dimming | COMPATIBILITY Understand triac dimmer issues to ensure compatibility The lighting industry must overcome differences in lamp load, lack of standards, and legacy wiring problems to optimally pair a phase-cut dimmer with an LED lamp write MARC GALLO and MICHAEL NEARY. nergy efficiency is driving consum- or automated dimming controls over stan- Varying loads, mixed results ers to replace standard incandes- dard light switches because dimmed light- Although there are applications where LED E cent lamps with LED-based retrofit ing can reduce energy use and offer ambi- lamps will operate with an incandescent lamps. Unfortunately, they’re often finding ance. The problem, however, is that nearly dimmer, in general, an incandescent dim- that the performance they’ve come to expect all dimmers found in homes today were mer will provide inconsistent performance for years isn’t being achieved – at least when designed for standard incandescent lamps. with SSL. A major issue is in the design of the solid-state lighting (SSL) products are When using an energy-efficient bulb, the each lamp type. The incandescent lamp by used with existing triac or phase-cut dim- homeowner generally expects an experience nature represents a simple resistive load mers. Let’s consider the dimmer compatibil- similar to what incandescent lamps pro- with a linear response to the dimmer set ity problem and the requirements of a uni- vide. Although some LED lamps are marked point (Fig. 1). Standard incandescent dim- versally-compatible dimmer. as compatible with incandescent dimmers, mers work particularly well with this type of We will discuss three main causes for the there are various degrees of what can be load by switching on at an adjustable phase compatibility issues encountered with dim- defined as “compatible.” Dimmable LED angle after the start of each alternating cur- mers. Legacy dimmers are not designed for lamps tend to interact quite differently when rent half-cycle, thereby altering the voltage LED loads. There are no industry standards used with these legacy devices. A number of waveform applied to lamps. By switching that guide performance requirements. And the existing residential wiring infrastruc- Supply current ture can limit the capabilities of modern Input voltage lighting controls. Supply Applied line current Current state of lighting voltage controls and lamps Today, there are more than four billion incandescent lamps in US homes according to the US Department of Energy (DOE). But FIG. 1. A simple resistor models an incandescent lamp. The input voltage and supply recent estimates by Philips Lighting predict current to such a load are linearly related. that LED lighting will capture as much as 50 percent of the consumer market by 2015. While LED and compact fluorescent (CFL) undesirable results may occur when you use instead of absorbing part of the voltage sup- lamps are drawing market share from incan- a dimmable LED lamp with an incandescent plied, minimal power is wasted, and dim- descent lamps due to energy and cost sav- dimmer, including: ming can occur almost instantaneously. ings benefits, a problem arises as consum- • Reduced dimming range In contrast, LED lamp loads can vary ers still need further education about how • Flickering or fluttering of the lamp greatly across different manufacturers and these new lamps will work with their exist- • Inconsistent performance based on the designs. But most can be characterized by ing incandescent lighting control devices – number and assortment of lamps being con- a diode-capacitor power supply feeding a most specifically, dimmers. trolled by one incandescent dimmer. constant current source (Fig. 2). The diodes Many consumers have turned to dimmers Let’s dig deeper into the main causes for rectify the applied AC voltage, allowing it to the compatibility issues, and discuss the charge the storage capacitor, while the LED MICHAEL NEARY is a Product Line Manager solutions the industry is currently offering. elements draw a constant current from the and MARC GALLO, Ph.D., P.E. is a Senior The solutions must meet the current tech- power supply that is related to the desired Electrical Engineer for Leviton’s Residential nological needs while being mindful of both dimming level and brightness. Lighting Controls business. past and future technological challenges. What makes this type of load significantly Reprinted with revisions to format, from the June 2012 edition of LEDs MAGAZINE Copyright 2012 by PennWell Corporation dimming | COMPATIBILITY different from incandescent lamps is the each uniquely corresponds with LED driv- mer. UL listing investigation will involve the non-linear relationship in which the applied ers – leading to a wide-range of inconsis- use of the specified electronic ballasts or a voltage and the current flowing into the load tencies among products. This lack of stan- synthetic load exhibiting the same in-rush are related. In incandescent lamps, as Fig 1 dardization can be seen not only in varying and steady-state characteristics in the over- shows, the applied voltage across the load characteristics between manufacturers, but load, endurance and temperature tests. and the resulting current flowing through also by product within some manufactur- the load are related linearly by Ohms Law ers’ product lines. Complications arise from Legacy wiring and synchronization (V = IR). In this case, the resistance sets the the fact that any given lamp can require a A third major issue that further exasper- scale, and the current waveform follows the set of electrical and electronic characteris- ates the inconsistency issues among dim- voltage waveform, differing only by scale. tics – current, voltage, amperage and con- mers and lamps is that most of the existing trol signals – that residential wiring infrastructure was built Supply current are vastly different without a neutral wire at the switch box. The from any other lamp. absence of the neutral wire is referred to as LED current While one lamp may two-wire lighting control while the inclusion Applied line depends on LED be able to be dimmed of a neutral at the switch box is referred to voltage brightness/dimming level by a particular dim- as three-wire lighting control. The need of ming device, others supporting two different wiring scenarios cannot. poses certain challenges that lighting-con- Efforts to estab- trol designers need to account for in plan- FIG. 2. The model of a typical LED lamp is significantly more lish a performance ning to control a broader range of lamp complex than that of an incandescent. standard for solid- types with a single dimmer. state dimming con- Some dimmers are designed to work with In LED loads, the applied voltage and trols have been guided by members of the one type or the other only, while some are resulting current flow are not related by National Electrical Manufacturers Associa- designed to work in both types of installa- a simple linear relationship. In the diode- tion (NEMA) and other lighting committees. tions. But, for all dimmers, even those that are capacitor power supply model of the LED NEMA, for example, has developed a stan- designed for both two-and three-wire instal- lamp, current flows from the applied voltage dard titled SSL 6-20104 which provides guid- lations, there are significant differences in to the load only when the magnitude of the ance to LED lamp manufacturers for suit- performance between these two installations applied voltage exceeds the stored voltage on able operation on standard, incandescent, in terms of how the dimmer circuitry is pow- the power supply capacitor. The stored volt- phase-cut dimmers. However, currently the ered and how the dimmer synchronizes with age on the power supply capacitor, in turn, NEMA standard does not provide a rating the line voltage. When used to drive incandes- depends on the current drawn by the LED scale or detail on what is considered com- elements themselves, which is a function of patible. While serving as a starting point, Input voltage the LED brightness. these standards are still incomplete and Therefore, the current flowing from the require additional amendments to ensure Supply current supply to the lamp depends both on the complete consistency among dimming instantaneous value of the input AC voltage devices and lamps. waveform and the brightness of the LED Under current UL standards, notably UL lamp. Changing the intensity or dimming 14725, intended to regulate the safety of level of the LED lamp affects where in the AC dimmers, an LED lamp is categorized as line cycle the load begins to draw current. an “electronic ballast.” One notable issue FIG. 3. The supply current to an LED lamp This inflection point also affects the amount addressed by UL 1472 is in-rush current is not linearly related to the voltage. of current that surges into the lamp. The which is generated at the start up of many relationship between these current peaks, LED lamp loads. High in-rush current can cent lamp loads, these differences are mostly and the times in each line cycle where these result in failure of switch contacts, which is negligible. But, when used to drive LED loads, current peaks occur, depends non-linearly a safety hazard in many field applications – they present significant challenges to stable on the lamp design, the LED brightness and such as dimmers – where the switch is serv- dimming and lighting control. the set dimming level. ing as the disconnect means. Regardless of the circuit type, all phase- To evaluate