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  1. #1
    L Gebhardt's Avatar
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    Extend the life of enlarger bulbs

    I just posted some details on extending the life of enlarger lamps. The 30,000 foot overview is that reducing the voltage can substantially increase the life of a bulb. This follows the formula of (Design Voltage / Actual Volatge)^12, so running a bulb at 110V instead of 120V can extend it's life by almost 3 times. This came from the May/June 2000 issue of Photo Techniques, which has an article “Optimize Enlarger Light Intensity” by Conrad R. Hoffmann. It focuses on reducing the light output by dimming, but touches on the lamp life benefits as well. Worth a read if you can find a copy.

    I figure this will be of use as more and more enlarger bulbs go out of production.

    Also, this is not a good idea if your enlarger has any type of electronics in it. I am only recommending this for simple condenser systems that have a bulb that plugs directly into the power outlet on your timer.

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    Most bulbs fail because of the sudden current surge when first turned on. A resistor in series with the bulb can extend the life by slowing the current surge. This has the advantage of not unduly changing the color temperature of the bulb which happens when the voltage is reduced. There are calculators on the web to determine the correct value for the resistor.
    A rock pile ceases to be a rock pile the moment a single man contemplates it, bearing within him the image of a cathedral.

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    Yes, a small reduction in voltage causes a large increase in life.
    The old radio transmitting tubes cost $5000 to $10000, so the filament voltage was closely monitored as 3% too high meant a 50% reduction in life.
    Furthermore, the inrush current when a tungsten lamp is switched on causes stress which leads to fatigue failure as the filament heats unevenly during start up.

    A trick I have done for many years is to add a resistor in series with a lamp, such that the voltage is reduced by 4 or 5 %.
    The lifetime increase is up to 5 times, as the resistor reduces both the inrush current and the operating voltage.

    Very early in my career, I was not remotely interested in photography, but i was designing voltage stabilizers, and i recall going to a photo printing facility where the owner was tearing his hair out with lamp failures, color and exposure inconsistency , etc, He was doing wedding photo prints etc and had the brides' mothers queuing up in his shops! I spent a morning with him and he kindly explained the processes in his smelly, crowded enlarger room. As I recall we were able to fix many of his problems with a few voltage stabilizers to power his enlargers.

  4. #4
    JLP
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    A resistor is not the best component to reduce inrush current, a coil is far better.
    The resistor will impact the overall light emission but the coil will only limit the current for a very short but important time and the light emission from the bulb will be nominal a few milliseconds later than without the coil.
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    Quote Originally Posted by JLP View Post
    A resistor is not the best component to reduce inrush current, a coil is far better.
    The resistor will impact the overall light emission but the coil will only limit the current for a very short but important time and the light emission from the bulb will be nominal a few milliseconds later than without the coil.
    OP has a 110 V AC lamp ( I don't know, maybe 75 Watt?)
    What inductance did you use to lay down for typical inrush time of 20~ 50 millisec yet retain nominal light emission?

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    A resistor is not the best component to reduce inrush current, a coil is far better.
    This would be true with a DC power supply, where the inductance would have no impact on the steady-state current. Restating/rephrasing what wombat2go said, with AC supply, the inductance value L would have to strike a balance between the time constant L/R being large enough to limit the inrush current, and the AC impedance L times omega small enough that the lamp still shines at near-nominal power.
    But, after all, a DC power supply might kill two birds with one stone, if it is voltage-stabilized.

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    It wont work un UK as we are 230Vac but I am sure that someone will come up with a solution, however what bulbs are going out of production? I only know of the ones the the last Leitz Focomat colour enlarger and there is a simpler solution involving a modification of the lamp holder to take a different bulb of the same value.

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    L Gebhardt's Avatar
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    Quote Originally Posted by BMbikerider View Post
    It wont work un UK as we are 230Vac but I am sure that someone will come up with a solution, however what bulbs are going out of production? I only know of the ones the the last Leitz Focomat colour enlarger and there is a simpler solution involving a modification of the lamp holder to take a different bulb of the same value.
    I think you would just need a variable transformer for your voltage range. Dropping from 230 to 210V should increase the lamp life a lot.

    I know the Durst bulbs for the 138s are out of production. If others aren't now, they will be at some time. I personally have switched my Durst over to LED, but still occasionally put the incandescent bulb back in test how well the LED systems I designed are working compared to the factory setup.

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    L Gebhardt's Avatar
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    Any tips on picking the right resistor? I know the bulb filament has a different resistance once hot. You also need to make sure the restoration can handle the heat. http://en.wikipedia.org/wiki/Inrush_current_limiter has a few options we should look at.

    It seems to me that adding an NTC thermistor along with a variac would give you the best of both methods.
    Last edited by L Gebhardt; 10-12-2013 at 06:27 AM. Click to view previous post history. Reason: Finish post

  10. #10
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    Rather than a resistor, I'd put a diode (of ample capacity) in series with the lamp. I'd wire one up, but there used to be little discs for that purpose that went into lamp sockets.

    It's not the end of the world if you blow a bulb. Keep a spare in stock, and if they are no longer available adapt to another bulb or fashion something with an LED bulb.

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