How do you choose dimmable LED light bulbs?

LED light bulbs isn't resistive load, LEDs are current-driven which means that the intensity of the light they generate depends on the amount of electric current flowing them. Nothing dimmable LED Light bulbs can suitable for all kinds dimmer which use for incandescent bulb. Because incandescent bulb is resistive load, The dimmer for incandescent bulbs is using different way to adjust the voltage. So when you choose dimmable led light bulbs, Be sure know that what kind dimmer are used in your lamp fixture. If you choose led light bulbs isn't for old dimmable lamp fixture replacement, I recommed that you buy led light bulbs with remote controller, Almost all remote controller is dimmable.

1. Triac dimmer, SCR dimmer:
TRIACs or SCRs are used to dim lamps by use of "phase control", This means you turn on the lamp during a certain phase of the AC sine, then back off when at the zero-crossing point. The output wave see below graphical

This type of dimmer is called a "Forward phase controlled" dimmer. The phase control dimmer has been the dominant dimming technology since the 1960's. Nearly all dimmer commonly mounted in the wall or for desk lamp. The triac dimmer can work perfectly with the incandescent light bulb, but not LED light bulbs or CFL lamp. Triac dimmer cannot drive LED directly, since LED is not pure resistive load. It needs a LED driver to drive LED. And most of LED driver cannot accept Triac output to adjust LED brightness. So for Triac dimmer, The led bulbs must be sepcial designed, When you choose led light bulbs, Be sure in the description said that it is for Triac dimmer.

2.IGBT Dimmer:
IGBTs (Insulated Gate Bipolar Transistors) have the ability of being turned off anytime during the phase (TRIACs and SCRs·can only be turned off during the zero-crossing of the waveform). Therefore, IGBT Dimmer is complicated, It can be phase controlled dimmer, sine wave dimmer.
a.)FPC IGBT Dimmer and RFC IGBT Dimmer, IGBT Phase controlled dimmer (advanced types of phase control dimmers):
All of Triac dimmer have one thing in common: the phase control waveform is far from ideal for dimming them. In many cases the load will perform poorly; in extreme cases the load (and possibly the dimmer) will be damaged. In the last decade, more advanced types of phase control dimmers have been produced using a type of power semi-conductor called an IGBT.
IGBT Phase controlled dimmer can be "Forward phase controlled" dimmer call as FPC IGBT Dimmer and "Reverse phase controlled" dimmer call as RFC IGBT Dimmer.

The output wave of "Forward phase controlled" dimmer is same as TRIAC dimmer. See graphical as below:

"reverse phase control" greatly reduces the EMI of the circuit, along with quieting the light bulb.· This approach has just recently been used commercially. Using IGBT drivers allows the dimmer to handle up to 500 W. By using IGBTs to control the lights, the dimming can start on the leading edge of the power cycle. This allows for a smooth turn on/off, thereby eliminating the rapid rise times that produce noise. This design eliminates costly noise filtering and extends the life of the lights by reducing thermal shock.

Reverse phase-controlled dimming of an a.c.-powered load is accomplished by switching power to the load on and off during each half cycle. In one embodiment of the present invention, power is switched on when line voltage is substantially equal to load voltage and off at a selected time later in the half cycle or at any time that the voltage across the switch exceeds a predetermined value. Below is a graphical representation of Forward and the benefits of Reverse Phase Control.

Some of these new dimmers operate in reverse phase control where the dimmer is turned on at the line zero-crossing point, then turned off at the desired output voltage. The bulky choke is eliminated in favor of a controlled turn-off by the IGBT. These systems still suffer from many of the same problems as any phase control dimmer. Reverse phase control dimming applies the voltage to the load at zero volts, and turns it off once the desired target voltage has been reached. This mode of operation is fine for a tungsten (resistive) or electronic low-voltage transformer (capacitive), but is unusable with inductive loads. Neon, ballasted loads, fans and small motors will generate de-structive inductive kickback energy when dimmed by a reverse-phase control dimmer. RPC dimmers must not be used with these loads, or must switch to forward-phase control to dim these loads. In all cases, FPC or RPC, the dimmers are producing triplen harmonics. So for IGBT Phase controlled dimmer, The led bulbs must be sepcial designed, When you choose led light bulbs, Be sure in the description said that it is for FPC IGBT Dimmer or RFC IGBT Dimmer.

IGBT Dimming Technology Features:
Fully Digital Dimming technology with Forward / Reverse phase dimming (automatic or user-selectable)
Compatible with new LED drivers that require reverse phase operation
No dimmer room required and no added cooling needed
Energy efficient - only a 2.5 volt (or less) insertion loss (depends on model)
Solid-state dimming technology with adjustable transition (rise) time up to 1,000 ms for quiet operation
Low Harm mode to reduce neutral harmonics
Automatic Overload Short Circuit protection
Products available for 120V, 277V, or 230V/240V applications

b.)Sine wave dimmer:
Sine wave Dimmer produce high frequency pulse width modulated control waveforms to the dimmer's IGBT power switches. The 47Khz carrier frequency is divided into 255 steps, providing a voltage resolution on the output of less than 0.5%, resulting in a flawless Sinewave output. The PWM waveforms cannot be directly used to control the load. They must be filtered to produce a Sinewave. So for
Sine wave dimmer, The led bulbs must be sepcial designed, When you choose led light bulbs, Be sure in the description said that it is for Sine wave dimmer. Below is a graphical representation of Sine wave Dimmer.


3. PWM Dimmer (Also call as DC dimmer):

PWM ( Pulse Width Modulation ) : A far superior method of dimming LEDs is to use Pulse Width Modulation (PWM). With PWM strings of LED bulbs can all be driven with the recommended forward current, with the dimming achieved by turning the LEDs on and off at high frequency - so fast the human eye cannot see the strobing effect. The longer the on periods are relative to the off periods, the brighter the LEDs will appear to the observer. Below is a graphical representation of PWM Dimmer. Duty Cycle is a percentage measure of the time that the LED is physically on. If, for example, the LED cycles ON for 9/1000 of a second, and then OFF for 1/1000 of a second, the duty cycle is 90%: 90% of the time it is ON, and 10% of the time it is OFF. Therefore, the intensity of the light will be approximately 90% of its undimmed level.

PWM dimming is good only for constant voltage LED light bulbs, Because when PWM output high level, The voltage of PWM dimmable LED light bulbs is same as his operation voltage. It will not work with LED light bulbs that with internal driver inside (mostly LED driver use for control the current of LED).
PWM dimmer is low voltage operation dimmer, It can be DC12V (Operation on DC12~14V), DC24V (operation on DC24~28V), or DC12V-DC24V dimmer. It is used in vehicles of carsm trucks, RVs,Boats, Marine and Aircrafts operate numerous panel lights and convenience lights. So for PWM Dimmer, The led bulbs must be sepcial designed, When you choose led light bulbs, Be sure in the description said that it is for PWM Dimmer or not driver inside bulb and choose it is for DC12V or DC24V Dimmer.