Multiple LED bulbs on one circuit - is the flicker synchronized?

[lumen aeternum]

If the flicker timing is random for each bulb, then would not an array of bulbs in a lighting fixuture usually average out to not much flicker? But sometimes they would randomly align & give you a "flash" effect ?

 

[hamhanded]

In this hypothetical situation, what's randomizing the timing?

If we're taking about multiple cheap LED bulbs that are driven straight off the mains with no true rectification (as in, they flicker at 60hz), then for all practical purposes the length of wiring between them doesn't introduce enough delay to phase shift them so that the flicker becomes noticeably offset. They'd all flash at practically the same period, so I don't think you'd be able to detect a beat frequency: while it's still technically there, the period and amplitude of the beat would be so long and weak respectively that your eye wouldn't see it.

 

[lumen aeternum]

I thought flicker was caused by the individual circuitry in each LED lamp unit. Nothing to do with the incoming voltage that the circuit turns into 5v or whatever

 

[hamhanded]

If you're referring to PWM in the driver causing flicker, then yes, they could beat like you describe, if multiple separate drivers (each with their own PWM) are running separate LEDs.

In a home fixture with a dimmer you are more likely to have the PWM being applied to the input to the bulbs' drivers rather than the output. If that's the case all of the drivers are seeing the same pulses. What they do with it in the way of filtering is up to their design.

 

[lumen aeternum]

So is a solution to bothersome flicker to have several independantly circuited LEDs ?

 

[LEDphile]

So is a solution to bothersome flicker to have several independantly circuited LEDs ?

Nope. If the flicker is line-synchronized, you will see some improvement by distributing the lamps across multiple hot legs (which requires multiple circuits, but multiple circuits does not guarantee multiple hot legs). But the better solution is to not buy bulbs that flicker in the first place.

 

[Dave_H]

From my opening up ordinary line-powered LED bulbs, I increasingly find series LED connection with 1-2 high-voltage low-current linear current regulators. This dispenses with the switching part so saves a number of components including large inductor and a lot of EMI filter components. For 120vac, LED vf's are stacked up to about 130-140v total. This means dual or triple LEDs per package (maybe quads?) to keep total number of LEDs realistic e.g. 16 triples (16 x 3 x 3v approx. = 144v). If bulb can be dimmed down you might be able to see individual LEDs (or use magnifier when off).

Consequence is that LEDs are driven by full-wave rectified line voltage, usually without large smoothing capacitor (though I saw one, in one case). Therefore flicker would be similar for all bulbs.

Dc voltage of switching-type bulbs up to 9W or so is typically high, 50-100v with single series string; in some cases two parallel strings of lower voltage.

There's an intriguing multi-substring type of linear driver which sequentially turns on/off 3-4 sub-strings of LEDs, following the ac waveform. It can reduce overall voltage overhead dynamically, and keep power loss manageable. I am not sure what type of flicker these create but would be neat to capture this on high-speed video. This one would be synchronized among bulbs.

Dave

 

[Dave_H]

But the better solution is to not buy bulbs that flicker in the first place.

Agreed. One quick way to check flicker is to wave your fingers quickly close to the light ("how many fingers do I have?"). I don't think it's usually all/nothing for flicker, just better/worse.

Dave

 

[LEDphile]

There's an intriguing multi-substring type of linear driver which sequentially turns on/off 3-4 sub-strings of LEDs, following the ac waveform. It can reduce overall voltage overhead dynamically, and keep power loss manageable. I am not sure what type of flicker these create but would be neat to capture this on high-speed video. This one would be synchronized among bulbs.

That's generally called a "tapped linear driver", and the flicker is very similar to a simple rectified string of LEDs without any filter cap.

 

[PhotonWrangler]

If you had three sub-strings of LEDs and each was fed by one phase of a three phase power connection, they would appear to chase in sync with the rotation of the generator at the power company. Not terribly useful and there are easier ways to get LEDs to chase, but conceptually it would be cool. Especially of the overall string of sub-strings was wrapped in a circle. You would be able to "see" the generator spinning.