Electrical – LEDs in a multi-bulb fixture only dim when installed alongside a single 60W incandescent bulb

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I have a 'Sputnik'-style ceiling light fixture that accepts about 12 bulbs. I installed 12 LED bulbs into the fixture. However, the fixture was just too bright with 12 LEDs, so I installed a dimmer switch later that evening. The LED bulbs are supposed to be dimmable and the Amazon reviews for the LEDs indicated that people were having success with Lutron and Leviton dimmers. I bought a Leviton Decora smart dimmer.

After installing the dimmer, I found that that the LED bulbs would not dim, they would only turn on or off. I verified that the Leviton app had the bulb type set to 'LED'. To troubleshoot, I decided to remove one of the LED bulbs and replace it with a 60W incandescent bulb. I knew that the incandescent bulb would be capable of dimming.

To my surprise, not only did the incandescent bulb dim, but the other LEDs started dimming as well. If I remove the incandescent, then the LEDs stop dimming. If I install a 20W incandescent, then the LEDs do not dim. It's only when I install the 60W incandescent that all of the LEDs start dimming properly.

How is it possible that the LEDs only dim when a single 60W incandescent bulb is installed in the same fixture?

Edit: these are the LED bulbs (6W each) and this is the Leviton Decora dimmer switch (300W max for LEDs, 600W max for incandescent.

Best Answer

In general, dimmers have both a minimum and a maximum load. The most common reason for low loads not to dim, is that they are below the minimum for the dimmer to function. By adding more load, the dimmer begins to function properly. This is further explained by lighting manufacturer Lighting Services Inc (LSI):

Minimum Loads

Dimmers have minimum load requirements because if the LED load isn’t drawing enough current to keep the dimmer’s switching elements closed, the dimmer will have unpredictable behavior. This behavior can include flickering, reduced dimming range, flashing when off, or not even working at all.

Dimmers that are designed for incandescent loads tend to have a larger minimum load requirement because incandescent fixtures generally have higher wattages than LED fixtures. This means that these dimmers will most likely require multiple LED fixtures to meet the minimum load requirement. Testing is necessary to determine the exact number of LED fixtures needed for each incandescent dimmer. Dimmers that are designed for LED loads usually have a lower minimum load requirement and typically only require one or two fixtures. The LED minimum load rating for LED load dimmers is usually given in number of lamps instead of Watts (W) or Volt Amps (VA).

According to Leviton's product page for the DW6HD-1BZ wi-fi dimmer:

It uses the latest generation of Leviton’s dimming technology for smart lighting control for a single LED bulb in a hallway or a strand of holiday lights, ensuring LEDs do not glow when off.

Based on this, it should be possible to perform low-range dimming of low-power LED loads with this dimmer.

The instruction sheet that accompanies the dimmer refers you to a list of compatible LED and CFL bulbs on Leviton's website, a 2015 version of which I found here: http://communities.leviton.com/servlet/JiveServlet/previewBody/4385-102-1-7048/Decora%20Rocker%20Slide%20Dimmer%20Compatibility%20Chart.pdf

Based on the Amazon product title for the bulbs you purchased, which contains the grammatically-incorrect phrase, "Light For Decorate Home", I can assume that these bulbs will not be from one of the manufacturers Leviton have tested and confirmed as compatible.

That being said, the same page lists Leviton IPL06-10Z, DSL06 and VP0SR-1LZ dimmers as compatible with those bulbs. Given that Leviton is a well-respected brand, my recommendation would be to trust their statements over those of the blb vendor/manufacturer and purchase new bulbs that Leviton lists as compatible. If you do not wish to do this, you may choose instead to contact Leviton and seek technical support based on the dimmer's inability to meet the description above.

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