LED Lighting and Compatibility Issues with Legacy Dimming Systems
LED technology is being adopted by numerous businesses around the world. Its energy saving benefits, long lifespan and modern features have won over individuals who are looking for reliable lighting solutions. But in order for the transition to gain full acceptance by consumers and commercial sectors, LEDs have to be able to support legacy dimming systems and controls.
LED Dimming Definition and Standards
Dimming is a crucial aspect of industrial lighting systems. NEMA 64-2012 defines continuous dimming as “a lighting control strategy that varies the light output of a lighting system over a continuous range from full light output to a minimum light output without flickering in imperceptible steps” (NEMA 2013). It is used to set the mood in large facilities, including movie theaters, conference halls, restaurants and outdoor venues. Such capabilities may also help manage energy consumption rates by providing just the right amount of light needed for the moment.
To streamline the dimming standards of LED equipment, several lighting institutions, such as Osram, Schneider Electric and ZigBee Alliance (ZigBee Light Link), have developed protocols and guidelines to ensure consistency in the marketplace. A key publication that covers best practices for dimming solid-state lighting products is “Dimming: A Technology-neutral Definition” (Volume 12, Issue 1) by the Alliance for Solid-State Illumination Systems and Technologies (ASSIST). In the report, the group establishes an industry-wide definition of dimming products along with updated dimming profiles and performance criteria for various lamp types.
ASSIST sets forth the following recommendations and criteria for a lamp-dimmer combination:
• Dimming profile: No limitation on the detailed shape as long as the dimming profile is monotonic and meets the dead travel and range requirements.
• Minimum light level: No more than 5% of the maximum light output.
• Maximum light level: No less than 90% of full light output at rated voltage (i.e., not on a dimmer).
• Dead travel: No more than 10% of dimmer travel at the top and bottom, or middle of the dimmer setting.
• Flicker: For frequencies below 100 Hz, less than 10% flicker; for frequencies equal to or above 100 Hz, a calculated acceptability rating greater than zero, following the procedure put forward in ASSIST recommends...Flicker Parameters for Reducing Stroboscopic Effects from Solid-state Lighting Systems (2012).
• Audible noise: Audible noise is an important parameter to consider in the definition of dimmable products; however, at the time of writing, no specific measurement method or noise criterion has been established.
• System efficacy: To be measured at maximum output when operated on a compatible dimmer.
Constant Current Reduction (CCR) and Pulse Width Modulation (PWM)
The two most common LED dimming methods are constant current reduction (CCR or analog) and pulse width modulation (PWM). CCR dimming works by manipulating the drive current that powers the LED unit. The option is simple, straightforward and easy to implement. When a full current is in use, the light is on full brightness or output. But when the current is reduced, the light is dimmed based on the level of current in use. PWM dimming applies a different dimming strategy that involves toggling the LED at rapid rates. The intense and rapid pulse rates (usually around 200 Hz or greater) cannot be detected by the human eye, while slow pulse rates could result in an undesirable flickering effect. It is important to consider that such dimming systems require the use of a PWM controller and a MOSFET switch. Many establishments prefer this method due to the production of consistent lighting under variable settings.
Compatibility becomes an issue when LED dimming methods are paired with conventional dimming devices currently available in the market today. Such products were ultimately designed for traditional fixtures, such as incandescent lamps. The first type of phase control device that is commonly found in residential settings is the TRIAC dimmer (also known as two-wire forward phase dimmer, leading edge dimmer or SCR dimming). The legacy dimmer was traditionally designed for incandescent fixtures that use A19 and PAR type lamps. It is also considered to be the most widely used and installed dimmer on the market. The system works by cutting or removing part of the leading edge of the AC waveform. During the process, it detects the AC input’s zero-crossing and turns on the TRIAC switch to deliver the AC to the load (which now has a portion missing from the leading edge of each half sine wave). The dimmer does this while waiting for a variable delay period.
It’s important to note that TRIAC dimmers should not be used with electronic low voltage (ELV) drivers. Such applications could result in buzzing, flickering or radio frequency interference. Additionally, some customers have reported a faint humming noise during dimming. Such observations have been linked to the rapid cycle of expansion and contraction in the filament. For ELV applications, reverse phase dimming is applied (also known as trailing edge or two-wire reverse phase dimming). As the name suggests, this type of dimmer cuts a part of the trailing edge of each half sine wave, which is the opposite of TRIAC dimmers. This option is generally more expensive compared to forward phase dimmers and requires the use of a neutral wire.