maximum voltage and current, and exceeding them causes the die to
overheat.  Pulsing takes advantage of both the LED's near
instantaneous response time(this is important) and the human
brain/eye's not-so-near instantaneous response time(but unlike a
camera, something that appears for a short time doesn't appear
"dimmer" to us, we just think it was on longer than it really
was...this is ALSO very important; maximum light output goes up, but
the OVERALL light output, averaged over a complete cycle, stays ABOUT
THE SAME.)  Note that you can actually get a little more candlepower
for your energy dollar in some cases, because LEDs are also more
efficient the colder the die is.

The limit, as above, is heat...if you pulse an LED at a 10% duty
cycle but crank up the current 10x, you get 10x the -instantaneous-
light, and the LED doesn't burn out.  We dumb humans see 10x the
light, but it looks kinda funny sometimes; ever get a weird feeling
about an LED light, especially when your eye/head is moving to look
at something else?  Or when the light is blinking?

LED manufacturers figured out this trick and created charts that
showed designers what duty cycles work best.  I messed around with it
a little, it was pretty neat.

Here's the problem...filaments are completely different technology.
Sure, they burn out too if you pump too much juice through them, but
unlike(and in fact opposite to LEDs) their heat is NECESSARY to
provide the light.  Further, filaments are VERY slow to respond(on
the order of hundreds of thousands of times slower)...AND they must
be designed to be used with A/C.  For example, theatrical bulbs are
all, for the most part, designed to be dimmed; spotlight bulbs are
one very notable exception.  They have very long filaments because of
their wattage, and if you try to dim them, the weird harmonics caused
by the dimmer pack chopping the A/C waveform causes the filaments to
vibrate and break.  It probably wouldn't be a problem with a
frequency in the 1kHz range with a bulb designed for automotive
use...all the filament is going to do probably is make a little
humming noise at 1kHz(next time you're in a theater, listen for noise
as the lights come on; the buzzing is the filaments shaking at
frequencies they're not "supposed to.")

Since the filament has thermal mass, it will be next to impossible to
get it to respond above a certain frequency; the "pulses" will get
smoothed out(you're not getting pulses of light!)  On theatrical
fixtures which are very high wattage(500W is not uncommon, 1000W is
about the limit for small, "normal" fixtures), the frequency response
is actually very, very low...they take around a half second or more
to cool down enough to stop producing light.

I'd guess that the response time of automotive headlights is still
well under 10Hz, but probably closer to 2-4Hz instead of .5-1Hz.
a
My semi-educated guess on this particular matter is that if you were
to hook up a scope or duty cycle meter to this rather expensive
widget, you'd find a voltage of 24V but a duty cycle of around 50%;
mostly likely a bit over 50%.  They probably tweaked it down from
something much higher until bulbs started lasting long enough for
them to claim something like "oh, some bulbs don't take the abuse as
well."  Etc...but still have it be brighter than without their
widget.  The end result is that you might be getting slightly more
light, but at a severe cost...reliability(cost of bulbs) and the cost
of this mystery widget(which is probably nothing more than a
switching transistor,transformer and a 555 timer; under $10 of parts,
probably.)

The kicker is the claim that you need to use new bulbs.  I would
imagine this is from one of a few different reasons:

a)used bulbs are too fragile from lots of use and instantly blow from
the higher wattage/current, versus a new bulb which will last a
little longer.

b)they're selling bulbs which are rated for higher wattages but are
unmarked as such so you don't know that you're really just getting
brighter bulbs with a worthless $200 electronic dodad. Their
"special" bulbs probably cost a lot more than they're worth, too.

All of what you described sounds very, very much like a sham.  I'd
strongly recommend you, and your friends, steer clear of them.

Brett
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