Characterizing Dimming of an LED Light Bulb
for Incandescent Replacement
Kevin Vick, Department of Physics
Case Western Reserve University, Cleveland, OH 44106
Advisers:
Dave Dudik and Kevin Payne, Design Engineers
GE Lighting, Nela Park, East Cleveland, OH 44112
Senior Project Thesis
April 27, 2011
I.
Abstract
Within the next few years, incandescent light bulbs will be phased out of production and
will no longer be available for purchase. LED, or light-emitting diode, lamps are one alternative
light source, and are a major part of the future of lighting with their high energy efficiency,
durability, and extremely long life. LEDs require a DC input and are therefore combined with
electronic drivers to convert the standard AC signal to the necessary DC signal. Issues arise
when this complex circuitry is connected with household dimmers, as the vast majority of
residential dimmers utilize phase modulation and distort the AC signal transmitted to the light
bulb. This project develops a statistical approach for characterizing dimmer compatibility of
various electronic drivers and correlates lamp performance with circuit design. Through
collaboration with power electronics engineers, potential driver designs are optimized for use
with dimmers. An automated experimental set-up and procedure is developed and implemented
for validating compatibility across the dimmer industry and benchmarking against a standard
incandescent lamp. The project will culminate in the down-selection of the most effective
electronic driver for use in a future LED light bulb design, which will be a valid replacement for
the standard incandescent.
Kevin Vick 2 Senior Project Thesis II.
Background
A. LED Light Bulbs
Starting in 2012, the U.S. government will have new energy efficiency requirements for
light bulbs as set by the Energy Independence and Security Act of 2007.1 As a result, the
standard incandescent light bulb will be phased out of production and will no longer be available
for purchase. Currently, the two most effective alternative light sources are compact fluorescent
lamps (CFLs) and light-emitting diode (LED) lamps. Compact fluorescent lamps have made
great strides in terms of energy efficiency and life when compared to incandescents; on average,
CFLs use 75% less energy and last 10-15 time longer. However, many consumers complain
about various features such as the mercury contents, the lower light quality, and the turn-on and
warm-up times often associated with CFLs. LED lamps, on the other hand, are the future of
lighting. They are as energy efficient as CFLs and last as much as 50 times longer than
incandescents. They are much more durable than both incandescents and CFLs as they do not
require a glass structure or a depressurized system. In direct comparison with CFLs, LEDs are
able to produce a better quality of light and do not contain any mercury.
Unlike incandescent lamps, LEDs require a DC input to operate and emit light. As a
result, they must be combined with an electronic driver housed within the lamp to convert the
standard AC signal from the lamp socket to the necessary DC signal. This complex circuitry
presents a much different load to the power source than the resistive load of an incandescent.
Figure 1 on the following page shows a sample block diagram of an LED light bulb in
comparison to an incandescent lamp.
Kevin Vick 3 Senior Project Thesis Figure 1. (a) The simple structure of an incandescent lamp compared to the
(b) more complex block diagram of an LED light bulb.
B. TRIAC Dimmers
There are approximately 150 million dimmers in residential applications in the US.2 The
vast majority of these are TRIAC (triode for alternating current) dimmers as they are inexpensive
and effective with resistive loads, such as the standard incandescent lamp. TRIAC dimmers
utilize phase modulation to cut out portions of the sinusoidal AC signal, thereby reducing the rms
voltage applied to the load. The most basic circuit of a TRIAC dimmer is shown in Figure 2.
Dimmers will often also include filters and
other elements to remove harmonics and
noise within the circuit.3 The main
components of the TRIAC dimmer are a
variable resistor (R), a capacitor (C), a
DIAC (diode for alternating current) and a
TRIAC (triode for alternating current). The
variable resistor is adjusted by the user
through knobs, sliders, buttons, or other
Kevin Vick Figure 2. Sample TRIAC dimming circuit
4 Senior Project Thesis interfaces. In combination with the capacitor, it forms a simple RC circuit with a known time
constant of
. This controls the speed at which the capacitor is charged; as the resistor
value is increased, the time constant increases, and the capacitor charges more slowly. The
charge on the capacitor represents the charge across the DIAC. The DIAC functions as an AC
switch and will only allow current to flow when the voltage across it exceeds a rated break-over
voltage. When the capacitor charges up to this break-over voltage, current is passed through the
DIAC and triggers the TRIAC. The TRIAC functions as a triggered gate and will not conduct
until current flows from the DIAC. Once this occurs, current then flows through the TRIAC,
thereby allowing current to flow through the lamp, producing light. The TRIAC will continue to
conduct until the current falls below a value known as the holding current, typically around
10 mA. Since the circuit is connected to an AC input, this entire process is repeated for every
half-wave in the sinusoidal signal.
Sample voltage waveforms undergoing phase modulated dimming are shown below in
Figure 3. The portion of the sine wave that is transmitted to the lamp is related to the phase
angle of the dimmer setting. A phase angle of 180° corresponds to a complete, unaltered sine
wave, while 0° corresponds to zero voltage and no light output. As the lamp is dimmed, the
phase angle decreases.
Figure 3. Sample voltage waveforms undergoing phase modulated
dimming with a TRIAC dimmer at phase angles of 120° and 45°.
Kevin Vick 5 Senior Project Thesis TRIAC dimmers are available as either analog or digital dimmers. While digital dimmers
only represent a small fraction of the dimmer industry, they are on the rise and slowly becoming
more popular. With a digital dimmer, the RC circuitry is replaced by a pre-programmed
microcontroller or discrete digital circuitry. This introduces more complex dimming control
beyond a simple knob or slider and usually features buttons or touch pads as the user interface.
Digital dimmers present one of the toughest challenges for dimmer compatibility with LED light
bulbs because the microcontroller requires power even while the dimmer is off. This results in a
small leakage current, typically around 20 mA, that needs to flow through the lamp when the
light should be off.
C. Current Issues
Many different issues can arise when LED drivers are connected in series with TRIAC
dimmers. The driver must be able to convert the chopped AC signal from the dimmer into a
steady DC signal for the LEDs in order to emit a constant light, while also interpreting the
altered waveform to the appropriate dimming level. Both dimmers and drivers have various
conditions that must be met for smooth operation, as well as consumer requirements for overall
performance.
To ensure proper operation and avoid permanent damage, TRIAC dimmers have several
conditions that must be satisfied. Firstly, dimmers have minimum and maximum load
requirements, in terms of watts. Most dimmers are rated to withstand 600W, but this value can
change depending on the model. Loads above this rating can result in permanent damage to the
dimmer in the long run. To ensure proper operation, dimmers also have minimum load
requirements that are typically around 25-40W for a 600W dimmer.4 For loads below this value,
Kevin Vick 6 Senior Project Thesis aesthetic issues often arise as the lamp can turn off prematurely or will not operate at lower
dimming levels. Another requirement for operation involves the holding current necessary to
maintain conduction of the TRIAC. Shortly after the TRIAC is fired, the holding current value
must be reached or else the TRIAC will stop conduction and the lamp will not light. From the
schematic diagram of Figure 1 previously, it can be seen that EMI/EMC (electromagnetic
interference and compatibility) filters are the first components in the driver. These are composed
of a network of capacitors and inductors that do not readily provide a path for current flow. An
alternative path for current must be available when the TRIAC fires to meet the holding current
requirement of the dimmer. Finally, digital dimmers also require a path for current flow while
the light should be off. Without this path, digital dimmers may not function at all as the
microcontroller is not being powered.
LED drivers have their own requirements that must be met for steady operation with
dimmers. Drivers must be designed to properly interpret a phase-modulated signal and transfer
the information to constant current output levels. As the phase angle is decreased, the output
current must also decrease in a relative manner. For constant light output, drivers must also be
able to draw current throughout the entire AC waveform, even though some parts may be cut out
by the dimmer. This requires proper storage of power in capacitors and inductors for use when
there is no signal from the input.
Finally, consumers have their own requirements when it comes to aesthetic performance
of the light bulb. They expect performance to mimic that of the incandescent lamp. This
includes flicker-free operation with smooth dimming over a wide range of light output levels.
Consumers also require quiet operation with a short turn-on time. If LED performance is
Kevin Vick 7 Senior Project Thesis noticeably worse than the standard incandescent, consumers will be dissatisfied, slowing
adoption of this new, energy efficient light source.
III.
Methods
A. Analysis Criteria
A test procedure was developed to determine dimmer compatibility of various driver
designs. The different performance characteristics were divided into three main criteria for
analysis and comparison between designs. For compatibility with a dimmer, the LED driver
must pass all criteria while under operation with the selected dimmer.
1. Light Output Curve
The light output curve plots relative light output (compared to operation without a
dimmer) as a function of the phase angle of the dimmer setting. Upper and lower limits were
chosen based on incandescent performance and input from various companies in the lighting and
dimmer industries that are members of NEMA, or the National Electrical Manufacturers
Association.5 A NEMA standard was published within the last year detailing the dimming
behavior for LED lighting. Figure 4 on the following page highlights the target region for
operation between the upper and lower limits. The dashed black line is an internal company goal
for a minimum light output level of 10%. The dashed green line in the middle of the target
region is the performance of an incandescent lamp for comparison.
Kevin Vick 8 Senior Project Thesis Figure 4. Target region for relative light output (RLO) as a function of phase angle.
2. Equivalent Incandescent Load
Equivalent incandescent load (EIL) is a measurement of how the dimmer perceives the
LED lamp compared to an incandescent. This value is determined through different current
measurements and determines how many lamps can be operated by a single dimmer based on the
maximum load rating. Although an LED lamp may only be consuming 10W of power, high
peak currents can result in an equivalent load over 100W.
Three current values are measured for an equivalent incandescent load, and the maximum
value is taken as the reported EIL. Inrush current, repetitive peak current, and rms current all
contribute to EIL. Figure 5 below depicts inrush current and repetitive peak current in the
current waveforms. Inrush current measures the instantaneous current flow when the lamp is
first turned on without a dimmer. Repetitive peak current measures the maximum current that
occurs with a dimmer every time the TRIAC fires. Finally, rms current simply measures the
Kevin Vick 9 Senior Project Thesis standard current during steady state operation. These three currents can be translated to an EIL
value based on the equations below.6
Figure 5. Depiction of inrush current and repetitive peak current in the current
waveforms that are used to determine equivalent incandescent load.6
3. Aesthetics
The third and final criterion for analysis involves aesthetic performance of the LED lamp
with the dimmer under test. Ideally, the LED lamp should mimic the performance of the
incandescent, however this is not usually the case. Table 1
lists the common aesthetic issues that arise when dimming
LED lamps. Turn-on time is a measure of how long the lamp
takes to turn on after the dimmer is turned on. Pop-on occurs
when the dimmer must be turned to a higher setting to turn
Lamp Failure Codes
A - Flicker/Shimmer
B - Doesn't turn off
C - Steppy dimming
D - Audible Noise
E - Turn-on time
F - Pop-on
Table 1. Common aesthetic issues
when dimming LED lamps.
the lamp on, compared to when the lamp went out. Each issue is a point of failure for
dimmer compatibility testing and the failure codes will be referenced in the results section.
Kevin Vick 10 Senior Project Thesis B. Automated Test Set-up
An automated test set-up was designed and constructed in order to efficiently test dimmer
compatibility and compile the data to validate the analysis criteria. Figure 6 below shows a
schematic diagram of the equipment set-up and connections. The dimmer control box was
constructed to easily switch between dimmers and add external load as needed. The power
analyzer measures and records voltage, current, power, and power factor values. It also
measures peak current values for use in calculation of the EIL. The data acquisition unit records
both the lamp voltage and a DC voltage output from the light meter that directly correlates to
light output. These values are used to produce the light output curves. The computer combines
all the information and calculates relative light output values and incandescent load
equivalencies, while plotting various electrical characteristics as a function of the phase angle of
the dimmer.
Light Sphere Computer
External Load Lamp PC
In Power Analyzer Out
Dimmer Dimmer Control Box Data Acquisition Unit Power Supply Light Meter Figure 6. Schematic diagram of the automated test set-up for dimmer compatibility testing.
Kevin Vick 11 Senior Project Thesis IV.
Results
Three different potential driver designs (A, B, and C) were tested and analyzed using the
criteria from the methods section earlier. Each driver is designed to deal with TRIAC dimmers
differently, and aspects of each design can be correlated to the overall performance
characteristics to determine its effectiveness as an incandescent replacement. Each driver was
tested with a set of five different dimmers – three analog, two digital – for initial
characterization. These dimmers were selected as a representative sample based on
manufacturer and type of interface. Before testing the LED drivers, however, two incandescent
light bulbs were first tested as a baseline for comparison.
Incandescent
Both a 40W and a 60W incandescent light bulb were tested by the method previously
described. The purpose of this test was to ensure the equipment functioned properly, and to
establish any issues that arise with the standard incandescent on the selected set of dimmers. For
comparison, all three LED drivers tested are designed as replacements for a 40W incandescent
light bulb.
Figure 7 on the following page plots the light output curve produced. Both the 40W and
the 60W lamp resulted in the same light output curve, so only a single curve is shown. As can be
seen in the graph, the measured data of the actual incandescent closely resembles the expected
behavior from the incandescent model. This confirms that the equipment is measuring
accurately and producing valid results.
Kevin Vick 12 Senior Project Thesis Figure 7. Light output curve for an incandescent lamp.
Tables 2 and 3 on the following page summarize the testing with the 40W incandescent
and the 60W incandescent, respectively. The summary tables detail the minimum and maximum
relative light output (RLO) levels, the calculated EIL, and any aesthetic issues in reference to the
failure codes listed earlier in Table 1. The red boxes denote areas of particular concern. Both
incandescent lamps produced similar results. The testing shows that even incandescent lamps
can experience pop-on at times, as seen with the third analog dimmer. Another area of interest
involves the maximum relative light output levels. In most cases, the maximum light output of
an incandescent light bulb on a dimmer is less than 90% of the light output without a dimmer.
Kevin Vick 13 Senior Project Thesis Dimmer Analog 1 Analog 2 Analog 3 Digital 1 Digital 2 40W Incandescent Performance Summary Max RLO Min RLO EIL 85.1 0.0 69.5 86.0 0.1 68.0 97.3 0.0 68.0 89.1 0.5 53.4 86.3 0.4 60.1 Failure Codes ‐ ‐ F ‐ ‐ Table 2. Performance summary for a 40W incandescent with a set of 5 different dimmers.
Dimmer Analog 1 Analog 2 Analog 3 Digital 1 Digital 2 60W Incandescent Performance Summary Max RLO Min RLO EIL 85.0 0.0 64.0 86.1 0.0 62.1 97.8 0.0 61.7 90.0 0.7 61.9 87.4 0.5 72.8 Failure Codes ‐ ‐ F ‐ ‐ Table 3. Performance summary for a 60W incandescent with a set of 5 different dimmers.
Driver A
Driver A is a single-stage circuit that features phase angle detection circuitry and an
inrush current limiter. The phase angle detection circuitry is designed to switch a resistor in and
out when the TRIAC fires. This creates a path for current flow to achieve the holding current
requirement of the dimmer. However, this also results in a small amount of wasted power
dissipated by the resistor. The inrush current limiter ensures that the peak currents do not get too
high, thereby maintaining a low EIL. Figure 8 on the following page shows the effect of the
phase angle detection circuitry and the inrush current limiter in the current waveform. When the
TRIAC fires, a resistor is switched in for a small period of time, as seen in the higher current
levels. This current value is limited by the inrush current limiter and then drops to lower values
as the resistor is switched back out of the circuit.
Kevin Vick 14 Senior Project Thesis Figure 8. Current waveform for operation of Driver A with a TRIAC dimmer showing
the effects of the phase angle detection circuitry and inrush current limiter.
The light output curve produced by Driver A is shown in Figure 9 below for an analog
dimmer with the widest dimming range. Due to the change in circuit when connected to a
dimmer, the lamp actually produces more light with a dimmer at the highest setting than without.
Figure 9. Light output curve for Driver A with a wide-range analog dimmer.
Kevin Vick 15 Senior Project Thesis Table 4 below summarizes the testing with Driver A, detailing the minimum and
maximum relative light output levels, the calculated EIL, and any aesthetic issues, again in
reference to the failure codes listed earlier in Table 1. The red boxes denote the areas of concern.
These results will be analyzed further in the discussion section.
Dimmer Analog 1 Analog 2 Analog 3 Digital 1 Digital 2 Driver A Performance Summary Max RLO Min RLO EIL 95.2 4.1 26.8 95.0 2.7 29.5 102.0 10.7 30.8 98.6 6.7 25.7 99.6 8.3 24.4 Failure Codes A, F A, F F B, C B Table 4. Performance summary for Driver A with a set of 5 different dimmers.
Driver B
Driver B is a two-stage circuit; the first stage acts as a constant current source, while the
second stage powers the LEDs. With this approach, the constant current source maintains
current flow through the dimmer without affecting the performance of the LEDs in the second
stage, thereby minimizing aesthetic issues and keeping the EIL low. This creates a path for
current flow to achieve the holding current, as well as the digital dimmer leakage current
requirement. Figure 10 on the following page shows the effect of the constant current source.
Instead of a sinusoidal signal, the current waveform appears more like a square wave during
operation. The peak in the center of each half-wave serves to improve power factor by
corresponding with the peak from the sinusoidal voltage signal.
Kevin Vick 16 Senior Project Thesis Figure 10. Current waveform for operation of Driver B with a TRIAC dimmer showing
the effect of the constant current source in the first stage.
The light output curve produced by Driver B is shown in Figure 11 below for the same
analog dimmer as the previous figure with Driver A. With the circuit design at the time of
testing, the light output curve for Driver B actually fell outside the upper limit. This issue will be
discussed in the following section, along with suggested improvements.
Figure 11. Light output curve for Driver B with a wide-range analog dimmer.
Kevin Vick 17 Senior Project Thesis Table 5 below summarizes the testing with Driver B, using the same set of dimmers.
Again, minimum and maximum relative light output levels, the calculated EIL, and aesthetic
issues are displayed.
Dimmer Analog 1 Analog 2 Analog 3 Digital 1 Digital 2 Driver B Performance Summary Max RLO Min RLO EIL 97.8 23.2 38.7 97.9 24.8 34.6 99.7 14.0 52.3 99.0 32.7 31.2 98.5 32.4 33.3 Failure Codes ‐ ‐ A, F ‐ ‐ Table 5. Performance summary for Driver B with a set of 5 different dimmers.
Driver C
Driver C is a single-stage circuit that features a specialized integrated circuit (IC)
designed for dimming. While the previous two driver designs, A and B, utilize an 8-pin IC,
Driver C uses a 10-pin IC with three built-in dimming pins. This offers dimming specific
controls with phase angle detection to more closely resemble the performance of an
incandescent. However, there is no circuitry to minimize the peak currents that can occur.
The light output curve produced by Driver C is shown in Figure 12 on the following page
for the same analog dimmer as with the previous drivers. The light output curve in this case
most closely resembles that of the incandescent due to the added dimming control in the circuit
design.
Kevin Vick 18 Senior Project Thesis Figure 12. Light output curve for Driver C with a wide-range analog dimmer.
Table 6 below summarizes the testing with Driver C, using the same set of dimmers.
Again, minimum and maximum relative light output levels, the calculated EIL, and aesthetic
issues are displayed.
Dimmer Analog 1 Analog 2 Analog 3 Digital 1 Digital 2 Driver C Performance Summary Max RLO Min RLO EIL 90.9 3.1 200.2 91.2 3.5 152.4 98.7 1.7 132.7 91.8 5.8 136.6 93.0 3.5 163.9 Failure Codes ‐ F F A, B, C A, B Table 6. Performance summary for Driver C with a set of 5 different dimmers.
Kevin Vick 19 Senior Project Thesis V.
Discussion
Each driver produced different performance results that can be attributed to aspects of the
circuit design. By comparing the varied approaches to dimmer compatibility, the effectiveness
of different driver components can be determined.
Driver A
Other than the points above 100%, the light output curve for Driver A falls within the
target region. The driver produced roughly a linear relation between the relative light output and
the phase angle of the dimmer setting. This single-stage design allows for a wide range of
dimming, achieving light levels as low as 2-3% relative to normal operation. However, the light
output curve drops straight to zero and does not level off on the low end. This often leads to the
lamp turning off before the dimmer has reached its minimum setting.
The inrush current limiter results in very low values for the equivalent incandescent load
– around 25-30W. This puts minimal stress on the dimmer, and allows for a greater number of
lamps to operate on the same dimmer; a single 600W dimmer could control up to 20 different
lamps without any load issues. However, the EIL is so low that it is at or below the minimum
load requirement of most dimmers. This is the source of many of the aesthetic issues
experienced during testing. For the first two analog dimmers, some flicker was seen at the low
end of the dimming range. For all three analog dimmers, the lamp exhibited pop-on.
Driver A also experienced issues with the two digital dimmers. While the phase angle
detection circuitry provides a path for current flow while the TRIAC is firing, there is no path for
current flow when the dimmer is off. This results in issues as those seen with the digital
dimmers. Since there is no alternative path for current flow, the driver misinterprets the small
Kevin Vick 20 Senior Project Thesis amount of leakage current from the digital dimmers as a signal to power the LEDs. For this
reason, the lamp remains on, even though it should be off.
By adding more lamps to the dimmer load in parallel, some aesthetic issues may
disappear as the minimum load of the dimmer is no longer an issue. This may also help in
meeting the holding current requirements and the leakage current requirements for digital
dimmers as the current can be distributed amongst the load lamps, minimizing the overall impact
on a single lamp. Some testing was done briefly to look into the effects of adding external load.
Table 7 below summarizes the performance results from these trials. A modified version of
Driver A was tested in a single lamp configuration, as well as with a total load of five lamps.
The same five dimmers from previous testing were used, along with the same procedure.
Dimmer Analog 1 Load 1 Lamp 5 Lamps External Load Performance Summary Max RLO Min RLO EIL Failure Codes 93.9 96.0 0.9 4.1 25.9 31.7 ‐ ‐ Analog 2 1 Lamp 5 Lamps 94.2 95.9 1.8 4.7 28.3 31.8 A ‐ Analog 3 1 Lamp 5 Lamps 100.8 100.1 2.4 2.3 23.0 31.2 F F Digital 1 1 Lamp 5 Lamps 99.4 98.2 2.1 9.7 26.2 31.4 A, B, C A, C Digital 2 1 Lamp 5 Lamps 97.0 96.3 2.7 8.6 25.9 31.2 A, B A Table 7. Performance summary for loads of a single lamp and five lamps with a set of 5 different dimmers.
With a total load of five lamps instead of one lamp, the overall performance actually
improved. The EIL increased slightly, however some of the aesthetic issues disappeared. For
the second analog dimmer, there was flickering with a single lamp, but showed no issues when
five lamps were tested. Another thing to note is that a single lamp load did not turn off with
either of the digital dimmers, but when more lamps were added to the load, they all then turned
Kevin Vick 21 Senior Project Thesis off. In this instance, adding external load for a total of five lamps eliminated many of the
aesthetic issues that originated from the minimum load requirements for the different dimmers.
Driver B
The two-stage circuit produces a much different light output curve compared to the
single-stage design of Driver A. Instead of a linear response, the slope of the curve becomes
steeper as the phase angle, and hence light level, decreases. However, this results in the curve
falling outside the upper limit, as well as a higher minimum light level. To correct these
deficient areas, circuit design work is being conducted to shift the entire curve down and to the
right, placing it within the limits while decreasing the minimum light level. This may be
achieved through a voltage divider within the circuit that will cause the driver to interpret the
incoming signal as a proportionally lower light level than the current design.
The two-stage approach solves many problems that other drivers commonly experience.
By implementing a constant current source, issues involving the holding current and leakage
current requirements are essentially eliminated. Current is able to pass through the lamp without
affecting the LED control. The only aesthetic issues that were observed occurred with the third
analog dimmer on the very low-end dimming range. The lamp showed signs of pop-on and light
flicker at the low end, however, even an incandescent lamp exhibited some pop-on with the same
dimmer, as presented earlier. In terms of lamp aesthetic performance for the consumer, the
Driver B design most closely resembles the performance of an incandescent.
The two-stage approach also helps to keep the equivalent incandescent load low. With
the constant current source as the first stage, peak currents are minimized while the TRIAC is
firing. Driver B produced EIL values around 30-40W, which is ideal as the lamp is designed to
Kevin Vick 22 Senior Project Thesis serve as a replacement for a 40W incandescent. If the EIL drops much below this level, other
aesthetic issues could arise due to the minimum load requirements of the dimmers as observed
with Driver A. There was one instance of an EIL above 50W with the third analog dimmer,
which occurred on the low end with the slight flicker as discussed earlier. The driver design may
have to be altered slightly to better deal with very low phase angles, as was evident with the third
dimmer.
Driver C
The light output curve for Driver C was the best when compared to the other driver
designs. The curve followed along the shape of the incandescent and leveled off at low light
levels, allowing for operation on the full range of phase angles. In all instances, the minimum
light level was below 6%, reaching as low as 1.7% in one case. The shape of the curve is more
complicated than previous drivers, which stems from the specialized IC for dimmer operation.
The extra pins in the IC serve as specific controls for dimming that utilize phase angle detection
and other circuitry to understand exactly where along the curve the lamp is currently operating.
However, the circuit design of Driver C does not contain any method for limiting the
peak currents that can occur. As a result, the measured values for the equivalent incandescent
load are well above the other drivers. The EIL reached as high as 200W with the first analog
dimmer, which puts significant stress on the dimmer. If more than three lamps were operated on
the same 600W dimmer, permanent damage could result as the dimmer would be overloaded.
This is a significant point of concern with the design of Driver C and extra circuitry must be
added to minimize the EIL values.
Kevin Vick 23 Senior Project Thesis In terms of aesthetics, the driver worked reasonably well with the analog dimmers, but
experienced issues with digital dimmers. Driver C showed some signs of pop-on with the second
and third analog dimmers, but even the incandescent lamps experience pop-on issues. Much like
the other single-stage circuit, there was no alternative path for the leakage current to flow with
the digital dimmers. As a result, the lamp remained on and flickered while both digital dimmers
were turned off.
VI.
Conclusion
By correlating aspects of circuit design with LED lamp performance, the driver design
can be optimized for dimmer compatibility based on the analysis criteria. The inrush current
limiter circuitry of Driver A is highly effective in minimizing the equivalent incandescent load,
as something must be included to reduce stress and avoid permanent damage to the dimmers.
The two-stage approach of Driver B presents a solution which meets both the holding current
and leakage current requirements of the dimmers, thereby eliminating many of the aesthetic
issues commonly seen with other designs. The added dimming control with the specialized IC of
Driver C produces light curves similar to that of an incandescent lamp, and allows the user to
dim to very low light levels. Each driver has its advantages for dimmer compatibility that can be
taken away for future designs.
By combining the knowledge gained through testing the different driver designs, a better
understanding of effective circuit design can be developed. This will lead to the design and
selection of the optimal driver for dimmer compatibility in the future lamp designs. Through the
work of this project, a light bulb will be produced that combines the energy efficiency,
Kevin Vick 24 Senior Project Thesis durability, and long life inherent in LEDs, with the dimming performance of an incandescent
light bulb.
VII.
Future Work
This report covers the first versions of Drivers A, B, and C. With the results and analysis
from these studies, much has been learned about the effects of driver design on lamp
performance. This information will be taken into account in a second revision of each of the
drivers to optimize performance and dimmer compatibility. With the automated test set-up in
place, these revisions can easily be tested to check for improvement upon the prior design.
Driver A must be redesigned for better operation with digital dimmers and to address the
aesthetic issues that arise at the lower dimming levels. Driver B must be altered to shift the light
output curve to the right and reduce the minimum relative light output level. Driver C must add
circuitry to limit the peak currents that result in such a high equivalent incandescent load.
The effects of adding external load can still be tested as well. Some initial testing was
performed as presented in the discussion of Driver A, however there is still much to learn. Only
the case of a total load of five lamps was looked at and showed improving results in
performance. Through further testing, performance correlations could be made with the total
load on the dimmer.
Once the driver designs have been finalized and the optimal driver has been selected, the
automated set-up can be used to verify compatibility with a wide range of dimmer models.
There are hundreds of dimmers available in the market, and the only way to verify the LED lamp
design will work is to test the dimmer and ensure that all analysis criteria are met. This can be
done efficiently to compile a list of all dimmers that the LED product is compatible with.
Kevin Vick 25 Senior Project Thesis Finally, the automated test set-up will be copied and implemented throughout the
company for greater accessibility to compatibility testing. Current plans involve reproducing the
set-up at sites in Canada, China, and Europe, based on the work outlined in this report.
VIII. Acknowledgments
I would like to thank my advisers, Kevin Payne and Dave Dudik, for their guidance and
support throughout the course of this project. I would also like to acknowledge the circuit
designers of the various drivers for helping me to understand how the circuits operate and
assisting me with any issues that arose while working with them. Finally, I would like to thank
many of the engineers and lab technicians at Nela Park as they proved to be a valuable resource
numerous times.
IX.
References
1.
J. Logan, “Lighting Efficiency Standards in the Energy Independence and Security Act of
2007: Are Incandescent Light Bulbs ‘Banned’?,” CRS Report for Congress, 28 February
2008.
2.
National Electrical Manufacturers Association, “Solid State Lighting for Incandescent
Replacement – Best Practices for Dimming,” Lighting Systems Division Document
LSD49-2010, 2010.
3.
J. Smith, J. Speakes, M. H. Rashid, “An Overview of the Modern Light Dimmer: Design,
Operation, and Application,” Proceedings of the 37th Annual North American Power
Symposium, pp. 299-303, October 2005.
4.
A. Beebe, E. Biery, “Controlling LEDs,” Lutron technical white paper 367-2035 Rev. B,
January 2011.
5.
National Electrical Manufacturers Association, “Solid State Lighting for Incandescent
Replacement – Dimming,” SSL 6-2010, 2010.
6.
E. Biery, “LEDs and Dimmer Basics,” Presentation prepared for GE Lighting, December
2010.
Kevin Vick 26 Senior Project Thesis