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When dimming LED lights with an LED dimmer, why do the LED lights at the end of a string of lights go off when LED lights at the beginning of a string of lights go dim but stay lit?

This application is in a kitchen. The LED dimmers were a Lutron Diva C.L Single-Pole/3-Way CFL/LED Dimmer White purchased at Menards. The LED bulbs were Exclusive EcoSmart 60-Watt Equivalent A19 Non-Dimmable LED Light Bulb Daylight (4-Pack)Model# B7A19A60WUL34 purchased at Home Depot. One dimmer controlled 4 bulbs from one location. When dimmed, the 2 bulbs in the string of lights closest to the dimmer did dim to a minimum but the 2 bulbs in the string of lights farthest from the dimmer shut off completely. The other dimmer controlled 8 bulbs from one location. When dimmed, the 6 bulbs in the string of lights closest to the dimmer did dim to a minimum but the 2 bulbs in the string of lights farthest from the dimmer shut off completely. All of the bulbs used were exactly the same. Perhaps the bulbs were not compatible with the dimmers. Originally I purchased the dimmers at Menards closer to my office. I installed the dimmers using the existing LED bulbs, not knowing they were the kind that would not dim. I then purchased the LED dimmable bulbs from Home Depot closer to home.

  • Can you provide more details about the application? This sounds like it might be a better fit for the Electrical Engineering SE, if you're talking about consumer electronics instead of residential LED lighting. The answer also depends on the type of dimmer you're using. – Hari Ganti Feb 27 at 21:46
  • @HariGanti, use of consumer electronics is specifically off-topic at EE.SE. – brhans Feb 27 at 21:49
  • @brhans "except when designing these products or modifying their electronics for other uses." Hobbyist electronics questions are pretty common there, including plenty of questions about LED strips, and this question leads me to believe it fits within the scope of "designing" or "modifying." Understandably, it's not about how to turn on your cell phone – Hari Ganti Feb 27 at 21:51
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    Hi, Russ. You now have two separate accounts; please use the SE contact form to request that they be merged. – Daniel Griscom Feb 27 at 23:46
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Because it's not enough to have an LED rated dimmer. You also need dimmer-rated LEDs.

What's happening is the dimmer needs to power itself. Some dimmers power themselves by using a neutral wire. But those which don't, power themselves by leaking a small amount of current through the light bulbs. Historically, incandescent bulbs didn't mind that. However, the more efficient LEDs see a small trickle of power as a command to "come on slightly". Many LEDs made for dimming have an extra circuit to cope with this.

You should also check your dimmer to see if it has a "hard off setting". Some dimmers have a smooth range in the dimming zone, but if you push harder, you will click past a detent, and into a "hard off" setting. That setting is to allow you to safely change light bulbs without getting nailed.

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The short answer without getting into the electrical engineering aspect is that not all LED bulbs are created equal, even if they say "dimmable" on them. There are literally thousands of variations on the electronics inside that make them work; what's called the "driver", which is an AC to DC power supply that provides low voltage DC for the LEDs themselves. But everyone has their own "secret sauce" so to speak on how to make those power supplies. So the effects of a dimmer, and what TYPE of dimmer, have different effects on different bulbs. The best advice if you don't want to live with this is to always use the EXACT same LED bulbs on any dimmed circuit; brand name, model, rating everything exactly the same.

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