The cost of LED Lighting has dramatically decreased over the last few years and LED lighting is becoming increasingly popular in both commercial and domestic applications. Not only does LED lighting provide higher energy efficiency than both incandescent and fluorescent lighting technology, It also an important additional feature which is the ability to be dimmed. Although incandescent light bulbs can be dimmed easily using TRIAC-based dimmer circuits, the poor energy efficiency of incandescent light bulbs has led to their ban in many countries around the world and production has ceased in many places. Meanwhile, fluorescent lights are more energy-efficient but less so than LED lights and they cannot easily be dimmed. Consequently, despite the still higher cost of LED lighting, the ability to dim coupled with the lower energy cost of use makes them a viable economic alternative in many applications. Note however that many fluorescent and LED bulbs come with a warning label stating they cannot be used in combination with dimmers as they are unable to handle the fast voltage rise time associated with TRIAC-based voltage waveforms or the reduced voltage level that results from this.
Already many examples exist of dimmable LED lighting installations. Although the number of LED bulbs that are dimmable is still relatively low, their numbers are growing rapidly and prices are starting to come down due to increased production volumes. This in turn will result in more energy savings going forward.
Examples of energy savings can be found in industrial building lighting systems where the use of dimmable LED lights allows lighting levels during the day to be adjusted to environmental conditions such as dimming the lights during hours of bright sunlight outdoors and increasing lighting level in early morning hours, during periods of overcast or rain and at dusk. Another example can be parking lot or parking garage lighting where lightened levels can be dimmed considerably when there is no one present as determined by motion detectors and increased as needed to provide security when persons are detected. In both examples, energy and thus cost savings can be substantial over time, offsetting the cost of using higher-priced LED bulbs.
Traditionally, TRIAC based dimming of incandescent light bulbs has been used for many years in residential and commercial applications. The same TRIAC dimmers can be used to dim LED-based lighting systems if needed. This not only accomplishes appropriate lighting levels for the prevailing conditions, It also saves energy.
The various way to accomplish dimming of LED lights and test this type of equipment with dimming control functions during product research, development and production are explained in this application note. With these techniques and this type of test equipment, the use of dimmable LED lighting is bound to increase in popularity.
Currently available lighting products use one of two types of LED dimming technology to achieve dimming.
The first type is controlled by adjusting the AC phase voltage the input power source. This method uses traditional TRIAC dimmers in series with the LED bulb to control the on and off time of the AC voltage waveform, allowing for several dimming level segments. The advantage of this method is that you can use an existing light switch dimmer to control the ON/OFF times of the power switch.
The second type is controlled by adjusting the LED load current level.
This simplest dimming control available is the analog TRIAC Based dimmer used for dimming incandescent light bulbs which has been on the market for over 20 years and has found widespread use for dimming purposes. This device controls the average AC voltage level by notching a segment of the AC sinewave at varying phase angles, thus acting like a variable resistor which limits the current through the bulb
reducing its output light level. A suitably design LED driver can detect the AC input voltage phase change and based on this adjust the current through the LED bulb, thus achieving the same dimming effect as for an incandescent bulb as shown in Figure 1. Figure 2 shows the voltage phase control achieved by a TRIAC dimmer. The example shows the waveforms for no dimming, a 30% dimming level and a 70% dimming level respectively.
As mentioned, the second method for LED dimming is controlling the LED load current. By adjusting the current level through the LED, you can adjust the LED light output from 0% to 100%. This can be done by using Pulse Width Modulation (PWM) switching at frequencies that are in the 100's of Hertz. By adjusting the pulse width between 0% and 100%, the average current through the LED can be controlled to achieve the desired dimming operation. The pulse width modulation can be controlled using a remote control device that applies a 0-to-12 V dc signal to the PWM circuit to control the pulse width setting from 0% to 100% LED current or 100Hz to 1000Hz PWM pulse width. Because the human eye exhibits persistence of vision, modulation below 100Hz will results in noticeable light flicker as shown in Figure 3. The same techniques are used to dim the backlight intensity of TV's or LCD computer monitors. One of the advantages of using PWM-modulation- based dimming control is that the color of the display is not affected by the brightness level, making it the most suitable dimming technology for these applications.
If the lighting application requires the most linear lighting and color, using a PWM- based dimming driver is the most appropriate choice. If the application is sensitive to switching noise or requires maximum efficiency, the use of an analog dimming driver is more appropriate.
Prodigit provides dimming LED light test solution
1. Model 5302A AC power source can simulate leading edge TRIAC dimmer and Trailing edge dimmer.
2. Model 3341G series LED Electronic Load built-in provides independently isolated 0-12V and output signal of PWM dimming.
Since a traditional TRIAC dimmer requires an operator to move the dimmer knob or slider to change the resistance setting by setting the phase angle position, this method is slow and not automated. Instead, the use of a programmable AC power source with programmable phase angle control from 0 to 180 deg of both trailing and leading edge voltage transitions allows computer control via a remote interface. This setup can accomplish fast and consistent automatic testing and verification of LED dimming operation as shown in Figure 4.
The programmable AC power source also has a built-in, high precision AC power meter allowing the efficiency of the LED driver to be measured when testing using a Prodigit 3341G Series LED electronic load as shown in Figure 5 below. This setup can automatically calculate overall efficiency by measuring both AC input power and LED Load output ratios as a function of dimming level. The LED Load's dimming control output can be used to provide the 0 to 12 Vdc analog control signal or a 500Hz, 0% to 100% pulse width modulation output signal to control dimming levels of the PWM LED driver. The use of the 3341G which comes complete with this isolated control signal built-in, can be controlled by the same computer that controls the AC power source.