LED Dimming: What You Need to Know

Why Dim LED Sources?

Dimming LED sources offers multiple benefits beyond simple ambiance control:

• Additional energy savings beyond what's achieved by switching to LED technology
• Increased visual task performance by adjusting light levels to specific needs
• Enhanced ambience and space flexibility
• Fewer light sources to specify, maintain, and stock
• Demand response load shedding capabilities
• Potentially improved efficacy and lifetime of the light source

Dimming Challenges

Despite the benefits, dimming LED sources presents several significant challenges:

• Wide variation in LED source and dimmer characteristics
• Little can be assumed based on historical practices with incandescent lighting
• Not all claims are equal due to lack of standard testing procedures
• Difficult to predict performance without actual testing
• Circuit-level interactions between LED drivers and dimmers create compatibility issues

LED Dimming Basics

Understanding these fundamental concepts is crucial for successful LED dimming implementations:

LEDs vs. Incandescent Sources

LED Drivers

LEDs require drivers to:

• Convert AC voltage to regulated DC current
• Compensate for manufacturing variations in LED forward voltage (Vf)
• Implement dimming functionality (CCR or PWM)
• Protect LEDs from overcurrent and overtemperature conditions

Dimming Technologies

The report categorizes dimming technologies into two main approaches:

1. Coincident AC Power and Control Signal

• Phase-cut AC sine wave (forward or reverse phase)
  • 2-Wire (hot, dimmed hot)
  • 3-Wire (hot, dimmed hot, neutral)
• Reduced amplitude AC sine-wave

2. Separate AC Power and Control Signal

• Fluorescent 3-Wire
• 0-10V
• DALI (Digital Addressable Lighting Interface)
• DMX512
• PWM (Pulse Width Modulation)

Dimming Methods

LEDs can be dimmed using two primary methods, each with distinct characteristics:

Constant Current Reduction (CCR/Analog)

• Method: Varying LED current while keeping LED always on
• Advantages:
  • Longer LED lifetime (lower current and temperature)
  • No noise generation
  • Potentially higher efficacy at lower dimming levels
  • Does not create flicker
• Disadvantages:
  • Possible objectionable color shift
  • More difficult regulation at deep dimming levels

Pulse Width Modulation (PWM)

• Method: Maintaining same LED current but varying on/off times
• Advantages:
  • Longer LED lifetime (less on time, lower temperature)
  • Good dimming regulation at deep dimming levels
  • No color shift
• Disadvantages:
  • Potential noise generation
  • PWM frequency is critical to avoid undesirable flicker
  • Minimum dimming level limitations

Flicker Issues

Flicker refers to the variation in time (modulation) of light output (luminous flux). While present in all traditional commercial electric light sources running on AC power, it can be more problematic with LED sources.

Who Cares About Flicker?

• Anyone who is sensitive to light modulation
• Those responsible for human health, well-being and/or performance in spaces with electric lighting
• At-risk populations:
  • Photosensitive epileptics (1 in 4000)
  • Migraine sufferers
  • Young people
  • Autistic individuals

Flicker Metrics

• Percent Flicker: (Max-Min)/(Max+Min) × 100%
• Flicker Index: Area above average / Total area

Test results in the report show flicker percentages ranging from 7.5% to 16.3% and flicker indices from 0.02 to 0.06 across different dimming levels and equipment combinations.

Power Quality

Dimming an LED source can change the behavior of the driver, potentially affecting power quality:

What is Power Quality?

Power quality refers to displacements and distortions to voltage and current waveforms, measured by:

• Power Factor (PF): Relates Active Power (P) and Apparent Power (S) by PF = P/S
• Total Harmonic Distortion (THD):
  • THD-V (voltage)
  • THD-I (current)

Who Cares About Power Quality?

• Electricity producers and consumers:
  • Increased current requirements
  • Electricity transport (I²R) losses
  • Wire, circuit breaker, transformer sizing
  • Potential equipment damage or degraded performance
• Lighting equipment manufacturers:
  • Voluntary requirements in ANSI C82.77-2002
  • System design tradeoffs
  • Cost and size constraints

Phase-Cut Dimming

Phase-cut dimming is the most commonly deployed dimming technology, with a large installed base in the U.S. (NEMA estimates >150 million units).

Phase-Cut vs. Sine-Wave Dimming

• Phase-Cut: Chops portions of the AC sine wave
  • Forward phase (leading edge)
  • Reverse phase (trailing edge)
• Sine-Wave: Reduces amplitude of entire sine wave

Designed for Incandescent Sources

Phase-cut dimmers were originally designed for incandescent sources, which:

• Behave like simple resistive loads
• Effectively only care about Vrms
• Are bidirectional (work with both positive and negative voltage)
• Have thermal persistence (light continues after current stops)

Compatibility Issues

The report identifies numerous compatibility issues that can occur with phase-cut dimming of LED sources:

• Dead travel: Adjusting dimmer setting without corresponding change in light level
• Pop-on: Dimmer setting needs to be raised above existing setting to get light output at turn-on
• Drop-out: No light output at the bottom of the dimming range
• Popcorn: Different turn-on times for different light sources on a dimmed circuit
• Flashing: Light source intermittently on when it should be off
• Ghosting: Light source at low-level on state when it should be off
• Audible noise from dimmer or LED driver
• Inoperability or premature failure

Sources of Compatibility Issues

• LED load cannot measure Vrms and/or conduction angle presented by the dimmer
• LED load doesn't draw enough current to keep dimmer switching elements closed
• LED load creates series impedance that disrupts dimmer timing elements
• LED load in off state doesn't pass dimmer current in a manner that keeps advanced features functioning
• LED load draws currents that create stresses beyond what dimmer's rated wattage indicates

Recommendations

The report provides several recommendations for successful LED dimming implementations:

Know Your Options

• Determine if the LED product is a lamp or luminaire
  • Lamps: Typically retrofit with standard base, constrained to phase control
  • Luminaires: Often have driver options yielding various control options
• Consider control technologies which separate AC power and control signal
• Use a phase-cut dimmer with a neutral if possible
• Consider using dimming controls designed specifically for LED sources

Take Advantage of Available Information

• Research the designed, claimed dimming performance of LED sources
• Look for recommended dimming control selection guidance
• Pay attention to specific makes/models recommendations
• Note dimmer loading requirements (max/min number of LED sources per control)

Ask the Right Questions

• What are the dimming transfer functions?
• Does the LED driver implement CCR or PWM?
• What is the PWM dimming frequency?
• Does dimming performance vary at different input voltages?
• Was the LED source evaluated for flicker and power quality over the dimming range?
• Does the dimmer require a neutral or trim-adjustment?

Weight the Trade-offs

• Application needs vs. wants
  • How much does flicker matter?
  • How much does power quality matter?
• Option 1: Only use LED sources and phase-cut dimmers with defined compatibility
• Option 2: Mock up installations with all intended components

Analyze Risk

• Common "fixes" for compatibility issues:
  • Change LED source, driver, or dimming control
  • Add incandescent or dummy loads
  • Add neutral wires
• Often there are no good solutions once products are installed
• Have a plan BEFORE products are ordered and installed