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DC on pilot lamp filaments = short life




 
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Author Topic: DC on pilot lamp filaments = short life  (Read 6782 times)
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k4kyv
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Don
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« on: November 22, 2006, 09:00:58 PM »

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?

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n3lrx
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« Reply #1 on: November 22, 2006, 09:48:12 PM »

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..
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w3jn
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« Reply #2 on: November 22, 2006, 09:55:36 PM »

Search function, although it works Fine Business, doesn't come up with any likely threads, Don.
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« Reply #3 on: November 22, 2006, 10:43:15 PM »

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.
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« Reply #4 on: November 24, 2006, 08:13:43 AM »

Hi Don,

Here is a reference to lamp life and DC. I did not write this !!  I am mearly posting this for reference Cool  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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« Reply #5 on: November 24, 2006, 08:36:13 AM »

Don,

From my old product catalog files:

GE Sub-Miniature lamps Catalog 3-6016 Revised May, 1976
Page 4   Lamp Life

Rated average life is that obtained in closely controlled laboratory testing of lamps on 60 Hz. alternating current at their design voltage.  Very long life lamps are generally rated on the basis of extrapolated laboratory test data.  Service conditions such as shock, vibration, voltage fluctuations, temperature, etc., may contribute to a shorter average life.

Ordinarily, for still-rack operation, normal tungsten filament evaporation is the basic force or mechanism controlling incandescent lamp life.  Where normal filament evaporation is the dominant failure mechanism, lamps should reach their design-predicted lifetimes.

In recent years, another life-influencing lamp filament mechanism has become more prominent.  This is commonly referred to as “filament notching”. Its prominence is due to at least three factors, primarily associated with sub-miniature type lamps:
1.   Low temperature filament operation, less than that for significant normal evaporation.  (Long life lamp designs, such as 10,000, 25,000, 50,000, and 100,000 hour designs.  This does not apply to filament temperatures below 1600 degrees C.)
2.   Small filament wire sizes, less than one mil (0.001”) diameter in many cases.
3.   Increased use of D.C. voltage operation (generally resulting from advances in solid state technology).

Notching is the appearance of step-like or saw-tooth irregularities, appearing on all or part of the tungsten filament surface, after some burning.  These notches reduce the filament wire diameter at various points.  In some cases, especially in fine-wire diameter lamps, the notching is so severe as to almost penetrate the entire wire diameter.  Thus accelerated spot evaporation due to this notching (as well as reduced filament strength), now becomes the dominant mechanism for influencing lamp life.  Because of its abnormal evaporation and/or reduced strength efforts, lamp lifetimes due to notching may be one-half or less of so-called ordinary or normal, predicted lamp lifetimes.

Photo below – “An example of filament notching”.


* filament_notching.JPG (120.15 KB, 640x480 - viewed 357 times.)
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« Reply #6 on: November 24, 2006, 10:25:48 AM »

If one is going to use DC on filaments that have been rated in AC terms then you should only use .707 of the pk AC rating. Does this seem reasonable?
Happy Thanksgiving all! We Am'ers have much to be thanlful for- such as the new gift from the FCC in terms of 80M reassignment! Dec 15 is just around the corner!
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k4kyv
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« Reply #7 on: November 24, 2006, 11:17:10 AM »

Thanks.  The key word is "filament notching."  I did a Google search, and found loads of information on the subject.  This about sums it up:


MECHANICAL PHENOMENON
      Question:
      What is filament notching?
      Answer:
      DC notching is a phenomenon that occurs to an incandescent lamp when operated on DC voltage. The Tungsten material flows in the direction of the current and stops when it has reached its elasticity point and then starts to flow again. This movement creates various weak points within the filament wire, shortening the average rated life by approximately 50%.

But:

Question:
      What are the benefits of using noble gasses (xenon, krypton, argon) for increased life, and how does their use affect cost?
      Answer:
      Halogen extends operating life by recycling tungsten, evaporated from the filament and deposited on the inside of the glass lamp, back on to filament coil. It keeps the lamp from turning gray/black and allows brightness to remain at or near initial brightness specs for up to five times that of standard incandescent lamps. Lamps using an inert gas such as argon physically suppress evaporation of tungsten from the filament. The heavier the atomic weight of the gas used, the greater its effectiveness. This also increases the cost of the lamp, but provides the added benefit of NO reduced life operating on DC voltage, and a whiter, more efficient light that is very desirable where color is a serious consideration.

http://jkllamps.com/techLibrary/index.cfm?action=techLibraryFAQs


The lamps in question are in my Gates BC1-T.  I run the relays on DC, and the same DC power supply lights up the filaments-on and plate-on indicators.  The original lamps are rated at 15 watts @ 220 v.a.c. but I run mine on 110v DC, and have not been able to find  replacement 220v  lamps, so I use the equivalent 110 volt ones.  In my case the solution will be to increase the series resistance, thus dropping the filament voltage below the rating, to compensate for life reduction due to the notching effect.  A series resistance also limits surge current, another factor that shortens filament life.

I run my 75A-4 with the equivalent 12-14 pilot lamps instead of the stock 6-8 volt ones.  Not only does this increase life from a few months to several years per lamp; it gives them a nice, warm, mellow amber glow instead of the harsh bright white glow of stock lamps.
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Don, K4KYV                                       AMI#5
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« Reply #8 on: November 24, 2006, 01:17:28 PM »

Not to steer this thread in a different direction as it is an interesting thread nonetheless but this now this begs the question of using DC on tube filaments.  The audiofools are sucking up the classic transmitting tubes for their audio projects and run them on DC from the few tube audio articles I've read to reduce noise and hum. Metal migration, just another factor to consider in the tube drain down by these folks.
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« Reply #9 on: November 24, 2006, 02:52:29 PM »

The mechanism is only a problem with filament wires around 0.001" diameter or less.  Tubes that the audiophools would use are not in this boat, unless they have fallen in love with 1R5's now - a 50 milliampere filament.

I don't have a table of tube and pilot bulb filament diameters, but I would guess that a filament greater than about 100 milliamperes is out of range of the problem.  Perhaps someone else has access to tube or bulb filament mechanical specifications.

12AU7, 12AX7   150 milliamperes
6SN7    600 milliamperes
6F5    300 milliamperes
6A3, 6B4  1 ampere
300B   1.2 amperes
45      1.5 amperes
805    3.25 amperes
810    4.5 amperes
811, 812  4.0 amperes

Shucks.
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« Reply #10 on: November 25, 2006, 10:29:36 PM »

the problem with bulbs is the cold resistance. The typical cold resistance is 1/10 that of hot.
This causes a large current transient at turn on. Ths shock is pretty hard on the fine wire inside. I suspect the reason for the coiled up wire. Also running bulbs at a slightly lower voltage will help it to have a long life.
RMS AC voltage .707 peak  is the same heating as DC..
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w3jn
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« Reply #11 on: November 26, 2006, 08:27:27 AM »

I've run DC on the fhyelliments of a quad of Russian 300Bs in my HB audio amp for a thousand or so hours with no failures.
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k4kyv
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« Reply #12 on: November 26, 2006, 01:44:02 PM »

I seem to recall reading somewhere that it is preferable to run directly-heated thoriated tungsten filaments on a.c., but if you have to run them on DC, to periodically reverse the polarity of the DC voltage applied to the filaments.

With both light bulbs and tube filaments, it wouldn't be a bad idea to soft start them with a series resistor. 

I use the lowest voltage primary tap available on my filament transformers, usually something like 105 volts, and place a filament rheostat in series, to bring the measured filament voltage down to its normal value with the rheostat set at midrange, at normal a.c. line voltage.  This gives me conrol of the  filament voltage when the line voltage is unusually high or low, and the series resistance limits the surge current at initial startup.

The same thing would work with pilot lamp filaments.  Put in enough series resistance to dim them down noticeably.  This will cause the bulb to give off a softer glow, extend the life of the bulbs, and limit the surge current.

Another approach that wastes less energy and produces less heat would be to use a variac, and install a shaft extension to the rear of the variac (most of them are designed to accomode this), and install a rotary on/off switch to the rear of the variac so that it works just like the classic on/off-volume control that was so common on broadcast radios from the mid-30's until vacuum tubes were phased out.

If you need to switch something over about 5 amps, it might be hard to find an appropriate rotary on/off switch.  Use a heavy duty contactor relay, and let the on/off switch activate the contactor.  Before installing the relay, test it to make sure it runs quietly without a loud buzzing noise.

Just turn the  knob all the way to the left until the power clicks off.  Turn on by clicking the knob to the right, and continue rotating until the filament or line voltage meter reads the correct value.
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Don, K4KYV                                       AMI#5
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« Reply #13 on: November 27, 2006, 11:38:47 AM »

Don,

Very enjoyable QSO 0n 75M last night-thanks!!!

Directly heated triodes should always be run with AC on the filament. This is desirable so as to maintain essentially a constant bias potental across the entire filament, relative to the grid. With DC on the filament, the bias potential will vary significantly from the positive to the negative leg of the filament.

The only reason DC would be applied to the filament of a directly heated triode would be to minimize the hum modulation caused by AC; but with either a reasonably well balanced center-tapped filament transformer or a hum-nulling pot installed across the filament circuit, the hum is usually down far enough so as not to cause audible noise in most applications.

I run the 833As in my modulator on AC, with the center-tap of each of the two filament xfmrs returning to ground, and there is zero noticable hum in this arrangement. The class A P-P 845 audio driver currently under construction to drive the 833As will use the same filament supply scheme. It was always done this way in BC xmtrs.

73,

Bruce
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