[rowinofwin]

If you want to have a range of brightness you have a couple of options. One option is to vary the voltage you supply to the led, leading to a reduced brightness at lower voltages. The alternative is to use pulse width modulation, basically turning the led on and off fast so that it is on for a percentage of the time at full power.

Going with lower voltage is less efficient as the LED will output less light per voltage but not linearly, it will still use most of the power of full brightness at half.

Choosing PWM allows you to skip this problem by keeping the voltage identical but by using human persistence of vision to get the ideal number of photons to your eyes per your perception frame. The problem comes from making a slow PWM signal, say 60Hz, or having two similar but not identical PWM signals near each other, such as two different TV screens with a different backlight PWM frequency. That can make you see flashing because of the out of phase brightness peaks and troughs lining up.

[throwaway821909]

I've noticed a couple of LED bulbs (one with warnings that it's non-dimmable) where the PWM (presumably) means that even when I'm not looking for it, I'll move my hand quickly to grab something and it looks like I'm in a 20fps computer game, fun novelty but it wears off. Fortunately there are plenty where I don't see this.

[jbay808]

The most jarring thing is seeing the flicker when you move your eyes. If you scan your vision past a flickering light, you'll have multiple discrete images of that light left on your retina.

Strobing can also be dangerous around rotating machinery, like a lathe, because at certain RPM the machine will appear to not be spinning.

I believe the solution is to use a buck converter with enough inductance to ensure that the current to the LED is not interrupted.

[bodhi_mind]

I get this effect when using a dimmed led headlamp in the rain. I can see the rain drop fall in discrete steps.

[derkades]

> Going with lower voltage is less efficient as the LED will output less light per voltage but not linearly, it will still use most of the power of full brightness at half.

I don't think this is true? Aren't LEDs more efficient at lower voltages/currents?

[tomn]

You're right -- look at any LED datasheet and you'll see the efficiency get lower at higher currents.

The real reason for PWM dimming is simplicity, expanse and size.

Turning an LED on and off is a lot simpler than a real constant-current LED driver: indicator LEDs can be driven directly from a digital output on a microcontroller, or switched with just a mosfet. Constant current is always going to be more components.

In AC-powered applications, making an LED not flicker at 100/120Hz requires capacitance for energy storage, which are bulky and unreliable.

[szundi]

They consume less for sure but the light they emit decreases faster, thus per light the efficiency decreases.

This is because leds have a fixed voltage drop and just a bit above that your light efficiency is almost zero.

[andreareina]

The other way is to have a constant current source, this gives you much better control versus the voltage source.

[briHass]

I don't think this is the flickering the original poster is referring to. Dimmers are a bit of a nightmare with LEDs, however. I usually buy high quality (pricey) dimmer switches and good quality LEDs that are listed by the dimmer as compatible.

The flicker that bothers me, and maybe the GP, is likely due to cost cutting in the LED power supply. Not enough filtering or ability to handle the normal voltage dips in a home electrical system. As above, the solution is usually to spend more $ on bulbs, but not a guarantee.

[xxs]

Triac based dimmers + LEDs are quite a dumb combo, effectively the dimmer cuts the sine wave and the LED driver tries to determine how much it was cut.

[terramex]

Is there no way to manufacture more persistent phosphor for white LEDs?

[xxs]

The white ones are actually 'blue' - but all that is not needed, the LEDs are current driven devices and even if they get a bit more relative output flux with higher currents, it's close to negligible. Overall phosphorus is 'bad' for the LEDs, reducing luminosity, increasing temperature.

Also the PWM can be in the high 30KHz w/o much of switching sacrifice loses, not even dogs would react on it.