By Dr Marthinus Smit, Shuttle Lighting
LED lamps were initially introduced as solid-state lighting (SSL) to provide significant energy saving combined with an increased lamp lifetime. SSL lamp technology is, however, continuously evolving and provide previously unknown lighting options to the market.
The maturity of LED ‘filament’ lamp technology as an example, pushed the envelope of innovation
into an evolution where the lamps can be stunning pieces of art that provide timeless elegance to any installation. The physical and visual appearance of the lamp itself is becoming the focal point of lighting.
An LED filament lamp consists of many individual LED components, sometimes hundreds, connected in series and are housed in a glass enclosure that is filled with a special heat conducting gas to keep the LEDs cool while a small voltage to current regulator is situated in the lamp base. Due to the large LED array, the internal operating voltage of the lamp is usually in excess of 150 VDC.
Oscillogram 1 shows a typical and characteristic waveform of the lamp current with a 230 VAC supply voltage. The yellow lamp current waveform rises very quickly when the voltage across the lamp reaches around 260 V and turns off when the voltage drops below this level again. The lamp thus effectively ‘operates’ only with the small blue shaded area of the total available supply voltage (per mains half cycle).
Lamps with a smaller LED array will turn on at a lower voltage while lamps with a larger array will turn on at a higher voltage than the sample measured. This characteristic can make the light output of the lamps rather sensitive to changes in supply voltage.
Graph 1 shows the measured lux of six different LED filament lamp models when operated on direct mains at 225 VAC and 235 VAC (a 4% difference) with a 230 VAC supply voltage as the reference. It can be seen that one model showed a change in light level in excess of 40%.
The dimming behaviour of LED filament lamps is usually very good due to the ‘linear’ type of voltage to current driver in the lamp base, which eliminates initial lamp inrush current and current peaks during steady state operation.
Oscillogram 2 shows the typical measured lamp voltage and current during trailing edge dimming. Dimming the lamp however increases the sensitivity of the lamp light output vs supply voltage fluctuations as is shown in Graph 2 when the six sample lamps are dimmed to approximately 30% intensity. When the lamp is at its minimum intensity, the sensitivity reaches its maximum.
Oscillogram 3 shows the waveforms when a sample lamp is dimmed to 10% intensity. The yellow lamp current waveform is then very narrow as can be seen in Oscillogram 4 (230 VAC supply voltage) when zooming in 50x on the time scale. When the supply voltage is changed to 225 VAC as per Oscillogram 5, the yellow current waveform has a significantly shorter duration than at 230 VAC supply voltage and a significantly longer duration as per Oscillogram 6 when at 235 VAC supply voltage.
Since there is a direct correlation between the duration of the lamp current and its light output, the sample lamps all showed significant changes in measured lamp lux vs supply voltage in Graph 3 when dimmed to a low intensity.
Graph 4 is a summary of the absolute percentage lamp lux changes vs supply voltage at various dimming intensities for the six samples tested. Since our eyes are much less sensitive to changes in light intensity at high lux levels, this lamp characteristic is not of a real concern. It may, however, be observed as a shimmer or even slow pulsing effect at very low intensity at installations where the supply voltage changes continuously by a small percentage. Setting the dimmer’s minimum intensity to avoid very deep dimming usually resolves this.
Oscillogram 3 illustrates another interesting aspect of LED filament technology: the lamps reach minimum intensity and thus turn off prematurely when the dimmer is still at a fairly high conduction angle. This is directly due to the lamp’s high internal operating voltage when the phase cut voltage from the dimmer is below the lamp’s turn-on voltage. When using a rotary dimmer, the operator tends to turn the knob until the lamp is on, but this can create the perception of a faulty bell-press dimmer when the lamp turns off prematurely and the operator thinks that the dimmer is off. It is thus advisable to use a bell-press dimmer that has a setting for the dimmer’s minimum intensity.
It is important to note that the behaviour of LED filament lamps described is not a reflection of the lamp quality, it is a fundamental characteristic of the lamp technology and can be observed, often to a lesser extent, in some other SSL lamps, depending on the lamp design.
It is always a good course of action to consult with the manufacturers or importers of LED lamps and dimmer modules to assure that the lamp behaviours have a fundamentally sound and technical reason for the observed characteristics, backed by the necessary compliance and certification adherence.
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