Plainsaw: LED research

Phase control devices temporarily shut off power to the light source and dim voltage. Also called phase cut dimmers because the interruptions in current create cuts in the AC sine wave.

Interruptions occur at a rate of 120 times per second, or twice the frequency at which alternating current delivers electricity over power lines
Because the tungsten filament in incandescent lamps is slow to heat up and cool down, the human eye sees the output as a constant level of decreased brightness.
The longer the interruptions, the dimmer the light.

TRIAC: triode semiconductor for alternating current, used to dim incandescent and halogen lamps, cuts from the forward phase, which begins just as the current changes polarity and the voltage running through the circuit is zero. Also referred to as forward-phase control dimmers
ELV: electronic low-voltage dimmers (reverse-phase control dimmers) were developed to enhance the performance of halogens that use electronic transformers. Cut from the latter portions, or trailing edges, of the AC waveform. By switching the light circuit on just as the current changes direction, they allow the voltage to rise gradually before turning it off later in the half-cycle

All LED devices require a driver in order to dim. Because they’re low-voltage, direct current (DC) sources, LEDs need drive electronics to convert the alternating current that flows through power lines into a usable and regulated direct current form. Two ways:

PWM: pulse-width modulation - the current sent through an LED is switched on and off at a high frequency. Reducing the amount of time that the LED is on decreases the time-averaged current, or the effective current, delivered to the device and, as a result, its brightness

More widely used PWM offers a broad dimming range, can decrease light output to values of less than 1 percent
Avoids color shift by operating the LED at its rated current level—or its maximum light output—and at zero current.
Because PWM dimming involves rapid switching, it requires sophisticated and expensive drive electronics to produce the current pulses at a frequency high enough to prevent perceptible flicker.

CCR: constant current reduction, or analog dimming. CCR maintains a continuous current to the source, but it reduces its amplitude to achieve dimming

CCR dimming is more efficient and simple to implement because of its less complex and less expensive electronic requirements
Unlike PWM, it does not have the potential to generate electromagnetic interference, which can result from high-frequency switching
CCR dimming allows drivers to be located remotely from the light source
CCR is not suitable for applications where dimming light levels below 10 percent is desired. At very low currents, LEDs do not perform as well and the light output can be erratic

The LEDs used predominantly in general illumination architectural applications are “Phosphor Converted Blue” LEDs - blue LEDs that have a yellow phosphor placed over the LED. The phosphor absorbs some of the blue light, and emits light in the yellow, green, and red portions of the spectrum. When the unabsorbed blue light mixes with the other colors of light it creates what your eye perceives as “white” light. The choice of blue LED and yellow phosphor, and the method of placing the phosphor, creates hundreds of colors of “white” due to differences in color temperature and CRI (Color Rendering Index).
LED chips are inherently low-voltage devices that need additional electronic components to convert the line-voltage 120/220/277V power to the low voltage needed for the LEDs. These electronics may also interpret control signals and dim the LEDs accordingly. These devices are referred to as LED drivers

Constant voltage drivers (12 V or 24 V) provide a fixed constant voltage to LED modules connected in parallel. Constant voltage drivers can only be dimmed via a PWM (see definition above) method
Constant current drivers provide a constant current, such as 700mA, to a specific LED module that is designed to operate at that current level. This is great for a down light, sconce or other LED fixtures that use only one LED module per driver (much like a fluorescent lamp with its associated ballast). For constant current drivers, there are two different dimming methods: PWM and CCR

Measured light: output in foot-candles
Perceived light: the amount of light that your eye interprets as a result of pupil dilation. The eye’s pupil dilates at lower light levels, causing the amount of light to be perceived higher than measured
The equation for determining perceived light is the square root of the measured light percentage

It is important to understand that the dimming range of any LED product is based solely on the driver

Different drivers may produce different dimming curves, even if they can dim to the same low-end light level.
No guarantee that relative light levels will match between LED products from different manufacturers, even at the same dimmer level

Remote mounting of a driver (mounting the driver external to the LED fixture, from ten to hundreds of feet away) could result in potential voltage drops, power losses, or noise susceptibility on the DC wiring that, if unaccounted for, will be reflected in unstable or lower-than-expected light output from the LED module in the fixture - is this something we should clarify in documentation?
Dimmer loads

While the LED lamp may only draw 15W continuously, it may have a start-up inrush current or repetitive peak current during every half-cycle that makes it appear much worse, acting like a higher wattage load
The 15W LED lamp can cause the same stresses on the dimmer as a 60W incandescent load
Lutron has observed that some LED loads can act as the electrical equivalent of up to a 100W incandescent, even for LED loads that are less than 20W

MLV (magnetic low-voltage: 2-wire forward phase control, typically used for incandescent and replacement LED lamps
ELV (electronic low-voltage): 2-wire reverse phase control; can be used for replacement LED
3-wire forward phase: typically used for flourescent; can be used well with LED with 3-wire control input
0-10V 4-wire: what we use for remote installs
DALI (Digitally Addressable Lighting Interface): digital control of fixtures and other building components
DMX512: typically theater lighting control. Allows control of color and temperature of lamps
RF: radio frequency control