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Comment by londons_explore | Hacker News Reader

I actually used this math at home! - one of my LED lamps was flickering in cold weather, and upon opening it up I saw a primary side controlled flyback transformer to act as a constant current power supply (for ~20 LED's in series).

The flicker was due to oscillation, due to a pole too close to the right half plane. I was about to stick a bigger capacitor in the feedback compensation circuit, but a bit of maths told me that that would make the problem worse (and probably blowing up my LED's) - so instead I used a smaller resistor value, and yay - perfect flicker free light!

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I noticed outdoor LED lamps flickering too, but only at dusk, and for what I thought was a feedback issue. These lamps are solar powered and turn on at night.

The cool part is that at twilight, when the light turned on, the little bit of reflected light bounced back to the photovoltaics, adding just enough light to turn the light off, which brought the system full circle. Amazingly, this behavior only occured for about 1 minute each day.

The normal solution to this is hysteresis. In most cases, it doesn't cost any extra to add hysteresis.

It does if you pay for a competent engineer in the first place who would know to do things like debounce switches and add hysteresis.

Thermostats would "flicker" this too if they didn't incorporate hysteresis. Hysteresis means the threshold at which the system transitions from OFF to ON is different from the threshold from ON to OFF.

A bizarre effect that I've noticed on LED flashlights that I own is as follows: The flashlights contain multiple LED elements, and as the battery depletes, some of the LEDs will flicker, but not all of the LEDs will flicker at the same rate! That is, LED A might flicker at half/twice/? the rate of LED B...

This is probably not related to Laplace Transforms, but I thought it was an interesting phenomena...

It probably is related to laplace transforms... Pretty much anything oscillating can be analyzed with a laplace transform.

I'd guess in your case it's because each LED has it's own constant current power supply, and when the battery voltage gets low, the power supply becomes unstable, but the exact flicker rate depends on small variations in the components it's made from.

What was the link to cold weather?

Aluminum electrolytic capacitors have higher ESR at colder temperatures, and lower ESR at higher temperatures. That is my guess at what went wrong. They generally perform better at higher temperatures (within reason), although a particular circuit may require the ESR to be within a certain range (don’t go randomly swapping out your capacitors for low-ESR capacitors, it may make the circuit unstable or destroy something).

Yes - the circuit had electrolytics on both the input and output side, and for my analysis I assumed they were both perfect, zero ESR capacitors, which is clearly an over-simplification...

When I did the stability analysis, it was marginally stable (ie. poles on the axis). I would guess whoever designed this lamp fitting used the wrong formula or didn't understand stability analysis.

By chance, it was mostly stable when warm (presumably because some resistance or capacitance slightly changed to push the pole to the left half plane), but when cold it flashed on and off at about 2Hz.

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