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I'm a layperson, not an engineer, so forgive the ignorance. I want to replace some dimmable 120V AC G8 xenon bulbs with LED bulbs, but so far three different LED bulbs I've tried, all advertised as "dimmable", flicker badly when the light fixture is switched to its "low" setting. I understand (more or less) that this flickering is the result of alternating current and the LED's binary nature, it's either on or off; and because it runs cool there's no "afterglow" of a hot filament to provide incandescence.

I've seen advertised some more expensive "flicker free" LED bulbs which I've yet to try. How do such LED bulbs solve the flickering issue? Does each bulb have a miniature onboard capacitor that keeps the current constant?

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  • $\begingroup$ I think the solution is to buy a different dimmer switch. I've seen some that claim to work with LEDs. $\endgroup$
    – Eric S
    May 7 at 16:10
  • $\begingroup$ there are two ways. flickering as you know (but maybe at even faster rates and an actual design to go with that frequency), and to map voltages into what is usually an extremely small voltage range. as the tech is changing, it still varies all over so you'll have to try it or get a set that is known to be compatible. $\endgroup$
    – Abel
    May 8 at 18:46
  • $\begingroup$ Are you using pure LEDs or are you using "LED bulbs" which feed the LED output to a phosphor, as niels' answer assumes? $\endgroup$ May 9 at 13:22
  • $\begingroup$ I don't know if a phosphor is involved since the sellers of these "flicker free" bulbs do not provide such technical details. $\endgroup$
    – Tim
    May 9 at 13:51
  • $\begingroup$ @Tim put the lights on full and switch off. How long does the glow remain? If it's enough to observe, there's almost certainly a phosphor involved. $\endgroup$ May 9 at 17:18

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Flicker can be minimized by choosing a phosphor in the design of the full-spectrum white LED that has a long persistence time. When you turn off one of these LED arrays, you can see that the light output does not instantaneously stop, but fades out exponentially.

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  • $\begingroup$ Well - put. I have some dimmable LEDs which take several seconds to go dark after being turned off. $\endgroup$ May 9 at 13:21
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I understand (more or less) that this flickering is the result of alternating current and the LED's binary nature, it's either on or off; and because it runs cool there's no "afterglow" of a hot filament to provide incandescence.

LEDs are not “binary” at all — the current can be freely adjusted (within the thermal power limits of the LED) to change the brightness. The problem with LEDs is entirely the second part: their light output is instantaneously proportional to the current, so unlike incandescent filaments, all of the “flicker” present in the input power appears as light.

Therefore, to avoid flicker, a LED driver (power supply) running on AC must include energy storage (usually in the form of capacitors or inductors) to maintain the same power input to the LEDs over the 1/60 or 1/50 second period of an AC cycle. If you see a LED bulb fade slowly when powered off, this is probably mostly due to those capacitors and not due to the phosphor. (Side note: Phosphor persistence would result in yellowish light, because white LED bulbs work by combining blue LEDs with a phosphor mix that converts some of the blue to other wavelengths that add up to yellow.)

Dimmer compatibility is another problem: conventional light dimmers work by cutting out an adjustable portion of the AC cycle (making it 0 volts instead of the usual waveform). This works poorly for “non-dimmable” LED bulbs because the output of their power supplies that drives the LEDs does not, like incandescent bulbs do, have a simple proportional(ish) relationship between the total input current and output current. Instead, there are various possible behaviors such as drawing a surge of power at the peaks of the AC waveform (simple rectifier-capacitor power supply) or shutting off entirely and “rebooting” every cycle (switch-mode power supply with insufficient storage capacitance).

Good dimming behavior comes not from simply throwing capacitors at the problem (which will make a bulb that can't be dimmed very much), but from power supply controllers that detect the incoming truncated waveform, derive the dimming % from that, and then produce a proportionally reduced output current to the LEDs. But that strategy then fails if the dimmer has an unusual output waveform the LED power supply isn't expecting.

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  • $\begingroup$ This is partly true. Nearly all LED bulbs - phosphor or not - are driven with a pulse stream. It could be rectified 60 Hz , or there can be a digital controller which adjusts the duty cycle within some base frequency to control the mean output brightness. $\endgroup$ May 10 at 11:18

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