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Monday, January 05, 2009

I have practically my whole loft on dimmers, which creates a very nice effect. The LA Times claims::
If you dim a halogen bulb to 50%, you will save over 40% energy and your light bulb can last more than 10 years.
My understanding was that dimmers work by increasing resistance along the line. So you simply heat up your dimmer, using exactly as much energy as if you let all the power flow to the bulb. But now the Wikipedia claims that "modern dimmers are silicon-controlled rectifiers" which, I think, switch power on and off, thus wasting no power. (But is that true of house dimmers?)

O Hive Mind, do you know the correct answer?

Incidentally, I've tried the new "dimmable" compact flourescents. They do dim somewhat. But they don't dim all the way. You can get them down to, oh, 50% of the maximum output. If you're trying to create a mood, 50% just feels like you have a couple of bulbs out. You really need the halogens; they go down almost all the way.



"If you dim a halogen bulb to 50%, you will save over 40% energy"

A Dim Sum.

By Blogger JamesHutchinson, at 9:39 PM  

You know, this brings up a general question I have about electronics. If V=IR, where V is voltage, I is current, and R is resistance, then an increase in R leads to a decrease in I, assuming a constant 120v. But the equation for powers (watts) is P=VI or P=(I^2)R. Which seems to imply a similar relationship -- that a great resistance would lead to less power being used. But that can't be right, surely, because, as you pointed out, the resistor is still using energy -- it's just dissipating it into heat. This bothers me.

By Blogger Seth, at 4:34 PM  

Huh, good point, Seth. That would suggest that resistors shouldn't heat up. Unless, of course, they both diminish current AND waste some of it.

By Blogger Alex Epstein, at 4:55 PM  

I think power is about the actual current flowing, 'thrusted' by the voltage. If you would have an infinite resistance, no current would flow, hence the used power would be 0, while no heat would be dissipated. Other way around, if the resistance would be nearly zero, the current would be almost infinite, thus boosting the power to almost infinite (short circuit).

I don't think the resistance is 'absorbing' the current or converting it into heat; I think it just prevents a certain part of the current from flowing.

By Blogger Avlan, at 3:37 AM  

Light dimmers haven't been resistors for the last 50 years or so.

This wasn't a desire to save money on electricity, but a much pressing issue - heat. Try it for yourself. Put a light on half brightness, and leave it on for about half a minute.

Then grab the lightbulb and hold it tight in your hand for a couple of minutes. Go on. Try it.

After you've recovered from the burns and decided never to follow advice on the internet again, you'll realise something interesting ... lightbulbs are hot. Very hot. That's why they are inefficient.

But remember that the light-bulb was on half power. So if the dimmer was a resistor it would be throwing away the same amount of power, but in a much smaller package.

Now put you hand on the dimmer. Is it hot? Nope.

So dimmers clearly work by another, much more clever method.

Instead of wasting the electricity, dimmers simply flick the light switch off, then on again. And repeats it about 120 times a second.

By changing how long it keeps the light on before turning it off, it can control the brightness.

(A normal light bulb can't dim that fast, so will just end up on partial brightness.... which is exactly want you want!)

Another mystery of life solved.


By Blogger Mac Harwood, at 4:21 AM  

Well, THANK YOU, Mac, for knowing your stuff and sharing it with us. I am blessed to have smart readers.

It makes sense then that the compact flourescents don't dim as well -- they stay bright on less electricity.

I wonder how well the upcoming dimmable LEDs will work.

By Blogger Alex Epstein, at 8:38 AM  

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