In recent years the use of LED fixtures and bulbs to replace incandescent bulbs has become widespread. Most modern LED bulbs and fixtures make the claim that they are dimmable, using conventional triac-based dimmers.
A variety of problems arise when using triac-based dimmers to dim LED fixtures.
They will not dim to low levels as incandescent bulbs will.
They may flicker unacceptably especially at lower levels.
They may not turn off all the way.
They may stay on 100% and not dim at all.
All of these problems relate to the fact that the conventional triac-based dimming circuit is designed to dim a relatively large resistive load, like an incandescent light bulb. Most 600 W wall dimmers have a specified minimum load that is typically 60 watts.
LED lights use a type of switching power supply that is very different that a resistive load. There are also many different types of LED fixtures that use power supplies that are very different. It is impossible for any dimmer manufacturer to design a dimmer that will work with all LEDs. It is impossible for a LED manufacturer to design a dimmable LED fixture that will work with all dimmers. If a homeowner or electrical contractor chooses a specific LED fixture and specific wall dimmer that is always some possibility the combination will have one or more of the problems listed above.
It is commonly known that adding an additional light bulb in parallel with the LED fixture may improve the dimming and may eliminate or reduce some of the problems listed above.
There are problems with this solution. We have discovered that the lower the impedance of the added load the more the load reduces the dimming problems. For 120 VAC fixtures a load of 10K helps marginally. A load of 5K ohms helps a little more. A load of 1K ohms helps greatly. The problem with lower impedance loads is that the power dissipated when the dimmer is on to a high level is too great. A 100% dim level a 10K load will consume 1.44 W, and a 5K load will consume 2.88 W, and 2K will consume 7.2 W and 1.0K will consume 14.4 W.
The most straightforward inexpensive place to install the load resistor is behind the dimmer switch in the single gang box.
A 600 W triac-based dimmer will dissipate 5 W in a single-gang US electrical box at full load. Adding a load resistor to stabilize the LED dimming in the switch box behind the switch is the best solution. Adding a 2 Watt load is OK since 2.0 W in addition to the maximum 5 W dissipated by the dimming switch is acceptable. Adding a more than 2.5 watts is NOT acceptable because more than 2.5 Watts in addition to the maximum 5 W dissipated by the dimming switch is adding over 50% heat dissipation. And adding a 2K (7.2 watts) or 1K (14.4 watts) resistor is completely unacceptable and dangerous.
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