I believe it is standard practice to wire flourescent lights in
factories off each of the 3 phases alternatively in order to counter the
strobe effect. I have seen strobing when a single phase supply is used.

Jeff Wisnia wrote:

Gary Coffman wrote:

On Wed, 15 Oct 2003 19:08:29 GMT, "AL A." wrote:

I am looking to build a "task" lamp (think dentist-type light) that uses
high output white LEDs for a friend who often does
work for extended periods of time in very remote places. I was looking at
the NTE electronics "lightwave" flashlights
http://www.nteinc.com/lightwave/ and they claim that they have circuitry
that makes it possible to run one of the 10
led lights for 24 days, continuously, on 3 D cells.

Assuming that the batteries are series connected, that's an average
current draw of 2.2 mA for standard alkaline cells.

Any idea how they accomplish that? Most of the high output LEDs I have seen
seem to want to run between 20 and 40 mA
current draw. Given the amp-hour ratings I find for D cells, I don't see how
that would work out. Do they modulate the power to the LEDs
in some way that helps extend the battery life so dramatically?

They must. LEDs don't have the thermal lag that filament type bulbs have.
So they can pulse the LED with a reasonably low duty cycle. If they use
a 5% duty cycle, they can get average current draw down to 2.2 mA while
still producing the same peak output as running continuously at 44 mA.

Of course *average* output is also only 5%, but the eye has persistence,
and is fooled into thinking it is seeing a much brighter light. Some strange
effects would occur for rapidly moving objects, strobing, but for nearly
stationary scenes you'd see about as well as if they were actually drawing
40 mA.

They pulse repetition rate would have to be higher than the persistence
duration of the eye to prevent you from noticing the strobing at ordinary
movement speeds. It probably operates in the kilohertz range.

Gary


Cool reasoning there Gary, I like it, and any "hazards" caused by strobing of
spinning stuff is not very likely, as you probably wouldn't have too much
rotating machinery around places where the only light source was your flashlight.
Except maybe under the hood of a car, where we all know enough to keep the hell
away from the general vicinity of the fan blades when the engine is running.

I remember a public school shop teacher telling me maybe 50 years ago to watch
out for strobing of spinning machinery parts under fluorescent lamps, but I can't
say I've ever noticed the effect. Maybe the phosphors used in today's lamps have
longer persistance than those in the early ones.

Jeff

--
Jeff Wisnia (W1BSV + Brass Rat '57 EE)

"If you can keep smiling when things go wrong, you've thought of someone to place
the blame on."