Five Dimming Pitfalls to Avoid

a guest post by Lisa J. Reed, Lighting Designer

Five Dimming Pitfalls to Avoid

We recently replaced two incandescent can lights in our bathroom with some LED retrofit cans. While the upgraded lighting looks beautiful, the controls are causing an ongoing headache.

At first our existing dimmers couldn’t control the new LED lights at all. The lights never turned all the way off, and raising the dimmer actually lowered the light level. We decided to replace our existing dimmers with the brand recommended in the new lights’ instructions – novel concept, read the instructions, right? Once the dimmers were replaced, we did have a modest level of success: the new lights can now be switched off, and they raise and lower appropriately with the dimmer. Unfortunately, our trouble-shooting days are not yet over. Now the lights strobe at low levels and they even strobe sometimes when the dimmer is raised to 100%.

Dimming has always added a level of complexity to lighting – even before the rise of the LED. Some common dimming problems include:

  1. Audible hum. With incandescent lighting the filament itself can hum when lights are dimmed to very low levels. With low voltage, transformers can be the source of audible noise. LED lights can hum on dimmers, too. It is important to carefully check dimmer compatibility with the LED driver and/or any transformers present in your light fixtures.
  2. Flicker. Besides “flicker” I’m really referring to a whole category of dimming challenges including the slow strobe effects, popping on, or dropping out before lights are dimmed to zero. The slow strobe was illustrated in the video above. Popping on happens when lights are dimmed to a low level and then switched off. When switched on at that low level, they won’t come on. The dimmer switch must be slowly raised until the lights “pop” on. Then they can be re-dimmed. This problem is common with but not exclusive to LED lights.
  3. Dimming range. This is another dimming challenge that is not new to LED. Fluorescent lights can be dimmed down to about 10% or maybe 5% or possibly even 1%, but not all the way to 0%. LED lights pose similar challenges. Check the low-end of the dimmability of your LED fixture, and remember that when a fixture is dimmed to 10%, the human eye adapts so that the light only appears to be dimmed to about 35%.
  4. Color shift. You might think that this is some kind of new LED problem, but not so. Incandescent lights have always shifter to a warmer color when dimmed. Metal halide lights have never been easy to dim, and when they are dimmed, the color tends to shift to blue. With LED, the lack of color shift is sometimes viewed as a problem.
  5. Number of fixtures. Dimmers are rated for a specific wattage. While this was straightforward with incandescent products, it can’t be calculated the same way with LED. A 500-watt dimmer can’t necessarily control fifty 10-watt LED fixtures! Inrush current from the LEDs can stress the dimmer.
    Conversely, sometimes dimmers require minimum wattages, too. This may mean that two LED fixtures on a single dimmer won’t work. The wattage may actually be too low for proper dimmer function. Consider a minimum of four LED fixtures per dimmer to be safe.

Read more about LED dimming. In the meantime, I’ll be trying to trouble-shoot my own dimming problem at home!

Lisa J Reed photo - Lighting Blog

Lisa J. Reed has been attracted to lighting (like a moth to a flame) for 20+ years. She is the Founding Principal at Envision Lighting Design, LLC in St. Louis, where she designs, teaches, and writes about architectural lighting.


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