dimming | COMPATIBILITY

Understand triac 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 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 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 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 - power supply feeding a most specifically, dimmers. trolled by one incandescent dimmer. constant current source (Fig. 2). The 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 at the 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 - 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 the safety of the combina- controlled dimmers need to synchronize State of regulatory affairs tion of dimmer and electronic ballasts, UL with the AC line in order to work correctly. Additional compatibility issues between has taken the systems approach by requir- Without the ability to sense the AC line and lamps and dimming devices often stem ing dimmer manufacturers to provide infor- its zero-crossings, a phase-controlled dim- from a lack of dimming performance stan- mation on the intended electronic load (i.e., mer would not detect the correct timing for dards within the lamp industry and how CFL, LED or electronic ballast) for each dim- switching the AC voltage, and it would lose dimming | COMPATIBILITY its ability to control and dim the lamp load. Wiring box wire solution for meeting all The end result is flickering and fluttering of Line Load the necessary requirements Dimmer the light output. control of today's and tomorrow’s Neutral lighting advancements, AC power Three-wire installations source Load but also remain backwards In three-wire installations (Fig. 4), you have Dimmer power and compatible to work effec- line, load and neutral in the electrical line sensing has its tively with all two-wire box. The line wire comes from the AC power own dedicated path solutions as well. source and supplies power for both the dim- FIG. 4. A three-wire circuit with a neutral wire is now mer and the load. The load wire is connected Universal dimmers required in switch boxes and improves dimmer operation. to the lamp load and provides a return path Early products designed to for the power delivered to the load. The third ular, and line synchronization becomes dif- offer compatibility for both wire, the neutral connection, provides the ficult. Similarly, the load current of LED two-and three-wire installations used an essential return path for the dimmer even lamps in their off state can be so small, that approach that did not derive the optimal when the load is disconnected or is in a state even obtaining a few milliamps to supply the performance from the lamps. The use of that doesn’t draw any current. internal dimmer circuitry can be challeng- basic, low-end trim adjustments and lim- The neutral is an important feature of ing. Without adequate supply and stable line ited programmability to address the differ- three-wire installations. It ensures that the synchronization, lamp flickering may result. ent loads resulted in less effective solutions. dimmer device has a direct connection to Advancements have been made, and device the AC power source regardless of the state Code changes manufacturers, such as Leviton, are offer- of the load. This third wire not only ensures In 2011, the National Electrical Code (NEC) ing newer options that better interoperate that the dimmer has power to drive its own added a requirement to new installations with and utilize the unique characteristics internal circuitry even when the load is dis- requiring a neutral wire in all switch boxes. of LED lamps. connected or off, it also provides a clean Although there was strong concern of the The responsibility for providing consum- signal of the incoming AC power source for additional construction costs, this require- ers with optimal lighting solutions falls to detection of zero-crossings and synchroni- ment was added after twelve years of debate both device and lamp manufacturers. Col- zation with the line. Both of these are essen- and effectively addresses advancements in laborative efforts are essential for not only tial to stable phase-controlled dimming, and lighting controls that require a neutral wire developing new, compatible devices that are easier to obtain in three-wire designs. for safe and effective operation. Many light- meet the changing needs, but to also pro- ing control devices require that the switch vide the necessary education to consumers, Two-wire installations be provided with standby voltage and cur- explaining how the inconsistencies between In two-wire devices and installations (Fig. rent at the switch in order to operate. current incandescent dimmers and energy- 5), only two wires are present in the electri- In the past, many electricians did not efficient lamps can affect their lighting cal box -- the line wire and the load wire. include a neutral conductor at switch loca- experience. In this case, the dimmer is simply placed in tions, and the result was the equipment Leviton collaborated with major lamp series between the line and the load. With grounding conductor being used as the manufacturers on joint testing of each com- only two wires, the dimmer must rely on neutral conductor. While the current on pany’s LED and CFL lamps for the release of the current passing through the load to the equipment grounding conductor is typ- the Leviton’s Universal Dimmers. As part of both power its own internal circuitry and ically less than 0.50 mA, the accumulation the formal product release for the Universal to detect zero-crossings for synchroniza- of many switches can result in an unaccept- Dimmers, lamp manufacturers participated tion with the AC line. able amount of current on the equipment in product evaluation consisting of dimmer When LED lamps perform poorly with a grounding conductors. With this change, performance with specific lamps. Measure- dimmer, often times, the blame is placed on gone are the days of using dead-end, three- ments included dimming range, pop-on volt- the dimmer circuit. But, more often than way switches and two-conductor switch age, drop-out voltage, appearance of flicker, not, the source of the problem really lies in loops. noise and in-rush current. how the LED load current Although the latest NEC requirements Understanding the variations among differs from the incandescent lamp in two- include the use of a neutral wire in all new manufacturers and lack of industry stan- wire applications. switch boxes, the vast majority of existing dards, Leviton developed its own internal If the load current is regular, as is the case installations, pre-2011, likely do not have a grading scale to rate lamp performance – for incandescent lamps, then stable line syn- neutral wire present. Understanding this, it a practice that other device manufacturers chronization and ample power for the dim- should be assumed that in all likelihood a are also adopting. These company-estab- mer’s internal circuitry are both easy to home’s system is a two-wire system. There- lished standards are subjective based on obtain. With LED lamps, however, the load fore, dimming products introduced into the specific products, and the variances may current is much smaller and much less reg- market must be able to offer not only a three- pose a problem for designers, installers and dimming | COMPATIBILITY consumers. As discussed in prior sections, enable dimmer functionality. The less power For LED loads in two-wire applica- now more than ever, the establishment of these ICs consume, the more compatibility tions, the simple zero-crossing technique industry-wide performance standards is the dimmer control circuit will have with is sometimes not enough. Fortunately, imperative for the lighting advances in microcontroller technology and controls industry. Wiring box have made putting advanced synchroniza- Rigorous testing proto- tion algorithms and approaches within the Line Load cols, however, help ensure Dimmer grasp of cost-sensitive designs with tight control that Leviton can continue power constraints. Utilizing a low-power to address the market microcontroller in the dimmer, advanced AC power demand for energy efficient source Load software can be set against the difficul- lighting with sophisticated Dimmer power and ties of measuring the AC line cycle with line sensing share load path lighting controls that are and depend on load current the irregular line currents present in two- compatible with LED and wire LED load applications. CFL lamps while remain- ing backwards compatible FIG. 5. Legacy installations with two wires makes Lessons learned with incandescent lamps. dimmer operations with LED lamps problematic. As energy efficient lamps continue to pen- etrate the lighting market, the availabil- Dimmer control design LED loads in two-wire configurations. ity of new lighting controls that meet the Leviton’s design team used several tech- Another important part of a universal specific needs of these lamps is increasing. niques to ensure that its dimmers work design is the utilization of advanced line Consumers can take fuller advantage of all with a broad range of lamps. For example, synchronizing techniques that resist the of the benefits of the newer, more energy effi- the dimmers minimize the supply current effects of irregular load currents. In older cient lamps. requirement of the control circuit. Having dimming devices, line synchronization is Designers, installers and consumers a dimmer control circuit that requires less accomplished using simple zero-crossing should become familiar with the nuances supply current means greater compatibil- detection. This method uses a circuit to of these energy efficient lamps and educate ity with lower load current applications, detect when the AC line voltage switches themselves on how they will interact with such as is typical with LED lamps. The new- polarity and crosses the zero voltage point. specific dimming controllers to ensure the est designs employ a micro-power circuit The zero-crossing method still is used in maximum benefits of energy savings and design, utilizing low-power control ICs and older designs because it requires simple cir- ambiance are achieved. modern, low-power microcontrollers. These cuitry, it is practical and it works for incan- are the essential internal components that descent lamps.

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