July/August 2018
What makes some LED replacement bulbs dimmable, while others are not? Some of the replacements I have purchased do not dim very smoothly. They seem to dim in steps and then just turn off before lowering to where I need them to be. The previous incandesants dimmed much more smoothly down to a soft glow. Is there a different, (maybe more expensive) type/technology that would more closely emulate the incandesants? I'm using an X10 controller for the dimming, could this be the problem?
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There is no such thing as a dimmable LED. The way we get around this is by using Pulse Width Modulation (PWM), which means the LED is turned on for shorter and shorter periods of time within a time frame. You can see that if the LED is turned on for 100% of the time, say 1/100th of a second, then it is on for 10,000 1/10,000ths of a second, the LED will be only 1/100th as "bright" if it is turned down to being on only 1/10,000th of a second for every 1/100th of a second.
I used 1/100th of a second as the time frame because the human eye can't see flickering when the flicker rate is around 1/50th of a second, this is why old analog TV's in the US used 1/60th of a second to build a picture frame, and in Europe they used 1/50th of a second for their TVs.
You can also think about this using a 1 second time frame if you want. In this case when the LED is on only 1/100th of a second for every second of time frame it would appear only 1/100th as bright. The problem here is that we would see the LED turning on and off, so we would lose the appearance of the LED dimming. We have to use a time frame that the human eye can't "see" such as a time frame of 0.01 (1/100th) seconds.
A LED is a beautiful dimmable device; lower the current, and the output brightness goes down. This can be done through simple resistors, or through PWM (Pulse Width Modulation). That is the good news.
In order to use a LED as a house incandescent lamp replacement, something needs to be done to make that possible. First of all, the voltage is WAY to high, it is AC (the LED prefers DC), and we need to feed it the correct current. So a power supply module is included to convert the 120 (or 220) Volt AC to some small DC current appropriate for the LED used.
Herein lies the problem. This PS module is designed to deliver x mA to the LED more or less regardless of the input. When you adjust the input, the PS module tries to compensate, to the point where it no longer can, often resulting in a flickering light. These are the now non-dimmable LED bulbs.
To make this “house” LED bulb dimmable, a radically different PS module must be built. It must sense that the input has changed (due to the dimmer setting), and adjust the LED current accordingly. Not easy to do! and of course, more expensive.
Several designs have been marketed, some better than others. In most cases, even with a “dimmable” LED bulb, a special dimmer is required. It is a mess, and most still don’t work perfectly.
I can’t answer specifically what is different about dimmable LED lamps, but will note that the type of dimmer is important. You need to use a dimmer specifically designed for LED lamps.
I have had good luck with Lutron CL line of dimmers that are made specifically for LED lamps. These are not X10, I have not seen any X10 dimmers for LED lamps. Also note that X10 light controls generally only work for incandescent lamps, they need a bit of current flowing through the filament to work correctly.
For LED lamps I have used the relay based X10 switch WS13A with good success, it also works with CFLs.
Different bulbs behave differently so you have try some options. There are some test results on the X10 Forum that you might find useful. X10 dimmers dim in 16 levels so some bulbs may not dim smoothly.
