Monday, 25 April 2011

Dimming Led

Standard dimming controls 

Typical home incandescent lamp dimmers are fundamentally electronic switches that toggle on and off 120 times per second, by delaying the beginning of each half-cycle of AC power (known as phase control), they regulate the amount of power to the lamp filament. Because this happens so fast, most people do not see the flicker, but they see continuous dimming. Although the general operation of such electronic dimmers is much the same, the specific electrical characteristic of home dimmers can vary considerably. These are immaterial to incandescent bulbs, but is significant when used with electronic devices such as CFL and LEDs.

Dimming CFLs
Some screw CFLs can be dimmed using line-voltage incandescent dimmers but must be manufactured to do so. They dim only to about 20 percent of maximum intensity, due to limitations of the low-cost ballast. More complicated electronic ballasts providing continuous dimming below 5% are available too, but they have a too big purchase cost for use in screw CFLs. Some lamps use successfully pin-based CFLs in combination with on-board dimming controls. Four-pin CFLs using separate dimming ballasts can be dimmed via line voltage or 0-10 volt DC controller, with dimming range as low as 1 percent, but more found with 5 or 20 percent.

Will LEDs solve the dimming problem?

Dimming LED is still a challenge similar to that of CFLs: their electronic parts are often incompatible with dimmers designed for incandescent bulbs. A LED driver connected to a line-voltage incandescent dimmer don’t get enough power to operate at lower dimming led levels or it may be damaged by current spikes. Some LED products can be used with line-voltage incandescent dimmers, but the dimmer and the LED driver electronics must be carefully selected. Because of variability in installed dimmers, it is impossible to guarantee that a LED fixture will work with all dimmers available. Some LED light bulb manufacturers publish lists of specific dimmer products tested and approved for use with their fixtures.

Complicated LED dimmers use low-voltage controls (either variable resistors or 0-10 volt DC control) connected separately to the electronic driver. Full AC power is given to the driver enabling the electronic controls to function at all times, thus allowing LEDs to be uniformly dimmed (usually down to 5% or lower). However, they may need additional low-voltage wiring for retrofit applications.

Flicker and dimming

Most of the LED drivers use pulse width modulation (PWM) to regulate the amount of power to the LEDs (dimming led). This method turns the LEDs on and off at high frequency, varying the total on time to make perceived dimming. Driver output frequency ought to be at least 120 Hertz (Hz) to keep away from perceptible flicker under normal circumstances. LED lighting fixtures may appear to flicker at the lowest settings, but only when the dimmer control is moved. This is due to the finite resolution of the digital electronics. Good-quality drivers feature 12-bit or a higher resolution to receive flicker-free operation throughout their dimming range.

Dimming LEDs with PWM

The simplest way to dim an LED array is to decrease the forward current. There's several effective ways to accomplish this including variable resistors or a voltage regulator with a variable programmable output. It is important to recognize the problem associated with dimming LED truck lights in such a way. The issue ultimately begins with the LEDs manufacturing method. LED manufactures are unable to produce the same LED lights. Therefore, the LEDS manufacture will check & separate dissimilar LED lamps basic on more characteristics. This is named as binning. Numerous LEDs within a specific bin have similar, but not identical characteristics. Some appear slightly brighter, while others have slightly different color. The voltage threshold at which the LED light will start to illuminate can also vary slightly. When driving an LED light at the recommended forward current rating, these variations ought to never become apparent. However, driving at unusually low forward currents will cause or more dissimilarities to become obvious. As a result, dimming LED truck lights by basically varying drive current may cause some LEDs to appear as a different color, while other may basically cease illuminating entirely.

Pulse Width Modulation

Since changing the forward drive current to accomplish LED dimming may show problematic, an alternative process is necessary. Pulse width modulation, or PWM, can effectively control the pulse width and duty cycle causing the LED light to vary its intensity. To produce an increased dimming effect, the LED will stay off longer time. In lieu, the eye fundamentally will detect the LED as a continuous light stream, but the light appears dimmer due to the short periods in the coursework of discontinued operation. To produce the appropriate frequency and pulse width, the LED drive circuitry requires some kind of programmable timer.

Lots of older PWM LED dimming circuits utilized the 555 led dimmer timer, and controlled the output pulse width using a simple variable resistor. This process works sufficiently. However, recent advanced in microcontrollers have created new opportunities for PWM LED dimming circuits. The microcontroller is custom programmable, and provides greater application flexibility when compared with traditional 555 circuits. In addition, lots of new microcontrollers actually need fewer outside parts than the 555 integrated circuit.

Changes in color and efficacy with dimming

When an incandescent lamp is dimmed, the filament temperature decreases, causing the emitted light to appear warmer, changing from white to yellow to orange/red. The luminous efficacy of the lamp also decreases: a 15 lm/W lamp at full power will be ten lm/W at 50% dimmed. CFL color temperature does not change with dimming as dramatically as with incandescent, walking counter to our expectation of significantly warmer color at low light levels. Luminous efficacy of fluorescent sources stays about constant with dimming until about 40%-50%; thereafter it decreases, but not as steeply as with incandescent lamps.

 Most white LEDs are actually blue LEDs with a phosphor coating that generates warm or icy white light. Their light does not shift to red when dimmed; some may well appear bluer with dimming. White light may even be made by mixing red, green, and blue (RGB) LEDs, allowing a full range of color mixing and color temperature modification. Overall LED luminaire efficacy decreases with dimming due to reduced driver efficiency at low dimming levels.

Future developments of dimming led

As LED lighting becomes more common for household applications, fully integrated LED dimming controls are becoming a reality in new construction. Meanwhile, LED products will need to be designed to make use of dimmers that were made for incandescent products, requiring manufacturers to indicate compatibility with specific dimmers. This will also continue to be necessary for retrofit products intended for existing homes, given the expense and trouble of changing out installed dimmers.
In summary, dimming LEDs will be limited in the near term by the installed stock of dimmers, which were designed for use with incandescent lamps. In the longer term, new design options are likely to emerge that greatly improve the dimming function of LEDs.

Are there dimmable Led bulbs?

This DIMMABLE LED light bulb can be dimmed with an ordinary dimmer switch and is designed to replace incandescent light bulbs used in the home with a light output approximately equivalent to a 50-60w incandescent (GLS) lamp. The diffuser cap of a dimmable led bulb is designed to provide a reasonably wide dispersal of the available light. The dimmed LED lamp is suited very well to uplighting applications including table lamps and standard lamps.  In a pendant fitting, the majority of the light output is naturally cast downwards.

Dimming led solutions:

Diagram of Infared Remote Control LED Dimmer
Diagram of 0/1-10V LED Dimming System

Diagram of DALI LED Dimming System

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