# Dimmer switch(s) - just curious

• posted on January 27, 2007, 10:38 am

I installed sixteen dimmer switches around the house, for various purposes. Don't have any problems at all, but am curious.
When a dimmer switch is turned down so the light is not as bright, does this save electricity or does the dimmer switch disperse the voltage drop in heat?
Thanks for the info. Bob

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• posted on January 27, 2007, 11:54 am
Bob wrote:

Modern home dimmer switch do save energy. Of course they do consume some heat and most lamps when dimmed are somewhat less efficient. So if you had 10 lamps in the room you could dim them all 50% or turn half of them off and save a little more energy.
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Joseph Meehan

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• posted on January 27, 2007, 1:15 pm

Wikipedia says the power used and light output of an incandescent bulb are proportional to V^1.6 and V^3.4, so light is proportional to V^(3.4/1.6) = P^2.125, no? Turning half off makes half the light with half the power, but dimming all to 50% power only makes 0.5^2.1225, ie 23% of the light.
Making 50% of the light requires dimming them all to 0.5^(1/2.125), ie 72% of the original power. That's a lot vs a little more energy, IMO.
Then again, CFs are 4X more efficient, and some dimmable CFs have a light output that's directly proportional to power :-)
Nick

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• posted on January 27, 2007, 1:38 pm
You do save alot but im not sure about nicks numbers, To get an incandesant to light when its output might be 1/ 100th or less I found mine took 6-8v. I believe at 50% power you get less than 50% light output. But incandesants can last 10 times more at 50% and nearly indefinatly at 10% power, but this does not apply to halogens.

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• posted on January 27, 2007, 3:53 pm
On Jan 27, 8:15 am, snipped-for-privacy@ece.villanova.edu wrote:

Another Nick classic. Quick with the numbers and clueless as usual. The guy didn't ask if a bulb that is at half brightness uses less energy. Any dummy knows that. What he asked was whether the energy saved at the bulb just shows up as energy wasted at the dimmer. Which of course it doesn;t because dimmers are Triacs.

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• posted on January 27, 2007, 4:15 pm
snipped-for-privacy@optonline.net wrote: ..

I guess that could be called a trader4 classic since not all dimmers are triacs.
His numbers are correct and I believe he properly addressed the question. Often a question calls for additional information.
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Joseph Meehan

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• posted on January 27, 2007, 4:54 pm
wrote:

Sorry, but I have to disagree. All Nick addressed was how much energy is consumed in the light bulbs. That was not the question. We all know a bulb that is burning at half brightness is going to use less energy. The question was whether the energy saved in the light bulb is just dissipated instead in the dimmer. And it would be if the dimmer were a resistor.

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• posted on January 27, 2007, 5:11 pm
snipped-for-privacy@optonline.net wrote:

But even if it were a resistor, the heat dissipated might not be much, because the total resistance of the (presumably) series-connected resistor and bulb would be greater than that of the bulb alone, so the total system dissipation would drop.
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• posted on January 27, 2007, 5:02 pm
wrote:

The OP says he installed 16 dimmer switches in his house. When is the last time you saw a house dimmer switch that wasn't semiconductor/ Triac based? Every one you can buy down at the home center is. And that was precisely the issue of the OP's question. Because these type that use semiconducotrs and have been around for decades do not just produce a voltage drop like a resistor would. If the dimmer was resistor based, then it would just dissipate the energy instead of the dimmed bulb and you would not save energy by dimming the bulb. That was the question, which asked if the energy that doesn't go to the bulb was not just dissipated as a voltage drop in the dimmer.

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• posted on January 27, 2007, 5:12 pm
snipped-for-privacy@optonline.net wrote:

Read my other post and think that through.
the

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• posted on January 27, 2007, 6:04 pm

I see your point and you are correct. Inserting a resistor as a dimmer doesn't mean that all of the power that would have been going into the fully lit bulb now gets dissipated in the resistor dimmer. But the amount the resistor has to dissipate as heat is still substantial and why that method is not used for AC switch dimmers.
Here's a simple example. Let's take a 120W bulb connected to 120V, which will draw 1 amp. So it uses 120Watts. To reduce the power at the light to 50%, 60Watts, means the current would have to decrease from 1 amp to .71amps. P=I**R That requires inserting a resistor of 49ohms in series. Inserting that, gives a total resistance of 169 amps, and a current of .71 amps through both the resistor and the bulb. The bulb dissipates 60 watts, while the resistor dissipates 24.5watts
Which is a big waste of energy. Thirty percent of the power is going to heat the resistor. Now consider that most of these switch dimmers are rated at 600W. If you put a full load on it, it will require the resistor dimmer to dissipate 122 Watts, which is a lot of heat for an wall box, not to mention a lot of wasted energy. And of course, this is to just dim the bulb from 120W to 60W. If you dim it more using a resistor, it only gets worse.
That's why Triacs are used.

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• posted on January 27, 2007, 7:45 pm

Wrong. Where did you dream that up? :-) The lower temp filament has a lower resistance. What does P=I**R mean? :-)

And a voltage of 31 ohms? :-)
Nick

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• posted on January 28, 2007, 12:00 am
On Jan 27, 2:45 pm, snipped-for-privacy@ece.villanova.edu wrote:

You can go ahead and factor in the decrease in resistance of the bulb at the lower light output if you want to. It doesn't change the answer to the OP's question, which was whether typical light switch dimmers disperse the power that would have gone to the bulb, which I noticed you never addressed at all. And if you do factor in that the resistance of the bulb decreases, it only makes the loss in a resistive type dimmer, if one were to be used, worse.
Triacs are used in dimmers, because instead of acting like a resistor, they act as an on/off switch on each current cycle, and hence waste only a small fraction of the power that a resistor would. Instead of harping on, for once just address the question that was asked and tell us if you diagree with the simple statement in the previous sentence.

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• posted on January 27, 2007, 1:34 pm

They do save, but not directly proportional to the voltage drop. Your bulbs will last longer also since they are not operating at full temperature.

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• posted on January 27, 2007, 2:50 pm
message

The type of dimmers under discussion are semiconductor based, using Triacs. They are not variable resistor based, which would just dissipate the energy that is not going to the lights, producing no energy savings. The Triacs work instead by delaying turning on the output voltage for part of the 60Hz sine wave, essentially chopping off varying portions of the leading edge. That chopping action, which turns a nice sine wave into something with a fast rising edge, is also what produces RF intereference that is common with these. There is of course some energy lost as heat in the dimmer, so they are not 100% efficient, as you can tell by touching one. But a light dimmed with one of these does save a substantial amount of energy.

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• posted on January 27, 2007, 4:20 pm
Edwin Pawlowski wrote: ...

I might add that while that is true for most all lamps, under some conditions (generally when dimmed between about 40-60% halogen lamps will actually have reduced life. Kind of knit picking. :-)
Also while and not sure exactly what trader4 was trying to say, but you are correct about the savings not being directly proportional.
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Joseph Meehan

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• posted on January 27, 2007, 5:05 pm
Joseph Meehan wrote:

And, as a further example, that should be "nit" not "knit", Joseph. <G>
Particularly relevant to this thread, given one of the definitions of "nit", dontcha' think?
http://en.wikipedia.org/wiki/Nit_%28unit%29
Jeff
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Jeffry Wisnia
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• posted on January 27, 2007, 8:09 pm
Jeff Wisnia wrote:

I knew I would get caught on that one. I just could not think of the correct spelling so I did SWAG and was wrong. :-)

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Joseph Meehan

Dia \'s Muire duit

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• posted on January 27, 2007, 5:09 pm
wrote:

I put in newer electronic dimmers, and part of their advertisement, was they reduce electricity, and extend bulb life. I don't remembering it be 1 for 1 relationship. Example, 50% setting saved 40% and extended bulbs life like 25%.
Just what I remember...
tom @ www.FreeCreditCheckGuide.com