Leading edge dimmers and power consumption

When you dim a load using leading edge dimmers (eg L5504D2 and L5508D1) do you save power? Eg if you have 4 x 50W downlights on a string and dim to 50% are you using approximately half the power (keeping in mind that transformer losses etc will be in play).

I have seen a lot of conflicting information on the net about this.

 

Hi MichaelK,

The short answer is yes, you can save power.

The amount you save will be less than 50% if you dim the lights to 50%, but may be not too much less than 50% if the lights are being used a significant proportion of the time.

There are some losses in the dimmers that occur when the load is turned on, plus some quiescent losses in the dimmer when the load is completely off. There is also some quiescent power consumption associated the C-Bus itself, associated with indicator lights and touch screen back lighting, processors running ready to respond/activate action, and communications, and some small power losses in the relatively efficient C-Bus power supplies.

If your group of down-lights were to be considered to be operated at diminishing percentages of the time, then the relatively small fixed quiescent losses would become progressively more significant in comparison to the diminishing total power consumption of the lights themselves. Conversely, the higher the total consumption of the loads being controlled, and the higher the percentage of time that the loads are on at a dimmed level, the better the total efficiency and energy savings you can achieve.

To be thorough, you would need to perform a quantitative analysis on a specific system and load configuration, but you would expect the results of the quantitative analysis to yield results consistent with the above qualitative outline.

If you were properly designing a C-Bus system specifically to optimise energy savings, there are a whole host of C-Bus system and load selection design choices that you could consider making that involve the system design and programming. You would also consider including energy monitoring in the C-Bus system, so you can review energy consumption, with a view to measuring that savings that your system is achieving. Then there are other aspects to the design of the building itself, and the usage patterns of the occupants, that may make very substantial contributions to possible energy savings. Often there are much larger potential savings to be made in heating and cooling (which you may also control with C-Bus) compared to those achievable in regard to lighting alone.

 

Thanks

Hi Brendan,

Thanks for the quick reply. The driver behind my question is that I am writing code to accumulate estimated power used by each string of downlights used in my house and office. This will be used in turn to work out the priority for replacement to LED downlights.

 

Dimming incandescents doesn't save as much power as you think. At the reduced dimmer setting the filament cools and this makes it emit less light. If you dim an incandescent bulb until you have half the light output you are using significantly more than half the power. As the filament cools it becomes even less efficient at emitting visible light. A ball-park figure is that 2/3 power input = 1/2 light output.

The opposite is true of LED-based light sources. At half the light output you are actually using slightly less than half-power. This is because at lower currents and lower temperatures LEDs become a little more efficient.