Whats causing this?
Dec 6, 2021 14:17:58 #
An LED emits light when there is current flowing through it. It's a diode, so current only flows one way and the LED will only light on one half of the line phase, depending on which way it is wired. In addition, the voltage has to get up high enough (maybe around 3 volts) before current will flow and the LED will light. That means there will be a small gap when neither color LED is lighted. (The gap is shown much wider than it would really be in the diagram below).
So if you have a string of LEDs with one group (red lights) wired in one direction and another group (green lights) wired in the other direction, the string will display red lights, then green lights, alternating at 60 Hz. Too fast to see by eye but you could probably see the effect if you swing the string of lights around. If the string is too long to swing conveniently, just turn your head (or just your eyes) across the string rapidly.
So if you have a string of LEDs with one group (red lights) wired in one direction and another group (green lights) wired in the other direction, the string will display red lights, then green lights, alternating at 60 Hz. Too fast to see by eye but you could probably see the effect if you swing the string of lights around. If the string is too long to swing conveniently, just turn your head (or just your eyes) across the string rapidly.
Dec 6, 2021 14:18:25 #
Ava'sPapa wrote:
Constantly on which is why I'm puzzled.
No light using 60~ power is truly constantly on. There is a 120~ blink even though our eyes can't see it. Neon and LED are more pronounced than tungsten filament, but even they have some blink.
Dec 6, 2021 14:27:18 #
PHRubin wrote:
No light using 60~ power is truly constantly on. There is a 120~ blink even though our eyes can't see it. Neon and LED are more pronounced than tungsten filament, but even they have some blink.
Incandescent lights are so slow to cool down and "turn off" compared to 60Hz that they never really go dark between cycles. I'd question if they even dim.
LEDs, being a semiconductor, are closer to instantaneous on and off. Look at the difference between LED and incandescent bulb brake lights. Incandescent "ramp up" and down while LEDs are "bam", on, bam, off.
Dec 6, 2021 14:33:32 #
Longshadow wrote:
Incandescent lights are so slow to cool down and "turn off" compared to 60Hz that they never really go dark between cycles. I'd question if they even dim...
Slow, yes. But they must dim, even if only slightly. They cycle at 120 Hz (they are heated by both phases of the 60 Hz line) and there is a period when the rate at which energy goes into heating the filament has to drop below the rate at which energy is lost to radiation (and some to conduction of heat to the surrounding gas). The period is so short that I would expect it to be very difficult to measure the drop without sophisticated detection apparatus, but it has to happen.
And, of course, it will depend on the construction of the filament. A thick filament will keep the surface hot by conduction from the inside of the filament, making the dimming smaller.
Dec 6, 2021 14:36:15 #
DirtFarmer wrote:
Slow, yes. But they must dim, even if only slightl... (show quote)
I wonder if anyone ever put a scope on a luminosity meter (sensor) to show any difference.
Dec 6, 2021 14:51:49 #
Longshadow wrote:
I wonder if anyone ever put a scope on a luminosity meter (sensor) to show any difference.
I wonder if anyone ever put a scope on a luminosity meter (sensor) to show any difference.
I think it would require a way to measure the luminosity/time curve to a precision of parts per million. WAG.
Dec 6, 2021 15:51:17 #
Longshadow wrote:
I wonder if anyone ever put a scope on a luminosity meter (sensor) to show any difference.
I wonder if anyone ever put a scope on a luminosity meter (sensor) to show any difference.
I think I would do just that. I don't think they dim because they are very slow.
Dec 6, 2021 15:53:48 #
BebuLamar wrote:
I think I would do just that. I don't think they dim because they are very slow.
If you have a scope, try it. I don't have access to a scope in retirement.
Dec 6, 2021 16:09:01 #
DirtFarmer wrote:
I think it would require a way to measure the luminosity/time curve to a precision of parts per million. WAG.
If the sensor response is fast enough, like a semiconductor sensor, a scope (oscilliscope) will show the difference in amplitude of the brightness sensed.
Dec 6, 2021 16:17:42 #
DirtFarmer wrote:
An LED emits light when there is current flowing t... (show quote)
I understand what you’re trying to demonstrate but that diagram is deceiving. It looks like each cycle is affecting each bulb individually. Besides the 60 cycles a second you need to take into account the speed of the wave itself based on resistance. Each cycle actually affects several bulbs at once which is why there are dark sections and light sections in those photos. If you look closely at the photo with the far right end dark you can see the last vestiges of light from the previous cycle in the last few bulbs.
Dec 6, 2021 16:29:41 #
All (likely more than you want to know) about LED light strings. Found this when two of last year's strings had exactly half the bulbs in the string not lighting! I think I'm going to go back to incandescent.
http://www.ciphersbyritter.com/RADELECT/LITES/LEDLITES.HTM
http://www.ciphersbyritter.com/RADELECT/LITES/LEDLITES.HTM
Dec 6, 2021 16:30:44 #
SuperflyTNT wrote:
I understand what you’re trying to demonstrate but... (show quote)
Electricity travels pretty fast, close to the speed of light.
I think what you see in that photo is the shutter travelling across the field of view. Typical shutter travel times are 4 msec so either the LED was on when the shutter started opening but shut off before it reached the top of the string of lights. The shutter is MUCH slower than the speed of light. The LEDs generate light when current is on. When the current turns off there is no residual light from the LED.
The diagram shows the light from the green bulbs (any or all of them) and the red bulbs (same). Not individual bulbs. All the bulbs in a given string (considering the way the bulbs are wired) act together.
Dec 6, 2021 16:31:31 #
DirtFarmer wrote:
If you have a scope, try it. I don't have access to a scope in retirement.
I have the scopes and I have sensor that is fast enough for that.
Dec 6, 2021 16:36:01 #
BebuLamar wrote:
I have the scopes and I have sensor that is fast enough for that.
That's great. Give it a try if you can. I suspect you will have to use a small bulb (thin filament) to maximize the possibility of seeing the very small amount of dimming of the light. And I think you will have to pay attention to 60 Hz noise in the system. Good luck.
As I said, I suspect the variation with phase will be extremely small so it's certainly possible you won't be able to see any effect.
Dec 6, 2021 16:38:56 #
MDI Mainer wrote:
All (likely more than you want to know) about LED light strings. Found this when two of last year's strings had exactly half the bulbs in the string not lighting! I think I'm going to go back to incandescent.
http://www.ciphersbyritter.com/RADELECT/LITES/LEDLITES.HTM
http://www.ciphersbyritter.com/RADELECT/LITES/LEDLITES.HTM
I'm using incandescent lights for my outdoor decorations. The LED strings were significantly more expensive and the incandescent lights are really cheap and last several years. I tried some LEDs a few years ago and they were 3-4 times more expensive but they did no better than the incandescent in terms of multi-year lifetime so I didn't see any advantage to them.
Haven't checked prices this year because I'm still using lights from a few years ago.
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