Does PWM follow the AC voltage or is it timed within each bulb?

[lumen aeternum]

Wondering if putting several LED light bulbs in series would result in synchronized PWM or if they would each have their own timing -- which would reduce the perception of PWM.

 

[Lexel]

If there are no electronics involved the lights follow all synchronized if you ignore the speed of light

 

[hellokitty[hk]]

Plug in one of them reverse polarity, then they'll be opposite synced.

 

[ssanasisredna]

If you mean AC light bulbs, they do not have "PWM" in the sense that I think you are thinking. They have ripple in the output in sync with the AC line.

 

[lumen aeternum]

I mean CFL or LED lights that you screw into an AC light bulb socket. They have circuitry to alter the 120v AC.

 

[ssanasisredna]

I mean CFL or LED lights that you screw into an AC light bulb socket. They have circuitry to alter the 120v AC.

The "pwm" that is inside CFL/LED bulbs is at a frequency well beyond what is visually noticeable to virtually any person. What they also have is flicker/ripple in the output that is almost perfectly in sync with the AC line (2x frequency). It's a byproduct of the topology of their power supplies and the amount of capacitance they can fit inside the bulb.

 

[ShineOnYouCrazyDiamond]

Plug in one of them reverse polarity, then they'll be opposite synced.

Actually I think you would get no light at all if you did this. You'd essentially have a reverse diode in the circuit which would block current flow in one direction with the other LEDs blocking current flow in the other direction.

The worst of exhibit of the AC line ripple form of PWM are Christmas lights. The real cheap ones with half-wave rectifiers flicker at a very visible 60Hz and a 50% duty cycle. The slightly better ones with a full-wave rectifier filcker at 120Hz, but at least they have a 100% duty cycle. (For those who are very nitpicky I do realize it's not quite 50% and 100%, and closer to 33% and 67% RMS duty-cycle because the AC line is a sine-wave and not a square-wave.) The newer ones I've seen out so far this year do seem to be producing much smoother light.

 

[ssanasisredna]

Wondering if putting several LED light bulbs in series would result in synchronized PWM or if they would each have their own timing -- which would reduce the perception of PWM.

To the original question, anything from 1/2,1/3,1/4 brightness (doubt any more than 3 or 4 would work), to uncontrolled flickering, to not working at all. Would all depend on the bulb, power supply architecture (if there is one), what protections are built in if any, whether it is dimmable, etc.

2x line frequency flicker would still be there.

 

[ShineOnYouCrazyDiamond]

For the sake of argument, I suppose you could use an AC phase shift circuit on each bulb in the circuit to shift the phase 45, 90, 135 and 180 degrees and then you'd end up bulbs with their own timing.

 

[MattPete]

I mean CFL or LED lights that you screw into an AC light bulb socket. They have circuitry to alter the 120v AC.

Led lights that you screw in, at minimum, have a rectifier that turns the AC to DC. No pulse width modulation is involved.

At minimum, it a single diode that chops off the negative part of the wave form (half-wave rectifier):

Even better is a full-wave rectifier (four diodes), but that will give you DC with considerable ripple (see 2nd image below). To get rid of the ripple, you add a smoothing capacitor (bottom image).

As for dimming...I don't know how the dimable ones do it.

 

[ssanasisredna]

In 2nd/3rd countries, that capacitor is on the AC side, the bulb is normally non-dimmable, but it may be. This is assuming a traditional driver type.

1st world with a traditional driver, cap on the input is small, and the big one is on the output and its impossible to make it small enough for no flicker. Additional circuitry, another stage, etc. may be added to remove flicker and that is getting more common.

With AC driver, it's rare they have filtering caps, but some designs have it.

Dimming --- variety of methods from measuring the phase angle of the dimmer to the equivalent analog voltage and using that to control output current ... again traditional. AC drivers have some inherent dimming capability and may enhance is as above.