LED bulbs and dimmer switches
September 15, 2018 6:37 PM   Subscribe

I have 3 light bulbs on a switch with a dimmer. I accidentally bought non-dimmable LED bulbs to replace the incandescents, turning my entryway into a discotheque (warning: strobing lights). However, when I put in a single incandescent and 2 LEDs, all the lights function fine - the LEDs take a half second longer to switch on, but no discernable flickering after that. Why does this work? Is it okay to keep them like this, or do I need to go back to the store and buy some dimmable bulbs? (I never use the dimmer so this is is a slightly annoying extra cost for me)
posted by btfreek to Home & Garden (9 answers total) 3 users marked this as a favorite
Update: Since posting this question I have been convinced by some very sensible people to just go ahead and buy the correct dang light bulbs. HOWEVER I am still curious as to why the 2 LED/1 incandescent setup worked, if anyone has an explanation!
posted by btfreek at 6:54 PM on September 15, 2018

The reason is that dimmers have a minimum load current for them to operate properly. Some dimmers have different circuity design and may have different minimum load requirements. This has to do with the operation of triacs. If you get the spec sheet for a new dimmer, it will tell you the minimum load current it needs to function. The three LEDs don't draw the necessary minimum current. By putting in one incandescent, you meet the minimum current requirements and the dimmer works properly.

So if it is working okay, and there is no annoying flickering, then you are okay. The ultimate solution is either to get dimmable LEDs or to replace the dimmer switch with a regular switch since you don't need the dimmer.

Replacing a regular light switch can cost less than a dollar, if you are competent in minor electrical work.
posted by JackFlash at 7:09 PM on September 15, 2018 [4 favorites]

However, even if you get so-called dimmable LEDs, there is no guarantee that they will work with your old dimmer. Some older dimmers just won't work with LEDs.
posted by JackFlash at 7:11 PM on September 15, 2018 [2 favorites]

Thanks! Unfortunately I am renting so I don't think my landlords would appreciate me doing any electrical work, however minor. I will keep my 2/3 LED setup until get my dimmable bulbs, and cross my fingers they work.
posted by btfreek at 8:18 PM on September 15, 2018

As you experienced, LED's don't work well with dimmers that aren't designed for them.

If you are renting, and still have the opportunity to return them, I would do that, to avoid being on the hook later if the landlord decides they need to be replaced again later.

An alternative might be to switch the new LED's with other places in the house?
posted by Gomez_in_the_South at 10:51 PM on September 15, 2018

By that logic, you could try buying a dimmable LED instead of the incandescent - it might have enough load to fool the dimmer into cooperating.
posted by a halcyon day at 11:33 PM on September 15, 2018

you could try buying a dimmable LED instead of the incandescent

Dimmable LED bulbs won't be loading your conventional dimmer any higher than a non-dimmable, so that's a waste of effort. LED bulbs contain a downconverter[1] to change the mains voltage to a level that the LEDs requires (couple of volts per LED element). Standard LED bulbs contain just that downconverter, which, by design, doesn't care much about the input voltage. Within limits, that is, but most standard LED bulbs are specified for 85-260VAC, and put out the same brightness over that range. Dimmables have an extra circuit that detects it's on a dimmed feed, and adjusts the power to the LEDs accordingly.

[1] A stonking big resistor would do the job too, but that would do away with a large part of a LED bulb's energy savings. It would make them dimmable, and working fine on conventional dimmer circuits.
posted by Stoneshop at 12:01 AM on September 16, 2018

Interesting - I swapped in some dimmable (?) LEDs from other fixtures to replace the incandescents and it (2 non-dimmable 1 dimmable) seems to have worked. What gives?
posted by btfreek at 3:41 PM on September 16, 2018

Same as before. You have an old dimmer that was not designed for LEDs. Typically these older dimmers require a load of at least 25 to 35 watts or more to turn on the triac that controls the switch. Your three original bulbs added together might be right at the edge where substituting a different brand with slightly different loading might make a difference. (It appears that the bulb in the picture is rated at 7.5W. It would be interesting to know what the other two look like.)

The other thing is that LED bulbs represent an inductive load and those older dimmers represent an capacitive load on the trigger input to the switch. When you have inductors and capacitors in the same circuit, they can interact is complicated ways. But it probably just comes down to needing a minimum current.

It seems you have found a combination that works for you. I wouldn't worry about any safety issues. You just want to avoid annoying visual flickering.

I wouldn't be surprised that if you try to use the dimmer in other settings except full brightness, that flickering would return.
posted by JackFlash at 4:29 PM on September 16, 2018

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