Are ac mains-dimmable LED lamps green?

Are ac mains-dimmable LED lamps green?

Technology News |
By eeNews Europe

This article provides a comparison between the 60 W incandescent lamp and two commercially available mains dimmable LED lamps including the light output versus power consumption under different dimming situations using phase-cut dimmers. In addition, it looks at the contribution of higher order harmonics to total system dissipation.

The lighting industry is undergoing a rapid transformation due  to the development and the improvement of LED technology. LED technology has become mature, and the performance of LED lamps has reached a threshold where efficacy ( amount of visible per unit of electrical power ) can be equal or  better in comparison to similar fluorescent lamp technology. LED’s already did outperform the traditional incandescent technology in terms of efficacy and lifetime over the past years. One of the major advantageous  of LED’s is dimmability. This article presents the results of a small study executed in order to verify the efficacy of dimmed LED lamps in comparison to the standard 60 W incandescent lamp. There are various dimmable LED lamps on the market that are dimmable with a conventional phase cut dimmer.  Two renounced types of different nominal wattages  ( 6 Watt and 12 Watt ) have been tested with various dimmers, and the corresponding light output was measured. Not only efficacy of the lamp itself, but also the efficacy of the complete system is considered, including losses in the dimmer itself and losses in the mains line due to different loading of the mains caused by dimmer and LED driver operation.

Initial Measurement results:

An initial measurement without use of dimmers was performed using a stabilized AC lab supply at 230AC 50Hz.

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Table 1: Initial measurements

As can be seen, the measured power of the LED lamps exceeds the nominal power as indicated on the lamps themselves. The resulting efficacy is a factor 3 to 6 better than the incandescent lamp. The LED lamps create an amount of higher order mains current, which result in higher THD figures ( Total harmonic distortion ). This in turn gives lower power factor and more apparent power. THD and other expressions like power factor are mathematically defined in the IEEE standard 1459.

Performance with Dimmer

During the next measurement, the lamps were controlled with phase cut dimmers. Three modes were tested: Fully open (undimmed), partially dimmed (Dimmed) and dimmed to the lowest position while still producing visible light.  A series of 13 commonly available European dimmers was used. Both trailing and leading edge operated dimmers were present. The electric power into the dimmer (Pin) and the power coming out the dimmer (Pout) was measured. The resulting power difference (dP) was calculated. Non-active power (Pnact) was calculated  in order to have an indication about the additional dissipation in the mains network.


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Table 2: Dimmed performances

 The last row in each subset is labeled estimated efficacy. This data was obtained by measuring the light using a Lux meter, in an enclosed environment at a fixed distance. The amount of lumen was calculated using the relative difference in Lux level measurement and applying this factor to the nominal rated light as specified on the initial product. There are three figures in these rows. Dimlevel represents the relative decrease in light level at the applied dimming position. The second figure represents the effective lumen per Watt number when the estimated lamp Lumen is divided by the average dimmer output power (Pout). The last figure is calculated dividing the estimated lamp Lumen by the apparent power. One of the most striking results are the very high THD values as measured under dimmed conditions. Even using an incandescent load, there are substantial mains harmonic currents present that cause additional mains load and network pollution.

This data can be analyzed in a number of graphs. The first graph displays the efficacy of the lamp over the dimming positions.


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Figure 1: Efficacy versus lamp power


Note that in the above figure , the vertical scale is logarithmic. For the LED lamps, both lines which calculate apparent power versus light output and real power versus light output are displayed. Dimmer losses are not included in this graph. From this figure, it becomes strikingly clear that both efficacy and power demand of LED lamps outperform the incandescent lamp. The efficacy of the LED lamps are 40 to 80 times better than the incandescent lamp under dimmed conditions . At lowest dim level, the 6 Watt LED lamp starts to outperform the 12 Watt lamp. This lamp is internally powered by the SSL2101, an IC from NXP. There are still few successful mains dimmable LED drivers in the power range below 10 Watt. Dimmer compatibility, application size and costs, efficiency and power factor, especially under dimmed conditions, all have to be balanced to reach a good solution. With the  SSL2101 this is the case. From energy conservation standpoint, using and dimming LED lamps for low light level applications is more effective than using incandescent lamps for the same purpose. Main reason for this difference is the physics behind the operation of the incandescent lamp. At dimming, the temperature of the tungsten filament will be reduced, and this in turn not only lowers the amount of dissipation but also lowers the wavelength of the spectrum. More energy is converted into heat instead of visible light. LED’s in contrast do not have this effect. In fact, efficacy of LED’s tend to increase at lower current levels because of reduction of operating temperature and less droop. In figure 1, this effect is countered by the driver losses in the LED driver. These driver losses are partly caused by the circuitry to enable correct dimmer operation.  It is assumed that  the primary purpose a  lamp is to produce visible light as seen by the human observer. The production of heat of incandescent lamps can partially compensate the energy saving of LED lamps. It causes reduced heating demand in households in a number of countries. If electric energy is produced with a method that is more environmental friendly than using fossil fuels, electric heating might be the preferred method.  On the other hand, when cooling is required ( air-conditioning ), the energy effectiveness of LED lamps is amplified by the lowered cooling demand.

Dimmer dissipation:

The next graph depicts the dissipation inside the dimmer:


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Figure 2: Dimmer dissipation

One dimmer in the test exceeded 1.6 Watts of dissipation (3.9 Watts) . This was an electronic ( transistor ) dimmer attached to the 60 Watt incandescent lamp under undimmed conditions. From the graph, it can be seen that attaching an LED lamp does not increase dissipation inside the dimmer. In fact, dissipation is lowered. On average the dissipation went down from 0.67 Watts for the 60 Watt incandescent to 0.4 Watt for the 12 Watt LED lamp and 0.31 Watt for the 6 Watt LED lamp.

Additional mains network losses:

The non-active power is the squared root over apparent power squared minus active power squared. In SMPS ( switch mode power supply ) converters, as present in LED lamps, this power is almost solely created by higher order harmonics currents. These higher order harmonics are made both by the LED lamp driver and the phase cut dimmer. The undimmed incandescent lamp acts like a perfect resistor, has very low harmonic current and a power factor that approaches  1. The category undimmed is with mains dimmer inserted, but at highest output.

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Figure 3: Non-active power

On the horizontal axis, the data is binned in non-linear categories. As can be seen from figure 3, the non-active power of the undimmed incandescent lamp is lower than that of the undimmed LED lamps. Under dimmed conditions however, the phase cut dimmer combined with the incandescent lamp causes harmonic currents which are bigger than that of the dimmed LED lamps. Replacing an incandescent lamp that is dimmed with  one of the tested LED lamps is expected to reduce both mains line pollution and additional network losses.

Harmonic distribution:

Next graph shows the relative amplitude of the 2nd to 20th harmonic currents under dimmed conditions. The lamps were measured with a single representative phase cut dimmer.

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Figure 4: Harmonic currents

The above graph shows a difference between the LED lamps and the incandescent lamp. The incandescent lamp has a successive decrease of amplitude of the higher order harmonics, while the LED lamps do not have this tendency. Note that these are relative figures. Also remarkable is the amplitude of the 6th harmonic. The resulting waveform of phase cut dimmers tends to cause this signal, which can be traced to the asymmetric loading caused by the phase cut dimmer.

Mains losses:

Last topic in this article discusses to what extend harmonic currents cause power losses in the mains network. Mains network analysis is by nature complex and can be influenced by many factors. The quality of the mains grid is one of them. Another phenomena that can occur is mains resonance, where there is an unwanted resonant flow of energy within the network. In order to provide some insight, the mains impedance of a typical Dutch mains connection is considered at the end terminal ( end of a LV feeder ). It is also assumed that resonance does not occur. The mains network can be represented by a single impedance, that  depends on the frequency:

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Figure 5: Mains network impedance of a Dutch LV cable network.

Data provided courtesy of KTI research (

 The RMS value of the total current squared  multiplied with a weighted impedance will provide a figure that is an estimate of the actual mains line dissipation. It is assumed that all customers in the LV networks have a similar installation with an equal number of LED lamps, 60 Pieces, representing an uniform distribution. From this calculation, next table can be made:

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Figure 6: Estimated mains network loss for 60 lamps

Due to the lower total power level of the LED lamps ( that produces similar light output ) , even the relative loss due to mains harmonic network currents is lower than that of dimmed incandescent lamps. In absolute terms , 30 to 50 Watts of mains network dissipation is avoided using LED lamps. As last example, the situation is taken where LED lamps are not applied to replace incandescent lamps. Instead, all mains groups are loaded with the maximal permissible real power. At 3680 Watt, 230 V AC 16 Amps, this corresponds to 306 12 Watt LED lamps or 613 6 Watt LED lamps.

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Figure 7: Estimated mains network loss at critical loading

This situation would occur when a new installation using LED lighting is applied and critically dimensioned. As a result, the apparent power exceeds the rated infrastructure capacity. One can now expect large dissipation in the network. Safety breakers are likely to become active. There is a major risk of overheating and fire hazard present. It is highly recommendable that sufficient information is provided with the product as to avoid this situation. One might for example provide the apparent power both on the product package and the product documentation.


LED lamps use much less energy than a comparable incandescent lamp, currently a factor 4 to 8 for the same light output. Under dimmed condition, this advantage increases to a factor of 40 to 80. Although higher order harmonics are present and increase to relatively high levels under dimmed conditions, they are still well below that of a comparable incandescent lamp using a phase cut dimmer.  There is evidence that these higher order harmonic current only marginally create additional losses in the mains network. The absolute reduction of total mains current has much more impact on mains network dissipation when replacing incandescent lamps with LED lamps. When an installation is designed to use LED lighting only, the apparent power of the device has to be taken into account as to avoid overloading. The SSL2101 IC from NXP was present in the tested 6 Watt LED lamp. More information on this device, and other parts for energy saving lighting applicatons, can be found on the NXP website:


[1] IEEE Std 1459-2010

[2] Costs and benefits of harmonic current reduction for switch-mode power supplies in a commercial office building. Key, Thomas , Lai, Jih-Sheng . IEEE October 1995.

[3] Bhattacharyya, S., Cobben, J.F.G. & Kling, W.L. Harmonic current pollution in a low voltage network. Proceedings of the 2010 IEEE Power and Energy Society General Meeting, Minneapolis, Minnesota, 25-29 July 2010.

About the Author

Victor Zwanenberg, Sr. Application Engineer, NXP

Victor has  over 25 years experience in various fields of electronics design and production. Since 2003 , Victor has grown expertise in the AC/DC LED driver electronics field  and retrofit LED lamp design.

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