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Why dimmable LED bulbs often flicker (at low power)

We take a lot of things for granted, especially in areas we are familiar with, but do not understand very well. One such case may be flicker on dimmable LED bulbs. My incandescent bulb did not flicker, so why this modern one?

There are two possible sources for this problem, I would like to show the one inside the LED bulb. Incandescent bulbs are basically just a resistor, a wire resistor, and resistors do what they can best (resisting) down to zero volt in a very smooth way.

LED_driver_circuit

LED bulbs are different, because they include driver electronics, they are electronic devices. You can see one in the above image. This is not the whole schematic of a bulb, but the part that is important to understand the problem.

LED_driver_block

This is a TS19450 by Taiwan Semiconductor, but other driver ICs work basically the same way: A pretty high voltage of sometimes up to 600V is connected to the pin Vin. As you can see in this image, there is a voltage regulator inside the chip, because the IC needs a stable operating voltage (VDD). This one requires about 7.5V at VDD.

So what happens when you use a triac dimmer to dim this bulb down? The voltage at Vin goes down. The IC needs at least 7V, the regulator needs a few more volt to operate, so when we get to about 10V at Vin, the IC can not operate properly any longer. It will switch itself off, because there is not enough voltage, then switch on again, because the capacitors had been charged a little again, then switch off again - the bulb will flicker.

Turn the dimmer down a little more and the bulb goes off completely, earlier than an incandescent bulb. This is because we are reducing the operating voltage for an electronic device, not for a resistor. A solution to this problem would be to add an interface to the circuit, always run it at optimal voltage, and use commands on the interface to regulate the power output - as we do in our Regulus smart luminaires.

Some manufacturers design the circuit so that the bulb will switch itself off completely before it can flicker. Look up datasheets or flyers for such bulbs, they will often advertise a dimming range of "10-100%". So don't be surprised if a dimmable LED bulb flickers at very low power, that is very normal.