AC or DC....that is the question

[propmonkey]

Ok this maybe a really dumb and pointless question but its been on my mine on and off(no pun intended) the last few days. but since lamps are running on AC, switching directions 50-60 times a second there for in fact dimming them 50-60 times a second does that cause them to wear out? what if they were running on DC so they are not dimming 50-60 times a second, and therefore extendign the life of the lamp?

sorry if this was a waste of time its just somethign i wondered and hopefully it makes sense to alteast oen person

[ship]

No question is a waste of time and your question is actually a very good question.

Should you wish for me to give you an immediate answer, please contact me off line, otherwise let's open this up for discussion. I will not anwser or voice opinion for a week. And this time I promiss I will be more nice.


Umm... filament notching.... grrr....

[koncept]

what you are saying does make sense. i would think it would extend the life since you are not "turning the lamp on and off" 50 - 60 times per second.

but i also wonder now about some of the new dimmers particularly the sld 96 from strand which has the sine wave technology in it and what that acheives?


edit:
it is amazing what you can find when something sparks your interest...
http://members.misty.com/don/bulb1.html#dc

[avkid]

It seems perfectly logical to me that if a lamp is dimmed fewer times it's life would become longer. My opinion is not based on any science or research,just what I think is a logical hypothesis.

[jumpjet]

I would think that the stress of changing temperature would cause a shorter lifespan so turning the unit on and off frequently would shorten the life, but at 60hz, there is going to be no significant thermal difference between cycles...

[SteveB]

Lamps have thermal inertia, which is why you don't see them flicker. The on/off cycle is so fast that the filament has no chance to respond. Note that even in a zero count blackout (the only kind of blackout), the lamp still has fade.

So generally, it doesn't harm the lamp.

Dimming quartz halogen (and incandescent) lamps can extend their life. G.E. use to put out a calculating wheel that shows the increase in life at variable decreases in voltage, along with the corresponding decrease in color temperature.

At some point though, the lamp temperature gets too low for the halogen cycle to function. This is where the burning tungsten reacts with the halogen gas and gets re-coated on the filament. If the temperature is too low, this cycle is stopped and I cannot remember at what point that occurs. Perhaps a post on rec.stagecraft could get a reply.

Remember also that lamp life is an average, and I believe that it's something like 50% of a test batch lasted at least that long, with some going longer and some less. They also assume that all lamps operate at exactly the rated voltage at 60 hz, the voltage varying greatly under field conditions.

SB

[SteveB]

Meant to add that there's a very good discussion over on the lightnetwork.com about SineWave type dimming.

In short, lamp life isn't one of the advantages.

I'm sure the ETC website etcconnect.com has some good info. as well

SB

[Mayhem]

Originally Posted by SteveB

Lamps have thermal inertia, which is why you don't see them flicker. The on/off cycle is so fast that the filament has no chance to respond. Note that even in a zero count blackout (the only kind of blackout), the lamp still has fade.

So generally, it doesn't harm the lamp.

Dimming quartz halogen (and incandescent) lamps can extend their life. G.E. use to put out a calculating wheel that shows the increase in life at variable decreases in voltage, along with the corresponding decrease in color temperature.

At some point though, the lamp temperature gets too low for the halogen cycle to function. This is where the burning tungsten reacts with the halogen gas and gets re-coated on the filament. If the temperature is too low, this cycle is stopped and I cannot remember at what point that occurs. Perhaps a post on rec.stagecraft could get a reply.

Remember also that lamp life is an average, and I believe that it's something like 50% of a test batch lasted at least that long, with some going longer and some less. They also assume that all lamps operate at exactly the rated voltage at 60 hz, the voltage varying greatly under field conditions.

SB

Remember that most dimmers will have a “pre heat” or “sensitivity” setting which actually means that at zero percent, they are not actually off. This keeps the filament hot enough to cope with the sudden increased intensity when it is operated.

Now I am by no means an expert on lamps but I am pretty sure that it is a reduction in voltage that extends lamp life. So placing a 118V lamp in a 110V fixture will result in an extended life. Running 110V lights in the same fixtures with one dimmed and one not dimmed, the lamps should last the same amount of time. Some fixtures have a “lamp save” switch on them that reduce the voltage going to the lamp. The older style rheostat dimmers work in this way (I believe)

SCR and Triac Dimmers actually turn the lamp on and off very quickly. Our eyes cannot differentiate between the on and off, so as a result, we see a lower intensity of light. The voltage remains the same. Take a white plastic ball and poke a screwdriver through the axis so that it can spin. Now, holding the screwdriver in the horizontal plane, paint the top half red and then the bottom half blue. If you spin it quickly enough, you will see a purple ball. Similar principle.

Like I said, I am no expert when it comes to lamps and especially when it comes to trying to understand how manufacturers determine lamp life. However, one thing is clear and that it the fact that there is no standard when it comes to the parameters used to determine this.

Anyone care to correct any misconceptions on my part?

[Mayhem]

Also noticed that there is another post on lamp life and pre heat, which cutlunch explained much better than I did here

[MSwan]

Even if using DC would allow lamps to last longer it is much more dangerous. Your muscles work on very low levels of DC current (along with lots of chemicals and the such). Defibrilaters use DC current and we all know how a human body reacts to that. If you were to touch a DC source at 120v 20A your body would freeze into place and you would not be able to let go, it can happen with AC as well but generally it doesn't as the changing current will at some point send your muscles the other direction causing you to release the source of voltage (hopefully before 0.6A passes through your heart).