Now the lights are TOO dim.
December 3, 2010 4:44 PM   Subscribe

Why do my dimmer switches dim so quickly? (I couldn't think of a better way to word it)

We got tired of the CFL recessed lights in our TV room, so I finally went and bought regular bulbs and two dimmer switches and installed them. They work great except that when you dim the lights, they are off by the time you have the dimmer switch halfway down the panel. It makes it difficult to increment properly because to get a slight dim you have to move the switch just the tiniest bit. They are 65 watt flood spot lights. Would getting more powerful bulbs solve this problem? Does it have something to do with the quality of the switches? I hope not, they were $22 a pop and I rather hope I don't have to get even more expensive ones!
posted by InsanePenguin to Home & Garden (14 answers total)
Response by poster: I just realized that I may not have bought the specifically "dimmable" bulbs. Well. That was a waste of a question.
posted by InsanePenguin at 4:49 PM on December 3, 2010

If your "regular" bulbs are incandescent (not CFL) then they should work with a dimmer. The switches sound like the culprit to me.

Maybe you can return the dimmers and get a different type?
posted by anadem at 4:51 PM on December 3, 2010

If they are regular lightbulbs, they are dimmable. If they are really 65 watt bulbs, I'm pretty sure you are in good shape. but it is possible they are CFLs in a spotlight form factor, and the packaging was misleading.

Look at the rating of the dimmer switches. If you have 130 watts on a dimmer capable of 750 watts, that might be the issue. Bulbs with higher wattage would solve the problem, sort of, but you'd spend more time dimmed, and that might not be good for the longevity of the bulbs or the switches.

Also, the switches you replaced weren't three-way switches, were they? Because two dimmers on one circuit probably won't work right. Might not even be possible. Each dimmer controls a different set of lights, right?

Not sure about any of this.
posted by gjc at 5:15 PM on December 3, 2010

Are these the dimmers that look like regular switches? I just remembered that I had the same problem. It buzzed a lot too. Might just be how they are made. An old school round one worked better for me.

There is another kind of dimmer switch that might be preferable if the round ones aren't to your liking. There is a little slider next to a regular switch, where on and off are controlled separately from the dimming.
posted by gjc at 5:17 PM on December 3, 2010

Response by poster: They are definitely regular bulbs, the rating on the dimmer switches is 600w, way higher than I need (180w) but I couldn't find any switches with a lower rating. It's two dimmers on the same set of lights, the oh-so-helpful Home Depot man said it would be fine because they are 3-way switches. I trust he was wrong. I will replace one of them with a regular switch tomorrow to see if it fixes the problem.
posted by InsanePenguin at 5:25 PM on December 3, 2010

Best answer: So you have two dimmer switches both controlling the same set of lights? If so, I can see how that might pose a problem. I think you would only need one dimmer and leave the other switch as a plain old toggle.
posted by Nothlit at 5:45 PM on December 3, 2010

Response by poster: Nothlit, that's right. I'll try switching it back and seeing how it is.
posted by InsanePenguin at 5:52 PM on December 3, 2010

Lutron specifically makes three-pole dimmers, BUT there's a master unit and a slave unit. I suspect that you've got two masters...or two slaves. (and as gjc mentioned, a lot of the lutron's are "intelligent" dimmers, but you would have paid a premium for those I suspect and would thus know about those settings.)
posted by TomMelee at 6:01 PM on December 3, 2010

Best answer: Yer typical household dimmer switches are phase-control dimmers, which should work fine even if rated for a higher wattage than their load. I suspect nothlit is right and the problem is having two dimmers on one circuit.
posted by hattifattener at 6:05 PM on December 3, 2010

Response by poster: All is well, I replaced one of dimmers with a regular switch. We're good. Thanks all!
posted by InsanePenguin at 6:21 PM on December 3, 2010

lower watt bulbs will dim better than higher watt ones.

The dimmer switch is a rheostat - which is a variable resistor. The increments of a rheostat will vary in proportion to the load, and higher wattage will result in a higher load to be modulated. Start with the lowest wattage incandescent bulbs you can handle. Buzzing from a rheostat is potentially problematic - it means that some of the energy is being converted to mechanical energy (fibrillation).
posted by yesster at 6:51 PM on December 3, 2010

Best answer: yesster: you've conflated two different types of dimmer there - old (and I mean "old", as in "haven't seen one newer than 30 years old except in very specific uses") style rheostat dimmers, and phase-control (effectively, pulse width modulation) dimmers. The old style did largely act as you describe (sans the word "modulated"); the rheostat itself was in series with the globe and, because the entire globe current runs through the windings, had to be a) rated to suit (i.e. capable of dissipating 65W of heat for a 65W globe at near-max brightness), and b) therefore big, hot, and not really suitable for domestic use. Variac / autotransformer dimmers still exist but, again, they're impractical for most domestic uses.

Modern domestic dimmers use phase control. Picture an AC waveform swinging above and below 0 volts - to control the power (& therefore brightness), the dimmer acts as a switch that turns off at 0V and turns back on later in the half-cycle. A potentiometer (wired as a rheostat, yes) controls the timing of that (by adjusting the voltage to a diac which, upon reaching its breakdown voltage, turns a triac hard-on until the end of the half cycle when the voltage across it drops to 0V and it stops conducting). The same thing happens during the next half-cycle, and so on, and so on - effectively, the globe is only powered for part of each half-cycle (almost all for full brightness; just a little bit for low brightness). The thermal mass of the filament averages this out and minimises/eliminates any flicker the switching causes.

The upshot of this is that the potentiomer/rheostat doesn't carry the entire current - only the few milliamps needed to turn on the gate of the triac - and therefore can be a normal carbon track pot (which doesn't have windings to buzz), and the whole circuit can be built into a block the size of a sugar cube or two. The other advantage of the phase control type is that they're pretty much immune to current variations due to the size of the load (the globe rating) - as long as the triac has enough current flowing through it to latch, and less than its maximum rating, there'll be almost no difference in operation (e.g. halfway on the control = half brightness for any given globe, etc). A typical value for minimum gate current would be something like 100mA, equating to a minimum globe rating of about 10W; a typical maximum triac rating in household dimmers would be 5A, equating to about a 600W globe.

Lower wattage bulbs will tend to dim differently (not necessarily "better") than higher wattage ones, but with phase control dimmers that's almost entirely due to the differences in filament structure. With rheostat dimmers it's a different story: stick a 65W globe on a 100W rheostat dimmer, and it'll actually dim worse - the range between "off" and "full brightness" will be cramped up towards the "full brightness" end of the control, compared to a 100W globe on the same dimmer.

The OP's problem was due to the 2 dimmers interfering with each other - either because they were wired in series and the second couldn't properly control the chopped waveform it was seeing from the first, or because they were wired in parallel and switching hash (because even at full brightness they can't switch right on the 0V-crossing point) meant that each interfered with the triggering of the other.
posted by Pinback at 8:25 PM on December 3, 2010 [6 favorites]

Response by poster: Thanks Pinback! I love big write-ups like that!
posted by InsanePenguin at 4:27 AM on December 4, 2010

Huh. That's probably why all the dimmers in my girlfriend's rental house are weird -- there's a lot of three way dimmers, and I suspect that (based on how screwed up everything else in the house is) why ALL of the dimmers are odd like that.
posted by SpecialK at 1:24 PM on December 4, 2010

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