In terms of LED characteristics, solid-state lighting can be smoothly dimmed, but in order to be compatible with traditional dimming technology, problems arise.
Dimming is often used in hotels and theatres to create an atmosphere, create an intimate feeling, and delight diners and spectators. For conference rooms and report halls, dimming also reduces energy consumption and space. Although dimming is used in a wide range of applications, there are still some problems with dimmers used in conjunction with conventional light sources, such as reducing the performance of incandescent lamps and damaging the life of fluorescent lamps.
Nadarajah Narendran, head of the Solid State Lighting Systems and Technology Alliance (ASSIST), director of the Lighting Research Center (LRC), said that most of the dimming systems installed today are phase controllers. Originally designed for incandescent lamps, they reduce the light output by cutting off the input current for each half-frequency cycle. The essence of dimming is that the phase controller temporarily cuts off the input voltage to reduce the power input to the light source. Because each cut-off of the input current occurs in an AC sine wave, they are also known as phase-cut dimmers.
The phase cut dimmer is turned off 120 times per second, or the number of cuts is equal to the frequency at which the alternating current transfers power to the power line. Since the tungsten filament of the incandescent lamp is heated and cooled at a relatively slow rate, the brightness of the light source seen by the human eye is lowered at a constant level. The longer the cut, the darker the light.
Not all phase controllers are switched off in the same direction as the AC sine wave. Triacs are used for dimming of incandescent and halogen lamps. They are switched off from the front phase of the sine wave. At the cut-off point, the input voltage just crosses zero and the input current is just reversed. Therefore, it is also called the pre-phase control dimmer, and the front-phase control dimmer will generate a current spike when it starts to conduct. This spike current will cause a buzzer and increase the pressure of the electronic driver.
The inverting control dimmer avoids the above problem because it cuts off the dimmer from the second half or the trailing edge of the AC sine wave. The inverting control dimmer turns on when the input current has just changed direction, allowing the voltage to gradually rise and then cutting it off during the next half cycle. Inverting control dimmers are also known as electronic low voltage (ELV) dimmers to improve the performance of halogen lamps that use electronic transformers.
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Figure: The triac (TRIAC) cuts off the power input to the source at the front phase, which occurs when the input voltage has just zeroed out and the input current has just changed direction. The inverting control dimmer (ELV) is switched off at the trailing edge of the AC sine wave.
How to dim LED?
As a constant current source, LEDs can be dimmed by nature. Narendran said: "The current flowing through the LED bead determines the output of the light." Their brightness can be adjusted very simply by controlling the current flow of the layer of semiconductor material attached to the substrate.
James Broderick, manager of lighting projects at the US Department of Energy's (DOE) Building Technology Office, said: LEDs are not like traditional light sources, and dimming does not affect the efficiency and longevity of LEDs. In fact, dimming can lower their operating temperature and thus extend the life of the LED.
In addition, LEDs have a wider dimming range than compact fluorescent lamps and high-intensity discharge lamps. 10%~30% dimming surplus compared to compact fluorescent lamps (data from NEMA), 30%~60% dimming surplus of high intensity discharge lamps (data from NLPIP) The dimming remaining of the LED can achieve less than 1% of the full load output.
Any LED device that requires a replacement light source or LED fixture requires a driver for dimming. Because LEDs are low voltage DC sources, LEDs require an electronic driver to convert the AC into a usable and adjustable DC current. These drivers are divided into two dimming methods. In Pulse Width Modulation (PWM), the current through the LED is turned on and off at very high frequencies, "usually thousands of times per second," says Narendran. "The current through the LED is equal to the average value of the current during the LED switching period." By reducing the power-on time of the LED, the average current or effective current can be reduced, thereby reducing the brightness of the LED.
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