dimmerswitches
I have this UBERHAUS floor lamp, with a single-filament, two-terminal bulb. They have a rotary switch in the tube (stem) that turns clockwise to three positions: off -> dim -> bright -> off.
My question is, how does a rotary switch (which must be small because the tube is only 1" in diameter) dim a 2-terminal bulb? I tried searching the internet for answers, but I don't even know what you'd call one of these switches. Also, would this lamp damage a non-dimmable bulb?
Wikipedia has a nice article on phase angle control which is used for most house hold light bulb dimmers. They usually don't work for halogen, LED, TL. This is a typical case where a single image says more than a thousand words:
It works by chopping a (varying) part from the sine wave using a triac. The image shows a rectified wave, but the mechanism works equally well with an unrectified one, the latter being most common in household light bulb dimmers. The dimmer is connected in series with the lamp and therefore needs to see a minimum load (in the order of 10W) to work.
This is the classical dimmer. Not sure how halogen / LED / CFL differs from this, but I know from experience (with CFL) that an unmatched dimmer can make the lamp flash at a really annoying frequency.
An 82 ohm resistor would dissipate about 175 watts; indeed, a 15 watt bulb may dissipate that much power briefly. Something the size of a 15-watt bulb that dissipates 175 watts is going to get very hot, however, and when tungsten gets hot its resistance increases by an order of magnitude. You may have been expecting your 1K resistor to dissipate about 1.5 watts, but in practice it would have dissipated about 17.5 initially. Most resistors don't nicely increase their resistance with temperature the way a lightbulb filament does, however, so your resistor may have continued to dissipate that much power until it melted, whereupon its resistance might have actually decreased, causing it to try to dissipate even more power, until things went downhill very severely.
I would suggest that you might consider a Christmas decoration that's designed for a C7 bulb; such bulbs draw about 1 Watt. If you wanted to use a resistor that drew less power, two 33K resistors half-watt resistors in series would probably be reasonably safe if they were somewhat ventilated (the resistors should dissipate less than 1/8 watt each, but half-watt resistors should be used so that in case one fails shorted the other would be able to handle the 0.44-watt fault power) but might not let through enough power to operate the touch switch (if it does work, keep adding 33K resistors until it doesn't work reliably, and then remove one). The fact that incandescent bulbs have a much lower resistance when cold than when heated allows a much smaller energy-wasting element to be used for a given application than would otherwise be necessary.
Best Answer
A dimmer module can be very small. They're efficient in that they don't waste much heat and so could be concealed in the lampstand quite easily.
simulate this circuit – Schematic created using CircuitLab
Figure 1. (a) A crude diode dimmer. (b) A fixed phase-angle dimmer.
Figure 1a uses a diode to allow only positive half-cycles of the mains through the bulb. This reduces the voltage to half but would not be a recommended solution as it will probably cause flicker and draws DC from the mains which the utility companies don't like as if there is enough of that going on there would be a DC bias on their transformers.
Figure 1b shows a TRIAC dimmer with a fixed trigger point delay. On each half-cycle C1 is charged up by R1. When the voltage gets high enough to cause DI1, a DIAC, to break down (typically around 20 V) the TRIAC will turn on.
Figure 2. The upper trace shows the trigger delayed close to the end of the cycle. The resultant effective voltage is low. The lower trace shows the trigger close to the start of the cycle. This will result in close to full voltage. The relationship between phase angle delay and resultant RMS voltage is graphed on the right. Image source: Dimmers for LEDs.
My linked article has a little more on the topic.