7
2.3.3 Interaction with Two-Wire Dimmers
Three-wire phase-control dimmers generally support advanced features and perform over a wide
range of external conditions better than their two-wire counterparts, due to the presence of a neutral wire
connection. LED sources amplify the challenges that two-wire dimmers must overcome. LED loads can
create series impedances which appear significantly different from the familiar resistive incandescent
behavior, leading to erratic dimming performance. This can also effectively limit the maximum number of
LED sources that should be used on the dimmer.
LED loads in the off state may not pass enough current to maintain proper operation of the dimmer.
This is most problematic for dimmers with advanced features, and can lead to dimmer malfunction or
even inoperability. Some LED loads in the off state can accept low levels of current, but behave
undesirably at higher levels. In some instances, the LED driver may initiate operation that it cannot
sustain, leading to strobing or erratic flashing behavior. Or, the LED driver may actually initiate and
maintain a low-level of operation, leading to “ghosting”, or the appearance of being on when it is
supposed to be off. Finally, some LED loads do not draw enough current over the duration of each half-
cycle to keep some two-wire dimmer circuit elements functioning properly, again leading to erratic
behavior. While this issue was significant in the early days of LEDs, it has been largely addressed in the
market; newer dimmers can operate at lower currents, and newer LED sources can avoid erratic behavior
by drawing extra current at critical timing points, sometimes at the expense of LED source efficacy.
2.3.4 Continuous product evolution
LEDs are still relatively new to the architectural lighting market, and innovation is still occurring at
all LED component levels. Product evolution is rapid and product generation life is short. Newer products
with more sophisticated LED drivers typically perform better with a wider range of older dimmers. This
continuous evolution can lead to its own problems, however. For example, an older generation LED
source may only work well with a small number of ELV dimmers, while a newer model works well with
a larger number of forward-phase dimmers, but no longer works well with ELV dimmers.
2.3.5 The NEMA SSL-7A Standard
A number of challenges must be overcome to successfully dim LEDs with phase-cut dimmers,
especially with two-wire configurations. The installed base of phase-cut dimmers in the U.S. is too varied
to be easily characterized, so it is a considerable challenge for an LED source manufacturer to design a
driver that works well with all these devices. Recognizing this as a growing barrier to LED adoption,
NEMA members decided that the best way to ensure some level of predictable LED source performance
would be to effectively reduce the variation over which LED sources and phase-cut dimmers must
operate, and put limits on some design variables that can lead to undesirable behavior. In late 2011,
NEMA formed the SSL-7 committee, which led to the 2013 publication of SSL-7A, “Phase Cut Dimming
for Solid State Lighting: Basic Compatibility”. NEMA SSL-7A contains a set of design specifications that
aims to ensure that one or more compliant LED sources (or, more specifically, LED light engines) will
work well with compliant dimmers. LED Light Engines (referred to in the specification as LLEs) are
defined as a combination of one or more LED modules and LED control gear (integral or remote)
designed for an AC mains circuit connection (i.e. one or more LED modules and a driver). LLEs describe
a wide range of lighting products, from screw-in integrated lamps to luminaires containing separate LED