I have installed dimmers on most of the light fixtures in my house, and I have made the following observations:
Standard household bulbs, when you turn them on with the dimmer set all the way down, glow, well, dimmly. As you run up the dimmer, they become brighter. Halogen fixtures, on the other hand, remain completely dark until you reach a certain threshold voltage (about 1/3 of the way up), at which point they begin to glow quite brightly. From this point, they can be dimmed further, but they will never start up below this threshold.
I surmise that there is a certain activating volage needed to excite the filament in a halogen lamp, but I can’t figure out why it works below this voltage once the process has begun. Does anyone have an explanation for this?
Back in the days before halogen car headlights were factory installed in the U.S., they were available as an aftermarket accessory. They always came with a warning to not have them on when cranking the engine, as the dropped voltage would shorten the life of the bulbs. If you haven’t already, you might want to investigate whether it’s a good idea to have a dimmer for a halogen lamp.
I’ve also noticed this, but I’ve assumed it was a function of the lamp dimmer, not the type of bulb.
Are all of the halogen lights in question those 6’ “torchieres” that became so popular in the 90s? My experience has been that most of them are very cheaply made, and probably use the cheapest dimmers available.
I find that a puzzling warning as virtually every halogen floor lamp I’ve seen has a dimmer circuit. I installed Euroipean style Hella halogen headlights in my car when sealed beam were still required in the US and recall no such warnings. I use halogen bulbs for modeling lamps in my studio lights and they all have a dimmer circuit so the intensity of the modeling lamp is proportional to flash power.
My WAG is that at very low voltage the filament is still cooled by halogen gas in the envelope and won’t heat enough to glow. that might account for the “startup threshold” which doesn’t seem to apply when reducing voltage. In a conventional vacuum bulb there is no where for the heat to go except by radiation.
I’ve just tried this with a couple of non-halogen bulbs I’ve got on a dimmer at home. They do the same thing; you can dim them down almost to the end of the dimmer’s travel, but once they go out, you have to turn the dimmer up 1/4 or 1/3 of the way to get the lamps to light at all again, even when the bulbs are hot from running for a long time.
The dimmer circuits in the wall dimmers and presumeably in the halogen torchieres are actually semiconductor based switching systems; I’m guessing that this is either a side effect or a deliberate feature of the way they operate, but hopefully some interested EE will come along to confirm or deny that.
I think caution is needed here, because some halogen fixtures (or rather, their transformers) are not supposed to be dimmed. With any luck Q.E.D. will be along shortly to explain exactly why, but I know that when I have tried to dim my halogen track lighting with X-10 controllers, the dimmer gets VERY hot.
Obviously, the dimmers that are built into the lamps are okay. But read the instructions that came with those halogen fixtures and make sure they don’t recommend against using dimmers.
Normal light dimers are designed for non-inductive loads, like incandescent lamps. They usually work by cutting the power to the lamp at a fixed point in its 60Hz cycle. The later the cutoff, the more power reaches the lamp, the brighter the lamp will be. This is usually done with a semiconductor element called a triac.
Normal dimmers don’t work well with halogen lamps because they are inductive like motors are. It can be very dangerous to drive a halogen lamp with a normal dimmer because of the instanteous change the dimmer tries to force the halogen lamp through. The dimmers that are used for halogen lamps are different than the normal dimmer. There are actually quite a few possible designs that can be used to dim inductive loads.
As your link points out, it’s low-voltage, transformer-based halogen lights - like track lights - that shouldn’t be used with a conventional light dimmer. The lamps used in most halogen torchieres, work lights, and as upgrades for conventional bulbs operate on 120VAC and require no transformer. They can be dimmed with an ordinary light dimmer with no worries.