Replacing an incandescent bulb with an LED causes another LED to be less bright

Question

I have a set of 2 recessed bulbs controlled by the same switch. After one of them burned out (it was an incandescent bulb), I replaced it with an LED bulb, which worked fine. Right after, I replaced the other incandescent bulb in the set with another LED from the same box, and now both bulbs are significantly less bright than the first LED was on its own when it was paired with the incandescent bulb. The first LED is also less bright when there is no bulb in the second socket, and in both cases the LED bulbs take longer to turn on than the LED plus incandescent set did. What could be causing this, and will I need an electrician to address it?

The are connected to a dimmer switch, and the replacement bulbs are both 65W equivalent dimmable LED bulbs.

Answer 1

It's the 'switch'

It's a timer, dimmer, motion sensor, smart switch, lighted switch, or other sort of dingus which requires electricity to power its own internal functions.

Normally when you want to power a thing, you grab always-hot and neutral, typically black and white at most receptacles. However, there is a thing called a switch loop which provides only always-hot and switched-hot to a switch; not neutral.

To cope with switch loops, the heritage way to power a "powered switch" was to leak a small amount of current through the incandescent bulb. Which by nature don't mind that. The current is too little to make them flicker or glow.

However now in the LED age, that does not work.

Our classic litmus test for this, in multi-bulb fixtures, is to use all LEDs except make 1 bulb a real incandescent. if the problem vanishes when you do that, then the above is the cause. Of course, you've already done this test in reverse, and got a positive (in the medical sense) result.

So cure #1 is come up with a path for the leakage current. Either use LEDs designed for this, typically marked as "dimmable", or add a LUT-MLC capacitor module which will do the same thing. Cure #2 is to modernize the "switch" or just get rid of it.