LED Dimming Problems
Yesterday I visited friends in their beautiful new home. As we were walking through the house it was obvious that there were some really irritating issues with their lighting. There was flickering, humming and buzzing going on in several locations. Of course they looked at me, sighing, and asked what they could do to fix it. How I wish I could just have played Superwoman, gotten on a ladder and fixed the problems on the spot. But unfortunately it was the dreaded LED dimming issue, a more fickle situation. LEDs are definitely dimmable. The problem is just that they often don't play well with conventional dimmers, no matter what the label on the box of the LED lamp says.
The only choices we have have historically faced when we went to the store to choose a dimmer were color and style. With the emerging LED technology this has changed overnight. There are the few lucky cases where LED lamps will operate with an incandescent dimmer, but in general you'll experience inconsistent performance if you use standard incandescent dimmers with LED lighting.
The dimmers that are found all over in millions of homes are so-called phase-cut dimmers that were developed for incandescent lamps. They might not be suitable and will very likely interfere with the performance of the LEDs. As the first step in ensuring dimming performance with your LEDs you will need to exchange your traditional dimmer for one that is built for LED dimming. The incandescent switching cycle is is built around the fluctuation of our household alternating current (AC) that is constantly changing direction.
The dimmer actually doesn't dim anything, but rather shuts the circuit off every time the current reverses direction and there is zero voltage running through the circuit. This happens so quickly (120 times a second) that it is not noticeable to the human eye. When the dimmer is turned to a brighter setting, it only cuts off very briefly, while it stays off longer when it is turned to a lower setting. Since the filaments take a while to cool down when the dimmer removes part of the input AC voltage the light doesn’t vary much over the 60-Hz cycle and the dimming appears smooth and even.
LEDs are operated with a constant current, where the electricity only flows in one direction. A driver is used to convert the AC power to low-voltage DC power, regulating the current flow direction and keeping the average current at a constant level. Since the driver circuits are optimized for full-power it is unfortunately still common to see flickering and strobing at low dimming levels. Compatibility between the LED driver and the dimmer and, for low-voltage circuits, the transformer is crucial. There is no filament in LEDs that glows, because they are solid state devices. LEDs are either on or off. So for dimming purposes the LEDs are turned on and off at very high frequency, the so called Pulse Width Modulation (PWM.) The longer the on periods the brighter we see the LEDs.
LEDs by nature react very quickly to changes in the current. That's one of the reasons that even a slight dimmer or transformer incompatibility can cause a lot of unpleasant side effects.
This goes for LED lamps (bulbs) that have integral non-replaceable drivers as well as LED light fixtures like pendants, cove lights and rail systems. Some dim perfectly while others exhibit an erratic range of issues from drop out (when there's no light output at the bottom of the dimming range), pop on (where the dimmer needs to be raised higher to get the light to turn on), buzzing, flickering and color shifts.
The frustrating thing is that there are few - if any - regulations as to what the manufacturers can claim on on the label of the lamps, so "dimmable" is a wide and vague term. Does it mean that the lamp dims down to 20%, which might be OK for an office or to 1% which you'd want in your living room? Does it mean that it will dim with any dimmer? - Not really.
Unfortunately the only real way to know if a product will dim without problems is to test it out.
If it's an LED bulb with its own built-in driver it's easy to test. If it flickers at low settings (and you now obviously can't return it to the store) move it to the utility room or any other place where you can use it without dimming. But what about a rail system with multiple heads and pendants? It is obviously not an option to spend $1000+ and then just hope for the best.
For the last years we have been so frustrated over this issue. We have been bombarding our LED lamp, dimmer as well as transformer manufacturers with questions, and it was always "the other guy's" fault when we had compatibility issues.
We therefore started testing the products we specify on our jobs on our own and have learned a lot - and are saving our clients and ourselves a lot of headaches. Once you know which dimmers work well with which lamps and transformers and load, you are fine, it's getting there that is a headache. Totally impractical for all do-it-yourself'ers, but just be bold and ask . Ask us or your local lighting experts. Dimmer manufacturers like Lutron have published compatibility charts. The problem is just that they can't include all LED lamps on the market.
Maximum Load for LED Drivers and Controls
One issue that popped up when we started testing LED lamps (bulbs) in all our rail systems was the maximum load. We were used to adding up the watts to determine the size of the transformer. So, for six fixtures with 50W MR16s you'd need a 300W transformer. So, it was a logical conclusion that we could get away with the smaller sized and less expensive 75W transformer if we used six 9W MR16s. Right? Well, no. Not if you want to dim the lamps! While these LEDs only draw 9W continuously they have a start-up inrush current that acts like a higher wattage load. This means that you might overload and "blow" the transformer. It also causes stress on the dimmer and could very well damage it in the process.
This kind of compatibility issues can be costly to fix. Been there, done that. Better to check compatibility from the get-go. So, we choose the transformer as if we used all 50W lamps in the fixtures if we want to dim them.
Another detail we have become aware of is the minimum load. Some electronic low voltage transformers just don't like to dim down to less that 10 or 20%. When we order low voltage track heads for LED lamps we specify that we want them with a 1% minimum load. Yet another thing that probably won't be necessary in the near future, but I am mentioning it anyway.
