Do LED lights flicker? (ac or dc current)

[eflat]

Just wondering. I am ones of those freaks that can detect flicker in fluorescent light bulbs, so like to use incandescent bulbs. How about LED's?

 

[eldorado99]

LEDs run on DC, so no.

 

[bobsmith1492]

Originally posted by: eldorado99
LEDs run on DC, so no.

LEDs CAN run on AC but they'll just be on somewhat <50% of the time (half-wave rectified or none) so they would indeed flicker. Full-wave rectified they'd be on more of the time but still pulsating. Add a cap to help filter and there would be no flicker.

It depends on how they're hooked up. If it's a commercial LED bulb for lighting it shouldn't flicker.

Also most modern fluorescent bulbs have electronic ballasts so there is no 60Hz flicker to see (it's more like 30KHz).

 

[eldorado99]

Originally posted by: bobsmith1492

Originally posted by: eldorado99
LEDs run on DC, so no.

LEDs CAN run on AC but they'll just be on somewhat <50% of the time (half-wave rectified or none) so they would indeed flicker. Full-wave rectified they'd be on more of the time but still pulsating. Add a cap to help filter and there would be no flicker.

It depends on how they're hooked up. If it's a commercial LED bulb for lighting it shouldn't flicker.

Also most modern fluorescent bulbs have electronic ballasts so there is no 60Hz flicker to see (it's more like 30KHz).

You know more than I do, sir.

 

[eflat]

Originally posted by: bobsmith1492

Originally posted by: eldorado99
LEDs run on DC, so no.

LEDs CAN run on AC but they'll just be on somewhat <50% of the time (half-wave rectified or none) so they would indeed flicker. Full-wave rectified they'd be on more of the time but still pulsating. Add a cap to help filter and there would be no flicker.

It depends on how they're hooked up. If it's a commercial LED bulb for lighting it shouldn't flicker.

Also most modern fluorescent bulbs have electronic ballasts so there is no 60Hz flicker to see (it's more like 30KHz).

And how does a cap interact with the dc current? What in the DC current is pulsating.

Say, AC = 50% flicker noticable rate

and DC = 80% or more like 99.99%

would one of those be true.

 

[EarthwormJim]

Originally posted by: eflat

Originally posted by: bobsmith1492

Originally posted by: eldorado99
LEDs run on DC, so no.

LEDs CAN run on AC but they'll just be on somewhat <50% of the time (half-wave rectified or none) so they would indeed flicker. Full-wave rectified they'd be on more of the time but still pulsating. Add a cap to help filter and there would be no flicker.

It depends on how they're hooked up. If it's a commercial LED bulb for lighting it shouldn't flicker.

Also most modern fluorescent bulbs have electronic ballasts so there is no 60Hz flicker to see (it's more like 30KHz).

And how does a cap interact with the dc current? What in the DC current is pulsating.

Say, AC = 50% flicker noticable rate

and DC = 80% or more like 99.99%

would one of those be true.

DC current doesn't pulsate, hence direct current (vs alternating current). If you were asking about AC, the current itself (voltage too) is alternating. Depending on the source it can alternate like a sine wave graph (sinusoidal) or a square wave, triangle wave all kinds of different waves.

The capacitor would basically run the led during the off cycles in AC power.

 

[dighn]

a lot of LEDs are driven by pulsed supplies either as a byproduct of voltage conversion or for brightness control. But that's usually in the kHz range, so too quick to notice.

 

[Jeff7]

They shouldn't.

However, I do sometimes see them flickering on the backs of cars or trucks when used as brake lights or other similar sorts of indicators. It might have something to do with PWM (pulse width modulation) used to control brightness.


As Bobsmith1492 mentioned, new fluorescent fixtures shouldn't flicker, as I think most of them use electronic ballasts now (CFLs definitely use electronic ballasts, don't know about tube-type fixtures), and they run at a much higher frequency.

 

[Rubycon]

Incandescent lights flicker off 60Hz but due to the attack and decay times of the filament, the AC carrier superimposed is buffered enough that human eyes cannot perceive it. A silicon cell connected to a small amplifier and pointed to a bulb will reveal a hum though.

LED's OTOH, have MUCH faster attack and decay times - the measure of how fast they reach brightness and how fast they go dark respectively. Thus if connected to a power source that fluctuates one is much more likely to detect aberrations in the incoming power. Many drivers will pulse the LED's to conserve power (or in the case of white LED's) to maintain a more constant tint over their operating brightness range. Most drivers will keep them in the near kHz range so the flicker does not become bothersome as well as the possible stroboscopic effect on fast moving objects.

An LED driven directly from a battery, will have no flicker whatsoever. If a battery light appears to flicker this happens because it's driven by a PWM converter which is often the case because it needs a higher voltage than the battery can produce and/or the builder wanted regulated (constant output) light over the life expectancy of the power cell.

 

[Jeff7]

Rubycon - are PWM drivers cheap to make? I built a small circuit based on a chip that Linear makes. They call it a "buck-boost" driver.

The circuit as I made it is able to convert 3-18VDC input to 35.9, and I can't see any flickering at all in the output.

I do see that "stroboscopic effect" in cars a lot though. Is that simply because lower-frequency drivers are cheaper, and thus more prevalent?



Always nice of you to drop by in these threads.

 

[Rubycon]

Originally posted by: Jeff7
Rubycon - are PWM drivers cheap to make? I built a small circuit based on a chip that Linear makes. They call it a "buck-boost" driver.

The circuit as I made it is able to convert 3-18VDC input to 35.9, and I can't see any flickering at all in the output.

I do see that "stroboscopic effect" in cars a lot though. Is that simply because lower-frequency drivers are cheaper, and thus more prevalent?



Always nice of you to drop by in these threads.

I guess it depends on how cheap labor is.

The drivers are cheap enough. Everything these days uses buck boost on it. A calculator with a fluorescent display is always fun to take apart.

I've seen the flicker on those third brakelights - and one would think with cars a constant light would be a priority. Problem is they are using a LOT of LED's. They should be using Luxeon's.

 

[Jeff7]

I built up an array of close to 100 T1 3/4 LEDs run off of a pair of these Linear-based things. I think the chip was an LT1618. No flicker. That was something I put together with very little electronics knowledge, and it was based largely on the specsheet for the LT1618. I'd expect a genuine electronics engineering professional, working for some company that designs such circuits, to be able to come up with something better.


Some truck lights appear to use larger LEDs in an array of 5-6 individual lights, and even those will flicker.

 

[Rubycon]

The quality of the design doesn't mean it flickers or not - it could be intentional to get your attention. That part I don't know. What's for certain is there is a very bright future for LED lighting. Oh the pun! :laugh:

 

[Jeff7]

Gee, you should write for a newspaper. They LOVE bad puns like that.

I believe they are also masochists, as this sort of thing causes pain in all who read it.

There was but one exception, about the tightrope walker who crossed a river, by himself, in China:

"Skywalker Crosses Han Solo."

 

[Rubycon]

Originally posted by: Jeff7
Gee, you should write for a newspaper. They LOVE bad puns like that.

I believe they are also masochists, as this sort of thing causes pain in all who read it.

There was but one exception, about the tightrope walker who crossed a river, by himself, in China:

"Skywalker Crosses Han Solo."

Ever read the novel Yellow River by I.P. Daily?

 

[Jeff7]

Originally posted by: Rubycon
Ever read the novel Yellow River by I.P. Daily?

Please don't make me hate you.

 

[Rubycon]

RE: PV cell detection...

If you want to try something interesting go find an old sealed beam bulb like the ones used in earlier model automobiles (before the halogen days). Connect it to a battery and shine it on a solar cell. Connect the output of the solar cell to your mic input on your sound card and record this. Now start tapping on the bulb. This will shake the filament slightly and these vibrations will modulate the light output. Listen to what you record, it sounds creepy! :Q

If you use a power supply make sure the DC is clean and ripple free. A battery charger is only rectified so it will have lots of ac ripple and this will bleed into your recording FYI.

PV cells are pretty efficient at detection of changes in amplitude. One mounted in the eyepiece of a telescope pointed at an active cumulonimbus cloud will reveal flashes of lightning and if an AM radio is connected (tuned to a blank space with no broadcast) and the PV cell is connected to the other channel and this is recorded in stereo the two when played back will sound strikingly similar. Oh the pun!

 

[Aluvus]

Originally posted by: EarthwormJim

DC current doesn't pulsate, hence direct current (vs alternating current). If you were asking about AC, the current itself (voltage too) is alternating. Depending on the source it can alternate like a sine wave graph (sinusoidal) or a square wave, triangle wave all kinds of different waves.

The capacitor would basically run the led during the off cycles in AC power.

Real-world DC that is produced by rectifying AC will have some ripple (the AC waveform "bleeds through" slightly), and will not match the ideal perfectly-flat voltage waveform. Shunt (parallel) capacitors are used as low-pass filters to get rid of some of this ripple. This is what bobsmith1492 is talking about.

In such a case, the LED's brightness might fade slightly, but it would not drop all the way to nothing. If any flicker whatsoever is detectable to a human eye, then it is either a severe design flaw or intentional. Alternately, using a switching power supply to go from AC to DC should produce ripple that is small in magnitude and at much too high a frequency to notice.

Conversely, fluorescents with a 60 Hz flicker are not all that hard to notice.

Originally posted by: Rubycon
The quality of the design doesn't mean it flickers or not - it could be intentional to get your attention. That part I don't know.

A study a few years ago found that brake lights that "pulse" (maybe 1 Hz or so) were substantially more effective in getting drivers' attention. But that they were also annoying to drivers, for the same reason. So it could be intentional.

 

[Squisher]

Originally posted by: Rubycon
The quality of the design doesn't mean it flickers or not - it could be intentional to get your attention. That part I don't know. What's for certain is there is a very bright future for LED lighting. Oh the pun! :laugh:

Oh so bad. Wince worthy.


I've converted all my 12v landscape lighting to LED. These CREE MR11's are super bright.

I looked at some 120v LEDs for AC lighting, but the price seems prohibitive still. $71 for a replacement of a 150watt bulb seems a little high. I wanted to put this in a floor lamp and with grandkids in the house I didn't want there to be a chance of knocking over one with a CFL in it.

 

[Triumph]

I bought an el-cheapie small LED light set from Ikea for 15 bucks, for lighting a book case, and they indeed flicker from time to time. So yes, it happens.

 

[sciencewhiz]

Originally posted by: Jeff7
I built up an array of close to 100 T1 3/4 LEDs run off of a pair of these Linear-based things. I think the chip was an LT1618. No flicker. That was something I put together with very little electronics knowledge, and it was based largely on the specsheet for the LT1618. I'd expect a genuine electronics engineering professional, working for some company that designs such circuits, to be able to come up with something better.

Considering that chip has a 1.4 mhz switching frequency, I'd hope you weren't able to see any flickering.

In consumer electronics, cost is generally design goal #1. When you're making millions of items, the difference between the $1.75 LT1618 and a 2 cent diode is millions of dollars. Thus the electronics engineering professionals are not designing the best circuit for performance but rather for cost.

When I was an intern in college (before there was single chip LED drivers like the LT1618) I designed a constant current LED driver with 2 op amps and a precision voltage source, and a few other discretes. My design goal was $5 (qty 1). Unfortunately, the product it was for never got off the ground but it was fun testing it over wide temperature ranges.

 

[Casawi]

The flickering comes from alternation of the current. DC does not alternate so no flickering. LED is simply a diode that emits light when current goes thru it, I think I am saying the obvious here ... so yeah no flickering.

 

[Special K]

Originally posted by: Rubycon
Incandescent lights flicker off 60Hz but due to the attack and decay times of the filament, the AC carrier superimposed is buffered enough that human eyes cannot perceive it. A silicon cell connected to a small amplifier and pointed to a bulb will reveal a hum though.

LED's OTOH, have MUCH faster attack and decay times - the measure of how fast they reach brightness and how fast they go dark respectively. Thus if connected to a power source that fluctuates one is much more likely to detect aberrations in the incoming power. Many drivers will pulse the LED's to conserve power (or in the case of white LED's) to maintain a more constant tint over their operating brightness range. Most drivers will keep them in the near kHz range so the flicker does not become bothersome as well as the possible stroboscopic effect on fast moving objects.

An LED driven directly from a battery, will have no flicker whatsoever. If a battery light appears to flicker this happens because it's driven by a PWM converter which is often the case because it needs a higher voltage than the battery can produce and/or the builder wanted regulated (constant output) light over the life expectancy of the power cell.

My laptop's power LED will cycle between fully lit and fully off when my laptop is in standby mode. The increase/decrease in brightness of the LED seems linear to me. Given the steep IV curve of diodes, how is it possible to make an LED that behaves this way? Are they using a high-frequency PWM here, and the duty cycle determines how bright the LED appears to be? Did they fabricate an LED with a linear IV curve?

 

[ICRS]

What is this lighting for if you don't mind me asking?

Also I wonder why MH (Metal Halide) lights aren't used to light homes and such, they have an efficiency rating that is higher or as high as any fluorescents. I think they give a nicer light too.

 

[SparkyJJO]

Originally posted by: ICRS
What is this lighting for if you don't mind me asking?

Also I wonder why MH (Metal Halide) lights aren't used to light homes and such, they have an efficiency rating that is higher or as high as any fluorescents. I think they give a nicer light too.

MH is more expensive isn't it?