May 2014
I've seen replacement LED light bulbs in the hardware stores that claim they are 'dimmable.' Is this a real feature that has a different internal design or is it just a way to get me to pay extra for an LED bulb? If there is a difference, what are they doing circuit-wise to make them dimmable?
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The type of dimmer switch and it’s minimum/maximum load range will indicate the compatibility with LED light bulbs.
The problem with LEDs is that their operating voltage is way below the 120VAC in the home. So you either string a whole bunch of LEDs in series to get up to the 120VAC (which is 165V peak) drop, or you put a resistor in the circuit (and waste tons of power as heat), or you need a step down voltage converter. It is the step down voltage converter that has problems with a typical dimmer circuit. The common triac based dimmer gives a badly distorted AC waveform that wreaks havoc on inductor based step down voltage regulators.
LED light bulbs are dimmable just like incandescent bulbs.
Fluorescent bulbs require a minimum voltage so as to be able to ionize the mercury vapor in the bulb. Without the ionization no current can flow and the bulb won't generate any UV light. It is the UV light that causes the coating of the bulb to fluoresce. So a special circuit is required to make a dimmable fluorescent bulb.
An LED will start to output light as soon as the minimum avalanche voltage is exceeded. The amount of light that is output is then determined by the current flow. The more current the brighter the light output. The current flow determines how hot the junction gets. When you exceed the maximum rated temperature the junction will melt and the LED will fail.
An incandescent bulb is a positive coefficient resistor. The hotter the filament the higher the resistance so the current is self limiting. You have to increase the voltage to increase the current flow and to increase the light output. When you exceed the maximum rated voltage of the bulb the filament will melt and the bulb will fail.
Have you ever noticed that most incandescent bulbs fail when you turn them on? That is because of the inrush current which overheats and melts the filament at the weak point. Sometimes you can fix an incandescent bulb for a while by lightly bumping the side of the bulb. The filament will weld back together, but it is always a weak joint that will fail again in a very short time.
So just like incandescent bulbs, LED bulbs should be dimmable whether or not the package says so. I may be wrong and I am going by my training and knowledge and not by empirical testing.
