Restaurants and theaters have long employed dimming as a way to create atmosphere, foster a sense of intimacy, and transport diners and audiences alike. Dimming can reduce energy consumption and enhance a space’s function, as in the case of a seminar room or lecture hall. But despite their ubiquity, dimmers used with conventional sources can still have problems, including a reduction in efficacy for incandescent lamps, and a reduction in longevity for fluorescent lamps.
The majority of dimming systems installed today are phase control devices. Designed originally for incandescent lamps, they reduce light output by “interrupting the current during each AC [alternating current] half-cycle,” says Nadarajah Narendran, director of research at the Lighting Research Center (LRC), in Troy, N.Y., and program organizer of the Alliance for Solid-State Illumination Systems and Technologies (ASSIST) program. Essentially, phase control devices temporarily shut off power to the light source and dim voltage. In fact, they’re also called phase cut dimmers because the interruptions in current create cuts in the AC sine wave.
The interruptions occur at a rate of 120 times per second, or twice the frequency at which alternating current delivers electricity over power lines. But 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.
Not all phase control devices cut from the same part of the AC sine wave. A triode semiconductor for alternating current (TRIAC), which is 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, TRIACs can produce spikes in current that cause dimmed lamps to buzz and add stress to electronic drivers.
Reverse-phase control dimmers avoid these problems by cutting 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. Also called electronic low-voltage (ELV) dimmers, reverse-phase control dimmers were developed to enhance the performance of halogens that use electronic transformers.
How LEDs Dim
As a constant-current source, an LED is inherently dimmable. “The amount of current flowing through an LED device determines the light output,” Narendran says. Their level of brightness is adjusted simply by controlling the current passing through the stacked layers of semiconductor material mounted on a substrate.
Unlike conventional sources, dimming does not affect the efficacy or longevity of LEDs, says James Brodrick, lighting program manager in the Building Technologies Office at the U.S. Department of Energy (DOE). In fact, dimming can extend the lifespan of LEDs by lowering their operating temperature.
Moreover, the dimming range of LEDs is broader than that of compact fluorescent and high-intensity discharge lamps. They can turn down to less than 1 percent of full output, compared to 10 to 30 percent of measured light output for compact fluorescents, according to the National Electrical Manufacturers Association (NEMA), and 30 to 60 percent of lamp power for high-intensity discharge lamps, according to the National Lighting Product Information Program.
All LED devices, be they replacement lamps or LED luminaires, 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. These drivers dim LEDs in one of two ways. In pulse-width modulation (PWM), the current sent through an LED is switched on and off at a high frequency—“often several thousand times per second,” Narendran says. “The current flow through the LED is the time-averaged value of the current when the LED is on and when it is off.” 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.
LEDs, as well as conventional sources, can also be dimmed through constant current reduction (CCR), or analog dimming. CCR maintains a continuous current to the source, but it reduces its amplitude to achieve dimming. “The light output is proportional to the amount of current flowing through the device,” Narendran says.
Both PWM and CCR strategies have their advantages and drawbacks. The more widely used PWM offers a broad dimming range, can decrease light output to values of “less than 1 percent,” Narendran says, and avoids color shift by operating the LED at its rated current level—or its maximum light output—and at zero current. However, 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 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 also allows drivers to be located remotely from the light source, which is helpful in the case of LED replacement lamps or in smaller fixtures where space is an issue. However, 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,” Narendran says.
Compatibility Issues
Although the driver dictates whether an LED product will dim, the driver’s performance largely depends on its compatibility with the dimming device, such as a phase control device. The driver must be designed to understand and interpret the signaling by the dimming device in order for dimming to occur.
Many of the dimming technologies used for conventional sources can also work with LEDs. These include zero-to-10V analog, DALI (Digital Addressable Lighting Interface), DMX (Digital Multiplex), and “other techniques that separate the dimming signal from AC Mains voltage,” Brodrick says.
Installing dedicated wiring that relays dimming information to the dimming device can alleviate compatibility issues because it enables the dimmer and source to operate with little or no interference from each other. However, these types of dimming systems also tend to be more complex and expensive, which may explain why they are more common in commercial applications than in residential.
The most common phase control device is the TRIAC dimmer. NEMA estimates that there are 150 million of these installed in U.S. homes, and that these legacy devices will represent the bulk of dimming devices for replacement LED fixtures as incandescent sources are phased out. Unfortunately, the compatibility of LEDs with TRIAC dimmers is problematic.
One reason for this stems from the difference in how incandescent lamps and LEDs are powered. Incandescents produce light through simple resistive loads that draw electricity directly from the AC grid. The relationship between current, voltage, and brightness is linear and straightforward. A change in the voltage affects the current proportionally.
Not so for LEDs. Because the diodes rely on drive circuitry to ensure constant current and to adapt power and voltage for their use, their interactions with TRIAC dimmers are less predictable. At low dimming levels, for example, an LED driver designed to supply constant current or constant voltage may try to compensate for the phase cut portions—or interruptions in the AC sine wave—by drawing in more current, causing the LED to stay bright or to flicker.
Moreover, not all drivers are built alike. Different circuitry means different ways of drawing power, converting it, and outputting it. Consequently, pairing a TRIAC with an LED product can be “hit or miss,” Narendran says. Also, “one lamp on a single dimmer might work but when several lamps are added in parallel—like in a chandelier—it may not dim well.”
The opposite can be true, says Jan Kemeling, founder and chief sales and marketing officer for Ledzworld, a Dutch manufacturer of LED lighting products. He advises against mixing different LED lamps on the same dimmer because of the variety of driver designs.
The wiring for a TRIAC dimmer further exacerbates matters. Many existing and installed dimmers are two-wire devices; that is, the same wire that provides power to the light source also conveys the dimmed voltage, or dimming signal. This can interfere with the functioning of both the LED device and the dimmer, Brodrick says. Dimmers, particularly those with additional features such as nightlights and light level indicators, have internal circuitry that require constant, albeit minimal, power even when the light source is turned off. With incandescents, this can be done without triggering illumination of the lamps. Because LEDs don’t require much to power up, this is a little trickier for those devices, which may also flicker, says Michael Skurla, senior product and market manager, Americas, Indoor Global Systems, Philips Lighting Systems.
The incompatibility between LED drivers and TRIAC dimmers can cause a host of problems. Six such problems are: pop-on, in which the LED source suddenly turns completely on as the dimmer switch is raised from fully off; drop-out, in which the light source shuts off completely as it is dimmed; dead travel, which occurs when changing the dimmer setting produces no visible shift in the light level; ghosting, where light is still visible when the dimmer switch is fully off; audible noise; and flicker.
Flicker, dimming, and color shift are some of the outstanding performance issues that may prompt professional and consumer wariness toward solid-state technology. However, the lighting industry is addressing the issue of dimming on multiple fronts. Released last year, NEMA SSL 7A-2013 Phase Cut Dimming for Solid State Lighting: Basic Compatibility seeks to minimize compatibility issues relating to LED phase cut dimming by providing design and testing guidelines for both dimmers and LED products. However, the standard only addresses future technologies and does not attempt to regulate past dimming and lighting devices.
Dimming the Right Way
The lighting industry has also developed protocols to bring uniformity to the marketplace. Ledotron is an open digital standard launched in Europe that aims to stabilize dimming performance in systems designed for CFL and LED lamps. The nascent standard results from collaborations between several European manufacturers, including Osram and Schneider Electric.
In North America, the ZigBee Alliance’s ZigBee Light Link is a standard for wireless dimming and control of LED products. Created for consumer convenience, Light Link certification ensures that lighting and light control products have plug-and-play functionality and interoperability; those that qualify bear the Zigbee Certified seal.
LRC’s 2013 publication ASSIST Recommends … Dimming: A Technology-Neutral Definition suggests performance criteria for dimming, regardless of lamp type, to ensure end-user visual comfort and satisfaction. It sets minimum and maximum light levels (5 percent and 90 percent, respectively), evaluates dimming profiles, and covers issues such as dead travel, flicker, and system efficacy.
In practice, LED dimming problems can be minimized by taking certain precautions. First and foremost, designers should specify dimming control devices that are designed for LEDs. Look for LED source and dimmer combinations that are recommended by the manufacturer of either product, or both. For wall-box installations, Brodrick advises selecting a NEMA SSL-7A-compliant dimmer and LED sources.
Designers should also perform a full mock-up of all lighting circuits, “including all LED sources and dimming controls, and test over the full dimming range.” If a mock-up is not possible, specify a proven LED source and dimmer combination, but make sure the information is no more than six months old.
When using LEDs with phase control dimmers, designers should decrease the maximum load rating of the dimmer, usually given in watts, to minimize stress to dimmer electronics. Although LEDs are considerably more efficient than their incandescent counterparts, determining the number of LED sources that can be connected to a dimmer is not as simple as dividing its maximum load rating by watts per source.
Instead, a decrease is needed to accommodate small spikes in power caused by driver functioning. “Typical de-rating percentages should be in the range of 25 to 30 percent of the dimmer-rated power,” Ledzworld’s Kemeling says. A dimmer with a maximum load rating of 1,000W would, therefore, be de-rated to 250W. This could then be used to calculate the maximum number of sources that the dimmer could accommodate.
Narendran says that manufacturers are also working to enhance circuitry in both LED drivers and dimming devices for better compatibility with TRIACs. Some drivers incorporate adaptive control processing, Kemeling says. This allows drivers to synchronize with any type of dimmer, but they do cost more. So while advancements in dimming have been made, optimal performance still requires a little more time and energy.
Note: This article has been updated since first publication to indicate that phase control devices reduce light output by interrupting the current 120 times per second, or twice the frequency at which alternating current is delivered in the U.S.
Resources
A list of introductory articles that discuss the process and common issues related to dimming in LEDs.
“Controlling LEDs,” by Lutron Electronics Co., 2011. Available at bit.ly/1kFPBlt.
“The Subtle Circuitry Behind LED Lighting,” by Bernie Weir, IEEE Spectrum, Feb. 27, 2012. Available at bit.ly/1nZxsEs.
“Dimming LEDs with Phase-Cut Dimmers: The Specifier’s Process for Maximizing Success,” by Naomi Miller and Michael Poplawski, Pacific Northwest National Laboratory, 2013. Available at 1.usa.gov/1g3cGfs.
ASSIST Recommends … Dimming: A Technology-Neutral Definition, by the Alliance for Solid-State Illumination Systems and Technologies and the Lighting Research Center, April 2013. Available at bit.ly/1fMM8EA.
LED Lighting Facts, by the U.S. Department of Energy, 2014. Available at lightingfacts.com.
