Cavechat.org

I have noticed that Christmas lights that are LED lights have some sort of flicker to them. I notice it when driving by some of them and I noticed it when putting up a string around my door. It only seems apparent when there's movement between me and the lights.

It's weird to me because it seems like the flicker is maybe 60 Hz? I don't see it with incandescent lights, so it doesn't make sense that it's related to the power supply.

Any of you smarties able to "enlighten" me? <pun intended>

They at controlling brightness with pulse width modulation (PWM). Inexpensive systems may just use reversed diodes to limit reverse current as the AC duty cycle reverses, which gives you an abhorently low visible PWM frequency.

PWM is used in MANY lights, including many caving lights, but you want to keep the frequency well above visible levels, definitely >500Hz, >2kHz is even better.

Marduke wrote:They at controlling brightness with pulse width modulation (PWM). Inexpensive systems may just use reversed diodes to limit reverse current as the AC duty cycle reverses, which gives you an abhorently low visible PWM frequency.

PWM is used in MANY lights, including many caving lights, but you want to keep the frequency well above visible levels, definitely >500Hz, >2kHz is even better.

I knew someone (probably you ) could answer my question. Sadly, I only kind of understood your answer. Could you explain what the heck you were saying? I'm electrically challenged!

Basically, the average current (and therefore brightness) is limited by switching the power to the LEDs on and off really quickly. If it wasn't doing this and just stayed on they would be brighter, or they'd see too much power and burn out.

Yep, see pulse-width modulation

OK, well, that sort of explains it.

Thanks!

Someone on our block has outdoor Christmas lights that seem to be flashing at a seizure inducing rate. I thought maybe they were set that way on purpose, but now I'm wondering if I'm seeing PWM. Interesting stuff.

Back in the day we used to simply wire a 330ohm resister inline with one lead or the other to drop 12-14 volts DC to a something an LED can handle. Given that this was 20 years ago what is different about LED's now that this wouldnt apply?

Mudduck wrote:Back in the day we used to simply wire a 330ohm resister inline with one lead or the other to drop 12-14 volts DC to a something an LED can handle. Given that this was 20 years ago what is different about LED's now that this wouldnt apply?

You could, but most LED christmas lights are running straight off AC current without being converted to DC. You just put a couple extra diodes reversed from eachother to power on both portions of the AC cycle. That's the simplest, cheapest way to do it, and produces the worst (and very noticeable) PWM flicker. Better lights up the PWM frequency to something far beyond what humans can see, or use a constant current source instead.

I was at Lowes the other day and noticed a Christmas tree with lights that would only flash while I was moving. I went to look at them and it is caused by a "lens" on the led that focuses the beam. When you move around, you get out of the line of one bunch of beams and move into others. It's like staring directly at a flashlight and then moving off to the side so your only seeing the light in the reflector.

MoonshineR DavE wrote:I was at Lowes the other day and noticed a Christmas tree with lights that would only flash while I was moving. I went to look at them and it is caused by a "lens" on the led that focuses the beam. When you move around, you get out of the line of one bunch of beams and move into others. It's like staring directly at a flashlight and then moving off to the side so your only seeing the light in the reflector.

You mean the faceted covers over some of the LED Christmas lights? That *seems* to be a source of flashing that occur with the lights I have hanging around my front door. But it doesn't seem to be the issue with the lights I drive by that are hung on a fence near my home (which don't have those "bulb covers.") And after seeing the problem with the lights along that fence makes me wonder if there's more to the flickering facet lights than just the facets.