I've heard it said that you shouldn't even try to dim a halogen bulb - if it runs below operating temp, the filament vapor will condense on the bulb envelope; I guess if you want to _permanently_ dim it, that'd be OK.
Why not just replace the bulb with one that's not so bright?
You could also get some neutral-density filter plastic, but you have to watch out for waste heat.
The only reason for dimming the halogen bulb is because different people want different brightness levels whilst working. This lamp would be for anyone as you are able to dim it to suit your needs.
Ive looked at several articles and i have mixed feelings about this topic. Some people say it is possible and others say it isnt.
I need a simple solution which can be incorporated into a desk lamp (a small unit)
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look at graph showing lifetime of standard car halogen lightbulb. I think there will be no problem with shortage of life with decreaseing voltage. Especially when you do not dimm it more than 20-30%.
I don't know about that. My wife has two of these fire hazards. One has a continuously variable dimmer knob, but the pot seems to have died in it. The other has a three position switch allowing for two brightness levels.
I once heard something like you describe though, but IIRC they also said that running it at full voltage again would restore the bulb. Anyone?
Running the lamp at full temperature may restore a blackened lamp, but there's no guarantee. It's called the 'Tungsten Halogen Cycle' and there's a neat animation here:
I used a lot of these types of bulbs back in my college theatre days. They work just find dimmed (all we ever did with them in the theatre) but are best if they at least occasionally brought up to full brightness and temperature.
Yes, i've found these comments useful. Some people say it can be dimmed, others say it shouldnt be dimmed. Do i just use a potentiometer and connect it to the 12v supply? Is there another way of dimming the halogen bulb using a potentiometer and a resistor of some sort? What kind?
I remember looking at some book in a university library over 20 years ago, showing lots of graphs and equations expalining what supposedly happens to the halogen cycle when a halogen lamp is dimmed.
Supposedly, dimming a halogen lamp slows down tungsten evaporation more than it slows down the halogen's returning of tungsten to the filament.
Now, for a couple things going wrong:
1) Dimming a halogen lamp is likely to not extend its life as much as dimming a non-halogen one does. The ends of the filament are cooler, and can be attacked by the halogen cycle, causing thin spots in the filament. These "end notching" sites can have temperature overshoot during a cold start. Soft starting can help here (and usually helps less with non-halogen lamps, where most fatal filament thin spots usually have excessive temperature in steady operation, and worsen at a rate accelerating worse than exponentially). Such filament end notches will worsen until failure occurs there one way or another. The bottom line is not dimming shortening life, but dimming extending halogen lamp life less than it does with non-halogen lamps.
2) Halogen lamps could contain contaminants that do the halogen cycle in reverse - transporting tungsten from the filament to the inner surface of the bulb. Such bad stuff may slow down less from dimming than the proper halogen cycle does. In such a case, dimming to the extent where a contaminant's "reverse halogen cycle" outruns the normal one will cause problems.
I suspect such problems are less with halogen lamps made by major brands such as GE, Osram/Sylvania, Philips, Thorn and Ushio. I suspect that such lamps will take mild to moderate dimming well and *usually* (maybe or maybe not reliably) not blacken with severe dimming. No guarantee from me!
Now, for fixing blackened halogen lamps:
I hear that this works. I suspect it works only if the blackening is simply tungsten, as opposed to an oxide (dusty grayish or colored). I would also be a bit leery of doing this, since with full power operation before the blackening is fixed the bulb can overheat. Every blue moon or something like that, a halogen lamp somewhere explodes even if nothing wrong is done, so I am leery of doing anything that increases stress on them.
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