Excuse my ignorance in the subject, below i will try to explain what i have and what i am trying to achieve.

In my living room I have about 15 led dimmable recessed lights on a dimmable switch.

From my research installing a motor in a dimmable switch is the recipe for disaster. So I wont install a ceiling fan because having dimmable lights is more important to me. Unless there is some sort of switch based on voltage that I can install on the individual light. That way the fan will only turn on when the dimmer is all the way up. I read of a device called voltage comparator but I am not clear on how it solves my issue.

Putting the fan aside, it will also be nice if when i dim the lights down some of them turn off, and on the way up they turn on when a voltage is reached.

If anyone knows of anything that will be able to reach my goals please let me know. Otherwise if there is a flaw on what i am asking for let me know also so I can learn more about the subject

  • 1
    You should look up how a modern dimmer works. They don't simply "reduce the voltage" - it's more complicated than that and a simple voltage test probably wouldn't give the results you want.
    – JPhi1618
    Commented Feb 12, 2020 at 19:56
  • You wouldn't want the fan to slow down when you dim the lights, anyway. You'd want independent control. Commented Feb 12, 2020 at 20:24

1 Answer 1


How dimmers work

It sounds like you have some very common misconceptions of how dimmers work. The usual assumption is they reduce the voltage; however that would require either a) a Variac (about a 30 pound transformer bigger than a soda can), or b) a large power resistor about the size of a shoe box, that makes lots and lots of heat.

That's not how it works at all. Actually, the dimmer supplies full voltage, but only part of the time. It waits until a certain point in the AC cycle, then turns the power on full. As the AC voltage follows the sinewave to zero, the switch shuts off automatically. Then, the dimmer waits a period and turns it on again. So the light only sees part of the sinewave. This scheme is done to keep the dimmer lightweight, small and cheap, so it's built around a 30 cent triac instead of a $40 transformer.

Motor speed control is a different kettle of fish

You can see where that would make a motor very, very angry. Further, many motors are designed to go one speed: the speed of the AC frequency, and to regulate their speed you need a Variable Frequency Drive, a sophisticated piece of electronics. So trimming voltage but keeping frequency just imbalances the motor and makes it hot and inefficient.

There are other schemes to control other types of motors, but none of them are anything like dimming.

So yes. Dimmers are for lights; motor speed controllers are for motors. Hence the names of the devices.

You can use any UL-listed control scheme that works

You are perfectly welcome to have two switches side by side; one being a dimmer and the other being a fan speed controller, each feeding its respective devices. That would, obviously, require a 2-gang box and some more cabling fished in the walls, but it's straightforward work for any competent electrician of the "specialize in fishing cables through walls" variety. That electrician could also make sure to install a fan-rated junction box which is capable of bearing the extreme vibration loads that will pull the nails out of a normal box, or metal-fatigue it.

Another option, these days, is to hardwire hot to the ceiling, and use "smart" fan and light controllers to respond to a smart switch. However, this setup must retain what every room in America has: a light switch in the usual location that a complete novice can turn on. That is to benefit guests, particularly EMTs, cops and firemen who need lights turned on fast to intubate you, clear the room, etc. It can be a smart switch; it just has to work.

  • 1
    Thank you, for taking the time to educate me in the subject.
    – DonO
    Commented Feb 12, 2020 at 23:25

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