In the lighting industry, the people often have a misunderstanding for LED light dimming that compared to other light sources (fluorescent, metal halide and sodium, etc.).the dimming LED relatively easier. But the reality is the application of LED light dimming technology Is not satisfactory for engineering .why is this? To answer these questions, we must first understand the volt ampere characteristic of LED.then to help readers to understand and master the LED light dimming and its application.
So from the volt-ampere characteristic of LED, we can know that LED light dimming can’t simply by reducing the input voltage and input current of LED to achieve, in addition to LED of a sine wave waveform is different from the incandescent lamp, therefore can not simple obtained by changing the conduction angle,so as to achieve changes in effective value (effective dimming) purposes.
LED Triac Dimming:
Ordinary incandescent and halogen lamps usually adopts SCR dimming. Because incandescent and halogen lamps is a pure resistance device, it does not require the input voltage is a sine wave, because it is always the same between the current waveform and voltage waveforms, so no matter how sine wave voltage waveform deviation, as long as the change of the input voltage, you can be dimming(adjust the light).However triac dimming to adjust the LED light source to produce unexpected problems, that is, the LC filter input will make oscillation with SCR. this oscillation for incandescent lamps is insignificant(lamp makes the eyes do not see this oscillation), but to drive power will be the production of LED audio noise and flicker. non sinusoidal waveform will produce serious interference signals in the circuit (EMI) power network pollution, serious will make the grid paralysis.
Hereto, you might ask: ” to reduce the voltage or current and the SCR dimming methods are not suitable for LED light dimming, then in what way is the most appropriate?.”
PWM (pulse width modulation) dimming can be a good way to solve the above problem, because LED is a diode, it can achieve fast switching, switching speed which allows may reach microsecond above, is unmatched by any light-emitting device. Therefore, as long as the power source into a constant current pulse, with the method of changing the pulse width,it is possible to change the brightness, this method is called the pulse width modulation (PWM) dimming method.This is dimming switch like a up to microsecond or switch gate, because the gate switch frequency soon,we are unable to recognize their state of the switch with the naked eye the result is that we can only pass much water downstream to recognition speed its switching frequency. In addition, as the gates flow changes the duty cycle of the output (effective flow rate), without changing the instantaneous pressure and the instantaneous water flow rate, so that the gates of microseconds up to a switching operation will not affect more than the work of flow, because the moment instant pressure and instantaneous flow does not change, change is the electric power generation of loader. and so on PWM (pulse width modulation) dimming method does not change the input instantaneous voltage and instantaneous current of LED PN junction, to change the duty cycle of the output current, thereby changing the brightness.
Thus, LED PWM (Pulse Width Modulation) dimming method has the advantages as following:
1, does not have any LED chromatography offset, because the LED is always working at full amplitude between current and 0V.
2, there is a very high dimming accuracy, because the pulse waveform can be controlled to a very high accuracy, so it is easy to achieve 1/10000 accuracy.
3, even, to dimming in a wide range, also won’t happen the flicker phenomenon. Because does not change the constant current source condition (step-up ratio or step-down ratio), the more impossible to geted the overheating problems.
4, can be with the digital (DALI / DSI / DMX 512) control techniques of combining, because the digital control signals are easily converted into a PWM signal.
Although LED PWM (Pulse Width Modulation) dimming has many advantages, but need to pay attention to the following two questions:
1,A pulse frequency of choice, because the LED is in a fast switching state, if the operating frequency is low, the human eye will feel flashes. To take full advantage of the human visual residual phenomenon, it should be higher than the operating frequency of 100Hz, preferably 200Hz.The heavy rain was the cause of the flood.
2, The dimming was the cause of whistling, although the human eye can not detect more than 200Hz, but they are all up to 20kHz range of human hearing. At this time it is possible to hear the slightest sound. There are two ways to solve this problem, one of the switching frequency up to 20kHz or more, out of the range of human hearing. Another way is to find sound devices and to be deal with.
the control signals are used DALI (digital Addressable lighting interface) technology, combined with digital lighting control system, fully digital LED control product line. have been very good to solve the above problems, Most important feature of DALI (Digital Addressable Lighting Interface) technology is a single lamp with separate address, or any single lamp lights set precise dimming and switch control via DALI system software,on whether those in strong electric lamps or different loop circuit lamps. Other words DALI system software has strong individually addressable for the led which same or a different electrical circuit loop.enabling individual control and arbitrary grouping. This idea brings great flexibility for lighting control,The user was delighted to be make the lighting solutions designed to meet their needs as required, even after the end of the installation,it’s can still modify any control requirements.Now, the DALI interface is not only limited to light adjustable fluorescent lamp ballast,electronic transformer tungsten halogen lamp, gas discharge lamp electronic ballast, LED uses the DALI interface dimming;control equipment also includes: a radio receiver, a relay switch input interface. All kinds of button control panel, including LED display panel has a DALI interface, which will make DALI applications more and more widely, the controller to expand from an office minimum to multi room building, expanding from a single store to the Stars Hotel.
Summary:
Although at this stage of the LED dimming applications was chaotic, and there are still some problems and obstacles, but there is no denying the fact that LED dimming prospects are bright. As long as we maintain a cautious attitude, develop science and psychology responsible LED dimming technology, to LED dimming technology promotion, to make good use of the LED dimming technology, then the LED dimming products bring us benefits, to bring the benefits of lighting industry, to bring the benefits of human beings in the not too distant future.