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One more reason for dimmable LED bulb flicker

I had already explained why a LED bulb itself can be the source for flicker at low power levels. There is another source: the dimmer.

triac_dimmer

The component that allows us easy dimming of devices consuming much power is called a "triac". The circuit above is a very simple dimmer, the blue line shows how the current flows through triac and lamp.

Pin 3 is where a triac is initiated, switched on. It can not be switched off there, only switched on. The only way to switch a triac off is by disconnecting the supply voltage. This is why you seldom see triacs in DC applications, mostly in AC devices.

triac_sine_wave

In AC, the voltage will rise and fall again, passing through a "zero crossing", where the voltage is zero. The image above shows those as "ZC".

So how can a triac reduce power consumption? It is switched on a while after the zero crossing, when there already is a certain voltage. It stays switched on until the next zero crossing. In the image on the right, the triac waits until the grey area has passed, then it is switched on. The energy in the grey area is not used now. The later the triac is switched on, the less energy a device can consume.

I must correct something: During the zero crossing, it is not the lack of voltage that switches the triac off, it is the lack of current. Every triac has a so-called "holding current". If the current through the triac goes below this threshold, the triac will switch itself off. And here again our energy consciousness is causing technical trouble:
Suppose we want to replace two 60W incandescent bulbs. At 220V, they may have consumed 550mA. Now we use two 8W LED bulbs instead, which may consume only 75mA, a more than sevenfold reduction. If we reduce the power on these LED bulbs to 10%, we may end up with only 7.5mA current.

So what happens then? The current going through the triac is lower than its holding current, it switches itself off. The capacitors in the bulbs are drained, so they are charged with the next wave and the triac stays on, only to switch off again when the current became too small. The result is flicker, again.

A few years back, Philips had a list on their website with recommended dimmers for their LED bulbs. If you went through the list you would find that they were all 250W or 125W dimmers, for very little power. The reason is simple: A smaller maximum current usually also means a smaller holding current.

A good way to avoid this problem is to supply LED light sources with a constant stable voltage and use an interface to tell the light source what output power to use, something we do in our Regulus smart luminaires.