On 2012-02-29, wrote:
On 28 Feb 2012 03:09:25 GMT, "DoN. Nichols"
wrote:

On 2012-02-28, wrote:
I have a bedside lamp with a 100W halogen lamp which runs directly off
110V. In the base of the lamp is a small rotary switch about 3/4" in
diameter. It has three positions: Full intensity, reduced intensity
and off. No other parts are discernible.

The question is: How does the lamp achieve the reduction in light
intensity (about half)?

The way I would do it is to put a diode in series with the lamp
on the low-intensity setting. I would guess that it is possible that
the diode could be inside the switch housing.

Correct. A dirt-cheap 1N5404, hidden inside the switch. Half-wave
rectifier properties etc. have already been mentioned.

I thought it was a neat trick. A resistor achieving the same thing
would have to be of the order of 17W or so.

And the resistor would be sensitive to the wattage of the bulb
installed -- assuming that it was a kind where various bulbs would fit
it. And the switch would probably get rather uncomfortably hot to the
touch after a while of operation. :-)

BTW neither the people who sold the lamp nor the experts in the two
specialist electrical shops knew the answer.

Of course not. They aren't members of this newsgroup, where
people care how and why things work.

Enjoy,
DoN.

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