# Why can't some lighting controls, like motion or light sensors, work with fluorescents? **URL:** \ **Category:** Factual Questions\ **Created:** [June 15, 2009, 11:25pm UTC](https://boards.straightdope.com/t/why-cant-some-lighting-controls-like-motion-or-light-sensors-work-with-fluorescents/499891 "2009-06-15T23:25:34Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![TheMadHun](https://avatars.discourse-cdn.com/v4/letter/t/53a042/32.png) [@TheMadHun](https://boards.straightdope.com/u/TheMadHun)\ **Post date:** [June 15, 2009, 11:25pm UTC](https://boards.straightdope.com/t/why-cant-some-lighting-controls-like-motion-or-light-sensors-work-with-fluorescents/499891/1 "2009-06-15T23:25:34Z")
Why can’t some lighting controls, like motion or light sensors, work with fluorescents? Is it because of noise on the line? A tendency to flutter on and off when changing modes? Or is it a RFI thing? ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [June 15, 2009, 11:31pm UTC](https://boards.straightdope.com/t/why-cant-some-lighting-controls-like-motion-or-light-sensors-work-with-fluorescents/499891/2 "2009-06-15T23:31:21Z")
I think it’s mostly because Fluorescent lights usually present an inductive load. Lighting controls usually use a Triac to switch the AC, and unless the circuit is designed to handle an inductive load, it won’t turn off correctly. ---
**Author:** ![HongKongFooey](https://avatars.discourse-cdn.com/v4/letter/h/c4cdca/32.png) [@HongKongFooey](https://boards.straightdope.com/u/HongKongFooey)\ **Post date:** [June 16, 2009, 12:40am UTC](https://boards.straightdope.com/t/why-cant-some-lighting-controls-like-motion-or-light-sensors-work-with-fluorescents/499891/3 "2009-06-16T00:40:50Z")
I think another reason is that fluorescent lamps are not self-limiting like resistive ones are. An incandescent filament heats up and limits the current because of the higher resistance so the control devices only see a short spike in current. Fluorescent lamps need the ballast to limit the current. The ballast has to build up to a higher voltage to cause the lamp to light in the first place before limiting the current and the controls may not be rated for that higher voltage. ---
**Author:** ![Lightnin](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lightnin/32/180_2.png) [@Lightnin](https://boards.straightdope.com/u/Lightnin)\ **Post date:** [June 16, 2009, 1:12am UTC](https://boards.straightdope.com/t/why-cant-some-lighting-controls-like-motion-or-light-sensors-work-with-fluorescents/499891/4 "2009-06-16T01:12:13Z")
Holy cow, I just ran into this situation last night. I installed a motion sensor on the lights in our laundry room, and had to replace the CFL we were using with a regular incandescent because the CFL flickered. A shame, 'cause we’re working on replacing all of our incandescents with CFLs. ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [June 16, 2009, 1:33am UTC](https://boards.straightdope.com/t/why-cant-some-lighting-controls-like-motion-or-light-sensors-work-with-fluorescents/499891/5 "2009-06-16T01:33:34Z")
In [this](http://boards.straightdope.com/sdmb/showthread.php?t=421934) thread I said, > [@](#): > > Some motion detectors use a mechanical relay. These are fine for any type of bulb. But some motion detectors use a triac. A triac-based unit is O.K. for incandescent bulbs, but it may pose a problem for fluorescent bulbs, depending on how the control circuit controls the triac. > > To be safe, if you want to use a motion detector w/ a fluorescent bulb (especially a compact fluorescent), use a detector with a mechanical relay. ---
**Author:** ![Lord\_Mondegreen](https://avatars.discourse-cdn.com/v4/letter/l/91b2a8/32.png) [@Lord\_Mondegreen](https://boards.straightdope.com/u/Lord_Mondegreen)\ **Post date:** [June 16, 2009, 1:42am UTC](https://boards.straightdope.com/t/why-cant-some-lighting-controls-like-motion-or-light-sensors-work-with-fluorescents/499891/6 "2009-06-16T01:42:03Z")
We have some meeting rooms at work which are lit by fluorescent lights (the old-fashioned type, not new CFLs). They have motion sensors and work fine. So it appears it is possible to get motion sensors to suit fluoros. Of course they are as irritating as hell because the sensor can pick up people walking past an open door, so they tend to flicker on many times a day. At least until someone gets fed up and turns them off completely. ---
**Author:** ![Tim\_T-Bonham.net](https://avatars.discourse-cdn.com/v4/letter/t/46a35a/32.png) [@Tim\_T-Bonham.net](https://boards.straightdope.com/u/Tim_T-Bonham.net)\ **Post date:** [June 16, 2009, 1:50am UTC](https://boards.straightdope.com/t/why-cant-some-lighting-controls-like-motion-or-light-sensors-work-with-fluorescents/499891/7 "2009-06-16T01:50:55Z")
> [@Lord\_Mondegreen](#): > > Of course they are as irritating as hell because the sensor can pick up people walking past an open door, so they tend to flicker on many times a day. At least until someone gets fed up and turns them off completely. Not if your electrician had positioned the sensor properly: above the door, pointing inward toward the room. ---
**Author:** ![TheMadHun](https://avatars.discourse-cdn.com/v4/letter/t/53a042/32.png) [@TheMadHun](https://boards.straightdope.com/u/TheMadHun)\ **Post date:** [June 18, 2009, 6:19pm UTC](https://boards.straightdope.com/t/why-cant-some-lighting-controls-like-motion-or-light-sensors-work-with-fluorescents/499891/8 "2009-06-18T18:19:21Z")
> [@Crafter\_Man](#): > > In [this](http://boards.straightdope.com/sdmb/showthread.php?t=421934) thread I said, > "Some motion detectors use a mechanical relay. These are fine for any type of bulb. But some motion detectors use a triac. A triac-based unit is O.K. for incandescent bulbs, but it may pose a problem for fluorescent bulbs, depending on how the control circuit controls the triac. > > To be safe, if you want to use a motion detector w/ a fluorescent bulb (especially a compact fluorescent), use a detector with a mechanical relay." That thread was quite useful. In particular that the Intermatic brand fried when put with a fluorescent bulb, which happened to me. And also that some people deny the problem exists just because it never happened to them. But now I have to ask, how can I find a mechanical type? Is there a brand perhaps? ---
**Author:** ![dracoi](https://avatars.discourse-cdn.com/v4/letter/d/90db22/32.png) [@dracoi](https://boards.straightdope.com/u/dracoi)\ **Post date:** [June 18, 2009, 6:47pm UTC](https://boards.straightdope.com/t/why-cant-some-lighting-controls-like-motion-or-light-sensors-work-with-fluorescents/499891/9 "2009-06-18T18:47:54Z")
> [@Lightnin](#): > > Holy cow, I just ran into this situation last night. I installed a motion sensor on the lights in our laundry room, and had to replace the CFL we were using with a regular incandescent because the CFL flickered. > > A shame, 'cause we’re working on replacing all of our incandescents with CFLs. From what I’ve read, you’re better off with a regular incandescent on a motion sensor anyway. CFLs do not appreciate being turned on an off frequently and wear out faster. They’re better for uses where it will be on for at least ten or fifteen minutes at a time. There was a thread about this earlier. ---
**Author:** ![Tim\_T-Bonham.net](https://avatars.discourse-cdn.com/v4/letter/t/46a35a/32.png) [@Tim\_T-Bonham.net](https://boards.straightdope.com/u/Tim_T-Bonham.net)\ **Post date:** [June 18, 2009, 8:58pm UTC](https://boards.straightdope.com/t/why-cant-some-lighting-controls-like-motion-or-light-sensors-work-with-fluorescents/499891/10 "2009-06-18T20:58:36Z")
> [@dracoi](#): > > From what I’ve read, you’re better off with a regular incandescent on a motion sensor anyway. CFLs do not appreciate being turned on an off frequently and wear out faster. They’re better for uses where it will be on for at least ten or fifteen minutes at a time. There was a thread about this earlier. Quite true. For something like a laundry room, or a closet, or any light that is only turned on for a few minutes, just leave a regular incandescent bulb there. The short period it is on only uses a small bit of electricity, and isn’t worth the effort to change it. An incandescent bulb will last for years in a location like that – when the time comes to replace it, we will have cheap, effective LED or LCD bulbs, or even something newer to replace it. ---
**Author:** ![Lord\_Mondegreen](https://avatars.discourse-cdn.com/v4/letter/l/91b2a8/32.png) [@Lord\_Mondegreen](https://boards.straightdope.com/u/Lord_Mondegreen)\ **Post date:** [June 19, 2009, 1:18am UTC](https://boards.straightdope.com/t/why-cant-some-lighting-controls-like-motion-or-light-sensors-work-with-fluorescents/499891/11 "2009-06-19T01:18:38Z")
> [@t-bonham@scc.net](#): > > Not if your electrician had positioned the sensor properly: above the door, pointing inward toward the room. What I didn’t mention was the expenditure reduction program in the company. No way we’d pay what a competent electrician would charge. ---
**Author:** ![Harry1945](https://avatars.discourse-cdn.com/v4/letter/h/e47c2d/32.png) [@Harry1945](https://boards.straightdope.com/u/Harry1945)\ **Post date:** [June 19, 2009, 1:33am UTC](https://boards.straightdope.com/t/why-cant-some-lighting-controls-like-motion-or-light-sensors-work-with-fluorescents/499891/12 "2009-06-19T01:33:47Z")
At school, our classroom flourescent lights are set up so they will turn off if there is no motion in the sensor’s field within ten or fifteen minutes. It’s a genuine pain to be sitting quietly at your desk correcting papers or working on lesson plans and have the lights go out. Most of us have desk lamps because of this. ---
**Author:** ![tomndebb](https://avatars.discourse-cdn.com/v4/letter/t/b9e5f3/32.png) [@tomndebb](https://boards.straightdope.com/u/tomndebb)\ **Post date:** [June 19, 2009, 4:13am UTC](https://boards.straightdope.com/t/why-cant-some-lighting-controls-like-motion-or-light-sensors-work-with-fluorescents/499891/13 "2009-06-19T04:13:08Z")
> [@Harry1945](#): > > At school, our classroom flourescent lights are set up so they will turn off if there is no motion in the sensor’s field within ten or fifteen minutes. It’s a genuine pain to be sitting quietly at your desk correcting papers or working on lesson plans and have the lights go out. Most of us have desk lamps because of this. The break rooms and restrooms at work have this arrangement, (along with fluorescent lighting). I have been reading a book at lunch, out of line from the sensor, and had the lights go out on me. ---
**Author:** ![Serenata67](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/serenata67/32/10646_2.png) [@Serenata67](https://boards.straightdope.com/u/Serenata67)\ **Post date:** [June 19, 2009, 4:25pm UTC](https://boards.straightdope.com/t/why-cant-some-lighting-controls-like-motion-or-light-sensors-work-with-fluorescents/499891/14 "2009-06-19T16:25:43Z")
Is this why flourescents don’t work with dimmers? I have a dimmer fixture in my dining room and put those flourescents-that-look-like-incandescents in, but they wouldn’t dim… so I had a super bright dining room. It suck; those bulbs are such an energy drain and we use the lights in that room a lot. ---
**Author:** ![Tim\_T-Bonham.net](https://avatars.discourse-cdn.com/v4/letter/t/46a35a/32.png) [@Tim\_T-Bonham.net](https://boards.straightdope.com/u/Tim_T-Bonham.net)\ **Post date:** [June 19, 2009, 10:02pm UTC](https://boards.straightdope.com/t/why-cant-some-lighting-controls-like-motion-or-light-sensors-work-with-fluorescents/499891/15 "2009-06-19T22:02:03Z")
> [@Serenata67](#): > > Is this why flourescents don’t work with dimmers? I have a dimmer fixture in my dining room and put those flourescents-that-look-like-incandescents in, but they wouldn’t dim… so I had a super bright dining room. It suck; those bulbs are such an energy drain and we use the lights in that room a lot. There are special versions of compact fluorescent lamps (CFL) that are dimmable, but the standard ones are not. You have to pay extra for dimmable CFL’s, but the savings in electricity still make it worthwhile. ---
**Author:** ![Kevbo](https://avatars.discourse-cdn.com/v4/letter/k/e47774/32.png) [@Kevbo](https://boards.straightdope.com/u/Kevbo)\ **Post date:** [June 19, 2009, 11:35pm UTC](https://boards.straightdope.com/t/why-cant-some-lighting-controls-like-motion-or-light-sensors-work-with-fluorescents/499891/16 "2009-06-19T23:35:20Z")
The function of a ballast is to keep the current through the tube constant, regardless of it’s temperature, age, and the line voltage. The dimmer lowers the line voltage, but the ballast compensates, so the the input current must increase to provide enough power for the tube. This might be enough current to damage the ballast, or the dimmer. Dimmable ballasts provide an output current proportional to input voltage. ---
**Author:** ![Balthisar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/balthisar/32/2825_2.png) [@Balthisar](https://boards.straightdope.com/u/Balthisar)\ **Post date:** [June 20, 2009, 3:59am UTC](https://boards.straightdope.com/t/why-cant-some-lighting-controls-like-motion-or-light-sensors-work-with-fluorescents/499891/17 "2009-06-20T03:59:08Z")
Maybe the ground. I can’t use my X10 switches with CFL’s for that reason. Well, dimmable ones, yet. The X10 circuit gets the neutral connection through the incandescent filament, even when the bulb is off. So aside from being an evil incandescent user, I’m also consuming energy when the switch is off. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [June 20, 2009, 4:10am UTC](https://boards.straightdope.com/t/why-cant-some-lighting-controls-like-motion-or-light-sensors-work-with-fluorescents/499891/18 "2009-06-20T04:10:40Z")
> [@Balthisar](#): > > Maybe the ground. I can’t use my X10 switches with CFL’s for that reason. Well, dimmable ones, yet. The X10 circuit gets the neutral connection through the incandescent filament, even when the bulb is off. So aside from being an evil incandescent user, I’m also consuming energy when the switch is off. I use X-10 lamp modules to control CFLs all over the house. ---
**Author:** ![sailor](https://avatars.discourse-cdn.com/v4/letter/s/a587f6/32.png) [@sailor](https://boards.straightdope.com/u/sailor)\ **Post date:** [June 20, 2009, 4:18am UTC](https://boards.straightdope.com/t/why-cant-some-lighting-controls-like-motion-or-light-sensors-work-with-fluorescents/499891/19 "2009-06-20T04:18:22Z")
> [@Kevbo](#): > > The function of a ballast is to keep the current through the tube constant, regardless of it’s temperature, age, and the line voltage. > > The dimmer lowers the line voltage, but the ballast compensates, so the the input current must increase to provide enough power for the tube. This might be enough current to damage the ballast, or the dimmer. > > Dimmable ballasts provide an output current proportional to input voltage. This is obviously not true of traditional magnetic ballasts so I will assume you are referring only to modern electronic “ballasts”. It is not true about electronic “ballasts” either. I put ballasts in quotes because they are really not ballasts at all. At any rate. The reason you standard control circuit does not work with dimmers is very simple. A dimmer is a switching circuit which changes the shape of the power voltage so that instead of being close to a sine wave it chops off the beginning and leaves the end. This changes the RMS value which is what counts with an incandescent lamp. But a fluorescent electronic control circuit works by first rectifying the line voltage and charging a capacitor so chopping of the front of the wave does nothing if the peak voltage is the same. The peak voltage is what counts here. At first it is not affected at all and then, as more of the wave is cut off, the peak voltage decreases and the result is that the bulb my decrease a bit in brightness but soon the circuit is not oscillating as it should and so the bulb first flickers and then dies. Simple circuits contained in the socket do not contain any stabilization circuitry although electronic “ballasts” for long tubes may contain them, mostly as an indirect result of power factor correction circuitry. ---
**Author:** ![Balthisar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/balthisar/32/2825_2.png) [@Balthisar](https://boards.straightdope.com/u/Balthisar)\ **Post date:** [June 20, 2009, 2:53pm UTC](https://boards.straightdope.com/t/why-cant-some-lighting-controls-like-motion-or-light-sensors-work-with-fluorescents/499891/20 "2009-06-20T14:53:56Z")
> [@beowulff](#): > > I use X-10 lamp modules to control CFLs all over the house. Lamp modules are self contained. I should have specifically mentioned light switches. [Next page](https://boards.straightdope.com/t/why-cant-some-lighting-controls-like-motion-or-light-sensors-work-with-fluorescents/499891.md?page=2)