” … energy saving is real when you dim LEDs – the more you dim, the more you save. The more you dim, the cooler the lamp operates resulting in reduced lumen depreciation as well as a longer lifetime of the LEDs driver.”
By Dr Marthinus Smit and Tracey Steyn, Shuttle Lighting
Dimming LEDs in homes, offices and even warehouses is a common requirement – mainly for the aesthetic value that dimming adds to a living or working environment.
Electricity consumption has been increasing rapidly, depleting natural resources such as coal and oil and, consequently, the cost of electricity has steadily increased. The result of this has been a global switch to LED lighting and energy efficient lighting systems.
The energy efficient revolution
In South Africa, we are acutely aware that our power grid is constantly under pressure, electricity prices are rocketing, and all lamps are becoming expensive. Fortunately, the very low energy consumption of LEDs provides some relief from high electricity costs. However, since dimmable LED lamps are slightly more expensive than their non-dimmable counterparts, buyers may think twice before they decide whether they actually want to dim the lights or not.
If the non-visual benefits – especially the additional energy savings possibilities and the increased lamp lifetime of the dimming LED lamps – are considered, it may be easier to make the decision whether to dim or not.
Figure 1: Measured light output versus energy consumption of halogen and LED lamps.
Figure 1 shows the measured energy consumption versus measured light level of a conventional 50 W GU10 halogen lamp, a typical 10 W GU10 LED lamp and a typical 10 W MR16 LED lamp with an electronic transformer. Besides the obviously much higher energy consumption of the halogen lamp and much lower initial light output, its consumption curve when dimming is very non-linear.
The LED lamps on the other hand not only show a much higher initial (non-dimmed) light output, but also exhibit a virtually linear consumption curve as the lamps are dimmed.
Figure 2: Measured energy savings versus light output of halogen and LED lamps.
LEDs have better light output using less energy
In order to better appreciate the implication of these characteristics, Figure 2 shows the energy savings as a percentage of the initial (non-dimmed) energy consumption versus the percentage of measured light output. If the lamps are dimmed to 50% lux output, the halogen lamp saves only 26% energy, very little considering that it consumes close to 50 W when not dimmed.
This is because most of the energy is used to ignite the gas and produce heat – the visible light output is only a small percentage of total consumption. LEDs are, however, an electronic component that has an inherent non-linear energy consumption versus light output, which is the opposite to that of a halogen or incandescent lamp.
Remarkable energy saving with dimming
With LED light sources, the energy savings are remarkable when dimming, as can be seen from Figure 2. When dimmed to 50% light output, the 10W GU10 LED saves a staggering 62% energy, effectively consuming only 3.8 W.
Due to the losses associated with the electronic transformer, the 10 W MR16 LED performs less efficiently but still saves a very respectable 48% energy when dimmed to 50%, effectively then only consuming 5.2 W.
Results will differ from brand to brand and even possibly between different models within a brand, but the general tendency will be the same or very similar for all LED lamps.
Interestingly, due to the high lumen output of the LED lamps, the MR16 measures around 400 lux (1 metre from source) and the GU10 approximately 450 lux compared to the a very low 150 lux of the halogen lamp when dimmed to 50%.
Figure 3: LED module lifetime versus temperature.
Dimming extends the lifetime of LED drivers
When an LED is dimmed, the current through the module reduces and the junction temperature correspondingly reduces (typically non-linear). This has a profound effect on the lumen maintenance of the LED as can be seen from Figure 3, which shows a graph of the expected LED module lifetime versus its internal temperature.
Additionally, the driver built into the LED lamp also suffers degradation due to increased temperature but dimming the lamp reduces the overall temperature.
A measurement of a typical 10 W GU10 LED showed that the aluminium housing temperature reduced from about 40 °C above ambient when at full intensity to about 5 °C above ambient when dimmed to its lowest intensity – this much reduced temperature has an obviously beneficial implication on the lifetime of the lamp’s driver.
Thus, the energy saving is real when you dim LEDs – the more you dim, the more you save. The more you dim, the cooler the lamp operates resulting in reduced lumen depreciation as well as a longer lifetime of the LED’s driver.
However, always ensure that the correct dimmer type is used for dimming a particular LED. If a dimmer is not matched or approved for a specific LED, it can result in reduced product lifetime.
Which brings us to the question:
Can you dim 100 x 5 W LEDs with a 500 W dimmer?
LED lighting technology provides numerous advantages for the end-user compared to the older incandescent and halogen technologies. But to the installer or specifier, LED technology often creates headaches when it comes to practical implementation. One of the problems often encountered, is the question of how many LEDs can be dimmed with one dimmer.
Surely a 100 W dimmer, for example, should be able to dim 10 x 10 W LEDs, since it is able to dim two 50 W halogen lamps? However, when implemented, premature dimmer failure is experienced or the dimmer gets extremely hot, if it works at all.
Dimmer rating and dynamic characteristics
Unfortunately, it’s an exception rather than a rule, that a dimmer can dim up to its full wattage rating of LED load for two reasons: the definition of a dimmer rating and the dynamic characteristics of a dimmable LED.
Phase cut leading or trailing edge dimmers are historically rated for Watts, which is strictly speaking incorrect, it should be rated for its VA capabilities. However, since halogen and incandescent (resistive) lamps have a unity power factor, which implies that their Wattage and VA are the same, it was logical to specify the dimmer rating in Watts.
Dimmer de-rating
Historically, only wire-wound transformers had a power factor of less than unity, which required a dimmer de-rating. A dimmer was thus typically specified, for example, as 500 W resistive and 400 W inductive loads. Importantly, the resistive type loads do not change characteristics when dimmed, the current always follows the voltage envelope and there are no significant current spikes or other non-linear characteristics.
Dimmable LEDs behave differently: Firstly, the power factor can vary from 55% to 95% and secondly, the driver (internal or external) characteristics can change when dimming, are different for leading and trailing edge dimmers, and often change dynamically according to specific intensity settings. The current ‘footprint’ of a particular LED can be very different at, for example, 30% and 80% light output.
Leading edge dimmer
Oscillogram 1 shows an example of a 9W LED (65% power factor) being dimmed with a leading edge dimmer. The single LED has a measured inrush current spike (yellow trace) of 1.2 Amps every mains half cycle (100 times a second). One can intuitively sense that it would be damaging to the dimmer to dim for example, 10 of these lamps with a 100 W dimmer, although the total LED wattage is only 90 W.
Trailing edge dimmer
Oscillogram 2 shows the same LED being dimmed with a trailing edge dimmer. In this instance the single LED has a corresponding measured peak current of only 0.2 Amps, which is clearly better for the dimmer, resulting in a larger number of LEDs per dimmer compared to the leading edge version.
Power factor
Both these measured examples illustrate the unique characteristic behaviour of dimmable LEDs which have a different power factor (mostly much lower) when being dimmed than that in their non-dimmed state.
The LED’s rated power factor can thus not be used in isolation to determine the dimmer rating. The only way to accurately determine the dimmer LED rating is to measure the particular LED’s current waveform at all intensities and match the results to the safe operating area of the power processing components (triac, mosfet, IGBT, SCR, etc.) of the dimmer as well as the EMI filter components and the complete dimmer thermal solution.
Since the above process is extremely time-consuming and requires intimate knowledge of a dimmer’s design to calculate the correct dimmer LED rating, LED suppliers do not usually provide the maximum number of LEDs per dimmer.
Rule of thumb guidelines
There are, however, some rule of thumb guidelines. For example: LEDs are marketed and purchased in Europe, not according to wattage but rather according to replacement of halogen or incandescent wattage. Leading European LED manufacturers often print the replacement wattage on the LED packaging and recommend that the replacement wattage be used to calculate the maximum dimmer load.
Thus, if a 5 W LED, for example, replaces a 50 W lamp, the rule of thumb is that a maximum of 10 x 5 W LEDs per 500 W dimmer can be installed. On the other hand, leading European dimmer manufacturers often recommend that their dimmers’ LED rating be approximately 10% of their resistive rating. Following this guideline, for example, a 500 W dimmer can dim a maximum of 10 x 5 W LEDs.
If the above guidelines are too limiting, it would be advisable to obtain a list of the maximum LED load per dimmer from the dimmer manufacturer.
An example of a comprehensive list can be found on the Shuttle Lighting home page under the Resources tab ‘Dimmer LED maximum load’.
More info: www.shuttlelighting.com
