DC on pilot lamp filaments = short life

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k4kyv:
I recall a year or two ago a thread appeared on this board this concerning the fact that running incandescent pilot lamps on DC results in greatly reduced life of the bulb compared to running the same bulb on a.c.  I recall it being related to some kind of "effect" with a proper name attached, and had somethig to do with segmented rings in the molecular structure of the filament wire.  I thought I had saved the text of the messages, but if they are still on my HDD, I don't  have a clue where they are, and I have tried several searches to no avail.

Can someone refresh my memory on this?

n3lrx:
I would think filaments would last longer on DC because it's somewhat stable, whereas AC is up down all over the place.. But now that I think of it; it does make sense.. I'm not sure of the exact scientific title myself but I would think it's along those same lines as to how Electroplating works.. While current flows through the wire tiny bits of metal are cast off and absorbed by the opposing polarity. Cant recall which way electroplating works whether it's the positive or negative molecules that fly away and stick, but each time they do they take a little meat with them.. Eventually it's all gone or too weak to do it's job.   

If I'm wrong I'd love to know the reason myself..

w3jn:
Search function, although it works Fine Business, doesn't come up with any likely threads, Don.

Tom WA3KLR:
I have never heard of DCV vs. ACV rms yielding lower bulb lifetime.  I have had my hands on GE Engineering bulb catalogs and don't recall reading such.  But I don't disagree with the possibility.

Now that I think about it, with the current flowing only in one direction all the time, you may get what is called metal migration.  This mechanism has been a killer in integrated circuits.

steve_qix:
Hi Don,

Here is a reference to lamp life and DC. I did not write this !!  I am mearly posting this for reference 8)  I have no idea if the information is true or not !!!  The info came from http://www.eaoswitch.com/about/lamps.htm

Hope this helps !!  Regards, Steve


A Guide to Lamps

All about illumination and lamps for switches and indicators. This short discourse explains the different types of lamps used in EAO switch products, helps you select the right lamp for the job, and discusses rerating incandescent lamps when they are used at different voltages.

--------------------------------------------------------------------------------
Light Sources
Incandescent lamps, neon lamps, and LEDs (light emitting diodes) are available for most EAO switch products. These three types of light sources have distinctly different characteristics. Each must be matched with appropriate lenses and other accessories for best results.

Lighted devices offer the designer a wide range of appearance options. Colors can be vivid or subdued. Lenses and legends can change color when illuminated. Legends can be hidden, appearing only when lighted, or be split, with one part always visible, another part appearing only when lighted. Components and application techniques must be selected for the effect desired.

All EAO light sources in a socket can be relamped from the front panel for quick and easy replacement. All EAO light sources are stationary within the switch assembly. By avoiding lamp travel, operating life is materially increased. All EAO light sources are designed for even distribution, with no halo or hot spots.


Incandescent Lamps

The miniature and subminiature incandescent lamp is the most widely used light source for illumination panel devices because of its great versatility. Its omnidirectional and broad-spectrum characteristics make its emissions readily diffusible for even illumination of relatively large areas. The broad spectrum permits filtering for attractive color effects. High intensity provides good brightness even after attenuation by diffusing and/or filtering materials. Good broad area brightness permits free use of legends (by engraving, film inserts, or both) with assured contrast for legibility. Finally, incandescent lamps are available for direct connection to most common system voltages and are inherently self-regulating when so used.

The less desirable characteristics of incandescent lamps are that they operate at relatively high current levels and generate considerable heat. Lamp filaments, unlike solid state semiconductor devices, must operate in a self-destructive mode to produce light. Service life, although often long, is limited.

The average life ratings published by lamp manufacturers (and included in the tables that follow) are based on tests performed under laboratory conditions: stable ambient temperature, no shock or vibration, filaments operated on AC voltage-regulated to plus or minus one percent. Average life is the elapsed time after which 50 percent of the lamps under test have failed. Failure includes a current increase to 10 percent above the initial current value and/or a brightness decrease to 20 percent less than initial brightness.



Average lamp life may be shortened by as much as 50 percent when lamps are operated on DC, which accelerates grain growth of the tungsten filament, leading to earlier development of hot spots. High ambient temperature (above 200 degrees F) shortens lamp life by increasing the rate of tungsten migration away from the filament wire. Lamps become increasingly vulnerable to failure from shock and vibration as they age and filaments are embrittled by grain growth. Failure most often occurs while no current is flowing, when the filament is most brittle.



When shock and vibration will be present it is best to use a low-voltage lamp. Higher-voltage filaments are generally longer and smaller in diameter, with a great number of resonant points. Derating lamps (operating them at less than rated voltage) improves life by lowering filament temperature. Maintaining current through the filament keeps the filament more resilient than if current is switched off entirely, and minimizes cold tungsten filament shock. (Less than 1 percent of rated voltage is required.) Operation of lamps in constant current or series circuit applications may reduce lamp life by subjecting filaments to excessive voltage.

Operating the pushbutton of an EAO device does not subject the lamp to shock or vibration forces. The lamp is fixed, its socket being integral with the body of the device. Moving parts of the pushbutton are all isolated from the lamp.

When a lamp is to operate at other than rated voltage, the following formulas may be used to predict the effects on luminous intensity, life, and current. The results are reliable for applied voltages close to the rated voltage. The further the deviation from rated voltage, the greater the percentage of error in the approximation.


 

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