Standard incandescent light bulbs, which is what most enlarger bulbs are, pass current through a piece of tungsten wire. The wire gets really hot, and glows. The "resistance" of the wire is much lower when it's cold, which means it will conduct much more current (amps) when you first turn it on. It heats up very quickly, increasing its resistance along the way, and quickly arrives at an equilibrium where it gives off constant light. This process takes maybe 0.05sec for most bulbs. (You can think of resistance as the diameter of a pipe. The wider the pipe, the lower the resistance, so more water can flow through it. A lightbulb is like a dynamic pipe where the diameter decreases rapidly as it heats up.)
Most enlargers bulbs are powered directly from house AC (alternating current). The voltage in your walls is AC - the exact voltage goes positive and negative relative to "neutral", but the "rms" value is 120V (or 110V, or 230V, depending where you are), and that's how we refer to it. [RMS is a method of mathematical power averaging. It doesn't matter for this discussion.] Because it's AC and varies with time, there are times when the voltage is very small, and times when it is large, it just depends where in the cycle you happen to press the power switch. For 120VAC in North America, the peaks are actually about 170V. As Gerald pointed out, bulbs typically fail if the switch is turned on when the voltage is near one of its peaks, which happens 100 or 120 times per second (50Hz vs 60Hz). If you get the timing just right, and we've all had this happen, you'll get an intensely bright flash as the bulb's instanteous power is much much higher than is rating, followed by darkness.
There's also an aging effect related to operating voltage. The higher the voltage, the shorter the life. The reason is that tungsten molecules slowly "boil" off the filament wire when the bulb is on and end up on the cooler bulb envelope (the glass). This causes the filament to get thinner as it ages. A thinner wire will be more prone to fail during turn-on if you happen to hit one of the line-cycle peaks. That's why new bulbs rarely fail on turn-on, while older bulbs are more likely to do so.
Using an in-line resistor to reduce the bulb's operating voltage will certainly extend its life, but will reduce its brightness and increase printing time. It also reduces the likelihood of popping the bulb when you turn it on because it will limit the current. A side-effect of this current-limiting is an increase in turn-on time. I don't know if this is a practical issue for enlarging, I've never tested it, but I would guess it shouldn't be too big an effect, maybe a tenth of a second or a bit more.
An inductor should limit turn-on current without affecting brightness. Because of this it won't extend bulb life. But inductors always have internal resistance as well, so in practice you'd need an inductor built with heavy-gauge wire if you didn't want to affect brightness. And because of the way inductors work, you need to think about what happens when you turn the light off, there can be a big voltage spike.
A diode will cut the bulb power in half, extending its life considerably, and reducing its brightness quite a bit. It doesn't do anything to prevent bulb failure at turn-on because it has essentially no effect during half of each AC line cycle (while completely blocking the other half-cycle).
The best solution, and something I've been meaning to get around to building for myself, is what's called a "zero voltage switch". Conceptually, it watches the AC voltage and only turns on the bulb when the voltage is small - it's not exactly zero, but is small enough that it doesn't matter. This protects the bulb against turn-on problems with no sacrifice in light output. This is more complex than just a resistor or inductor, but with today's electronic components it's fairly easy to do. But since my enlarger uses a cold light it's not something I need.
Or just stock up on bulbs.
Mark Barendt, Ignacio, CO
"We do not see things the way they are. We see things the way we are." Anaïs Nin
A diode in series would be a half wave rectifier, - not suitable, with no effect on inrush and an rms voltage of 78 volt ( on a 110 V circuit).
I would say that while it seems simple to add a series resistor ( or inductor) , the calculations must be done properly or it won't do the job, and it must be rated and istalled safely to avoid shock or fire hazard. Not so simple to implement. Resistor will run hot and generally, a resistor should not be used at more than about half its rated value, to reduce high temperatures on its leads and surface.
My vintage Federal condensor just has a standard lamp holder. It had a now obsolete Sylvania 40 Watt 110 V lamp which failed , so have been using a standard 75 Watt lamp with the lettering rubbed off.
I modified the bellows to take M39, and I use a 90mm for 6 x 9 and a 50 mm for 35mm. Using Ilford Multigrade RC Pearl 8 x 10 (inch), the results are good so far.
I have here some "Hitlights Afford 1X LED light bulb" " rated 9 Watt with color temp 6000 K. Next time I use the enlarger I might try one, to see if it gives even light across the frame ( I use a dslr to check) and still have the nice tonality on the prints.
-Anybody using household LEDS ??
The problem with the original lamps in the old enlarger is that the negs heat up while I fool around focussing etc.
So LED might help that too.
CFLs are not so good because they take a few seconds to even out, also they can have residual glow when off.
Have you tried getting 150w es enlarger bulbs? Like trying to find rocking horse shit.
Originally Posted by BMbikerider
Norman is an island.Time and tide wait for Norman.
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My experience with three strateges
I have a dozen bulbs for V35
I used to run mine with the domestic power supply supplied by Leica for $125. Bulbs seemed to last forever at reduced brightness. It was all I could afford in1985 when the better one was $500.
I have since bought the better V35 power supply. It slowly warms the bulb to full brightness in 1/2 sec to 1 sec.
Buy 250 watt bulbs and run them on around 90 to 100 volts. You get effective 150 watt output. Or buy the 150 and run reduced voltage for effective 75 watts. Been doing this for decades and it works. Wire a wall dimmer to a box or use the really nice dimmer that came from Aristo. This is a beautifully made 1950 quality electric device. It is a Cadillac. The volt reducer is nice if you need the 250, but it heats the enlarger too much, and they do, for focusing.
Diodes don't do the right job as discussed above. I have the knowledge but not the mechanical skills to fashion something so I bought an LED light source from this fellow:
Originally Posted by jp498
Seems to work great for black and white. I doubt its spectrum is good for color but I've enough 211s for when I start that again.
I may eventually get his complete VC head when I have the iPad to run it, or a revised iPhone, but that's a chunk of change right now. The light source works great on my D2.
I have no expertise in electrical engineering, but have wondered about this for a while. Would there be any benefit(s) to running an enlarger lamp on 12-volt DC current only? Like marine or RV vehicles do it?
I would think that an inexpensive automobile battery and a trickle charger to keep it topped could easily handle a normal darkroom session. The bulb would not be subjected to the normal AC current swings. There would be no power grid fluctuations to mess with those delicate highlight values. And heck, if you used a mechanical timer you could even print during power outages.
But would the bulb last any longer?
[Edit: Oops. I see that 'bernard_L' already mentions this in passing in post #6...]
Last edited by Ken Nadvornick; 10-12-2013 at 02:40 PM. Click to view previous post history.
Reason: Added [Edit]...
"Hate is an adolescent term used to stop discussion with people you disagree with. You can do better than that."
— 'blanksy', December 13, 2013
A 12V bulb doesn't care whether it's running on DC or AC. You wouldn't get the higher turn-on peaks with DC that you get with AC so it'd be less likely (but not impossible) for it to pop at turn-on. But don't forget that a car battery is really something like 13.6V, a bit higher with a charger running. Bulb life goes inversely with the twelfth power of the voltage so the lifetime would be reduced by 80% running at 13.6V relative to 12V.
Originally Posted by Ken Nadvornick
A 120V bulb running on 12V would be very dim indeed, but would probably outlast your great-grandchildren.
I've gotta build that zero-voltage-switch thing, maybe over Christmas as my employer kicks us all out for two weeks.
You could get an isolation transformer line voltage in, line voltage out. Using one would prevent the current surge and would not effect the color temperature of the bulb or its output.
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~Antoine de Saint-Exupery