On or about 2007-10-20,
Mike Williams illuminated us with:
Wasn't it Mark Ayliffe who wrote:
On or about 2007-10-19,
Mike Williams illuminated us with:
Wasn't it Les Hemmings who wrote:

Well, LED's only work with one polarity (all diodes work like a one way
valve, letting current flow one way but not the other) so on alternating
current they will flicker at the same frequency as the mains (50 Hz in the
UK i think) which means there are 25 periods of darkness per second.

I'd guess that they'll probably use flicker-free LED lights for street
lighting. Such things do exist. Some people (e.g. me) find the flicker
from LEDs extremely annoying. I find it painful to look at the new LED
traffic lights that have been installed around here, and would go
bananas if whole streets were lit that way.

By the way: 50Hz would mean 50 periods of darkness per second, not 25.

Errm, sorry to be a paid, but wouldn't it be 100? 2 zero crossings per
cycle, no?

No. All diodes work like a one-way valve, letting current flow one way
but not the other. So they're dark for the whole half cycle where the
current flows in the opposite direction.

Fair enough. My mistake. It's too many decades since I studied this
stuff obviously!

I guess one way to make flicker free LED lights would be to use pairs of
LEDs arranged with opposite polarity, so that both half cycles are used.

That or use a full wave rectifier. But your suggestion is more likely
I'd agree. Presumably unless you're spending more cash to supply them
with DC, you'd pretty much do that anyway for more than one LED?

I also guess that for street lights, they'd be using "white" LEDs. The
way most "white" LEDs work is that the LED itself actually emits blue
light, but it is coated with a phosphor that absorbs some of that blue
light and fluoresces with a broad spectrum yellow light. The blue and
yellow give the appearance of white light, with loss of colour fidelity
in the red and green. The fluorescence effect lags behind the light
emitted from the LED.

The Cateye one I use on my bike does look very white. More so than the
blue tinged headlights you see on some cars these days and definitely
not green or yellow as some of the early ones were. But I've not
inspected it carefully enough to see whether I can see any phospher.

--
Mark
Real email address | "Did you sleep well?"
is mark at | "No, I made a couple of mistakes."
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