Russell Holton
(AVSIG Member)
12/15/17 06:20 PM
Re: Strobing Runway Lights


I would speculate that the Pulse Width Modulation (PWM) dimming system of the LED runway lights was phasing with my propeller, causing the observed effect.

Maybe. Doing the math, 3 blades x 2400 Rev/min x 1/60 min/sec = 120 cycles/sec. Exactly what you'd expect for a light powered by the 60Hz mains.

Yes, standard dimmers use a type of PWM for dimming, but I don't think that's the real issue. The problem is the LEDs "flicker" in time to the AC mains that the old incandescent (being a heat-based device) couldn't.

The problem here might be retrofitting dimable LED bulbs on a conventional circuit. I haven't looked to see how dimable 120V LED bulbs work, but I'd imagine they have to be responsive to the "PWM"/dimming of the conventional system to function properly. Simply rectifying and smoothing the incoming voltage is going to result in a bulb with really strange dimming characteristics. (Normal brightness until below 50% and then rapid dimming as the cycle drops.) As a result, a strong 120Hz strobe is quite likely.

An LED system designed from the ground up to be LED may run at a higher PWM speed and not cause that problem.

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