light bulbs

I have some naive questions about light bulbs that maybe someone here knows how to answer.

(1) What is the theoretical voltage-current relationship for a light bulb? I realize this depends on aspects of its construction, including the material the filament is made of. (2) The resistance of a light bulb apparently increases with voltage but not linearly. What is the theoretical voltage-resistance relationship of a light bulb? (3) What mechanisms explain the relationship among voltage, current and resistance of a light bulb and how do I compute them? For example, how do the temperature and the work function of the filament and the resulting electron cloud around the filament behave and how do I compute their effect, if any, on current and resistance.

These questions derive from my continuing attempts to read Kloeffler's book, Electron Tubes. I realized that the problems I was having with it were all my own fault, caused by plunging into the chapter I was interested in instead of reading the book carefully from the beginning. Often I can get away with it, but Kloeffler's book is written a lot more carefully than I realized and even the parts that I considered too trivial to read contain some information that is necessary for understanding the conventions of rest of the book. So, now I'm starting to appreciate the book. In particular, it is nice to see that he starts with light bulbs to illustrate his graphic techniques. I'll refer to a light bulb as a "unode". But he has no theoretical discussion of the characteristics of unodes and I would like to fill that lacuna.

Ignorantly, Allan Adler snipped-for-privacy@zurich.ai.mit.edu

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Reply to
Allan Adler
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    Boris Mohar
Reply to
Boris Mohar

The resistivity of tungsten (the material normally used for lamp filaments), as for most metals, increases with temperatures. This change in resistance is a property of the material, and not something that the light bulb designers specifically designed.

I suspect that tungsten is used for lamp filaments because it has a very high melting point, and a fairly high resistivity. This means that the filament can be heated white-hot without fear of it melting, and you don't need a long wire, or very thin wire, to make a useful lamp.

Ohm's Law.

As I recall, the filament or cathode in a vacuum tube is specially treated to make it emit useful quantities of electrons. The filament of a light bulb will not be so treated, because it doesn't need an electron cloud around it.

A light bulb is not a "unode" (whatever that may be). A light bulb has very little in common with a vacuum tube.

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Reply to
Peter Bennett

At various points in the evolution of incandescant lights, the envelopes have been evacuated rather than filled with (relatively) inert gas.

For historical perspective, the Edison Effect was discovered by Thomas Alva when he was messing around with light bulbs.

This was long before John Ambrose Fleming & Lee deForest put it to use.

Reply to
JeffM

At various points in the evolution of incandescant lights, the envelopes have been evacuated rather than filled with (relatively) inert gas.

For historical perspective, the Edison Effect was discovered by Thomas Alva when he was messing around with light bulbs.

This was long before John Ambrose Fleming & Lee deForest put it to use.

Reply to
JeffM

Indeed. For that matter, a light bulb does not even enclose a vacuum.

Harry C.

Reply to
Harry Conover

Some do. Roghly on an average, it is bgetter to use a vacuum than to use the usual argon-nitrogen mixture if the wattage is less than 10 watts per centimeter of apparent filament length (or maybe length plus diameter, with these dimensions being the aparent-"visible" overall dimensions). With near or over 10 watts per centimeter, light bulbs usually get a gas fill.

Some examples among 120V ones in the USA:

Tubular refrigerator bulbs of 25 and 40 watts have a vacuum, and the (not especially easy to find) 60 watt one has a gas fill.

"A19" "regular" bulbs with the coiled-coil filament about 2-2.5 cm long normally get a gas fill from 25 watts on up. 120V bulbs with the multi-supported C-shaped filament 40 watts and up get a gas fill. 120V bulbs 15 watts and less generally have a vacuum.

The reasoning: A gas fill reduces filament evaporation by having gas atoms "bounce" evaporated tungsten atoms back onto the filament. This permits the filament to operate at a higher temperature for a given life expectancy. (Note that gas filled ones have a trend of producing whiter light than vacuum ones.) The higher temperature makes the filament's radiation more in the visible and less in the infrared (although still much more infrared than visible). The drawback of a gas fill is that it conducts heat from the filament - and this means energy going in that is not radiated at all. Heat conduction is nearly enough proportional to filament visibly apparent length but surprisingly independent of visible apparent diameter, since a wider filament has a thicker "boundary layer" of gas around it and the thicker boundary layer has a lower temperature gradient to nearly cancel the greater circumference. Thinner filaments in a gas fill have almost as much heat conducted from them (per unit length) as thicker ones, which makes gas heat conduction a higher percentage of input power with the thinner filaments. This is why filaments with more wattage per centimeter of apparent length do better with a gas fill and ones with lower wattage per centimeter of apparent length do better with a vacuum.

- Don Klipstein ( snipped-for-privacy@misty.com,

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Reply to
Don Klipstein

Thanks to snipped-for-privacy@manx.misty.com (Don Klipstein) for all the interesting information about light bulbs. Is there a book you can recommend that has all this design information?

Here is another question, on a lower level: the schematic for my EICO 460 oscilloscope calls for a #47 lamp somewhere. What is a #47 lamp and where would one read its specs? The specs themselves are not a vital concern for understanding the workings of the scope but I am curious to know where one looks up stuff like that.

Regarding my original question, which pertained to how one can give a complete accounting of various processes taking place in the bulb (how the resistance of the filament depends on voltage, how much heat is generated, the structure of the electron cloud and how its presence affects the energy inventory, etc.), I haven't gotten any answers or any references but I have made some progress on my own. First, I found that Tolman's Principles of Statistical Thermodynamics has a discussion of "conduction electrons", which discussion I've now read and am trying to digest. It is also discussed in van Vleck's book on electric and magnetic susceptibilities, which I've since also read briefly. There is also a book of Samsonov on his "configurational model of matter" which applies the model to electrical conduction and thermionic emission, among other things. So, hopefully I'll learn a little more about what is going on that way. That doesn't mean I'll get the "inventory" I was asking for, but it might help.

At any rate, thermodynamically, we can think of the electrons as a kind of gas (obeying Fermi-Dirac statistics), some of which manages to get outside the filament when the velocity is sufficiently high, and which otherwise manages somehow to maintain an electric current. Athough this is nominally about electrons, voltage, current, heat and clouds, maybe I should be asking about this on a newsgroup devoted to thermodynamics.

Ignorantly, Allan Adler snipped-for-privacy@zurich.ai.mit.edu

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Reply to
Allan Adler

This URL may help:

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Unfortunately, the above covers only the older classic type pilot lamps and completely omit more recent types (of which there are hundreds).

Both the Newark Electronic and Allied catalogs list virtually every type of pilot lamp that is manufactured, including some specifications. Their is also a table of pilot lamps and characteristics in older editions of the ARRL Radio Amateur's Handbook (along with extensive listings of vacuum tubes, crts, etc. Beyond these sources, you may need to consult manufacturer's literature.

Harry C.

Reply to
Harry Conover

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Reply to
John Fields

#47: T3-1/4 bulb, miniature bayonet base, design voltage 6.3 volts, drawing .15 amp at that voltage, .5 MSCD (approx. 6.3 lumens) light output, design life expectancy 3,000 hours. Variants include the 1847 (.38 MSCD or approx. 4.8 lumens, 5,000-plus hours life expectancy), the 40 (same as 47 except screw base instead of bayonet base), and the 755 (.33 MSCD or 4.1 lumens, 20,000 hour life expecyancy at 6.3 volts).

Varies from one bulb to another and with voltage, but in most cases within voltage ranges where the bulbs visibly glow and don't burn out within a second the resistance is not too farr off from proportional to the square root of voltage.

Power in becomes heat. Where the light mostly fails to escape the room that the lamp is in, power into the lamp becomes heat in the room. Light (and infrared) that escapes the location becomes heat wherever the light eventually goes. It seems to me that roughly 40-65% of the power going into an incandescent lamp is radiated, and the remainder becomes heat at the site of the lamp.

Sorry, I can't help much there.

Energy stored in the electron cloud is small and reasonably constant during lamp operation. By any accounting of energy breakdown in input and output during continuous operation, energy storage in the electron cloud should be insignificant. I would not worry about electron clouds, thermionic emission nor the like in any pie charts for input to (100% electric power) nor output from an incandescent lamp.

- Don Klipstein ( snipped-for-privacy@misty.com)

Reply to
Don Klipstein

Can you tell us how two light bulbs compare: the U.S. 120VAC light bulb compared to the Euro 240VAC light bulb. I assume that the U.S. filament is shorter and heavier than the Euro. So is it more efficient?

I have an old Tensor lamp that uses the #91 (IIRC) light for autos. Thing was always needing a lamp (maybe because I used it a lot!), but they were readily available at the store. In that case, it didn't seem as if the heavier filament was helping it last longer.

This weekend I bought a light bulb from the auto store. It's a regular tubular light for a car dome light, but it has a blue coating on it to make the radiated light blue. It draws more than .83A at

12V, so it eats up more than 10W, and gets hot (I'm surprised the blue coating doesn't melt). Yet it puts out surprisingly little blue light, in fact I'd say a dozen blue LEDs would easily outshine it.
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Reply to
Watson A.Name - "Watt Sun

In article , snipped-for-privacy@nestle.ai.mit.edu mentioned...

The #47 lamp is an industry standard, the most common miniature light bulb; it was used in just about every all american 5-tube radio in the days of tube radios. It's 6.3VAC at 150 mA, which matched the current of the filaments of a tube, so it could be run in series with the tube. It should be easily found at any place that sells light bulbs, maybe even Radio Snack. There should be an abbreviated list of the various common miniature lamps available on the web. A search for it should turn up something. Here's one that i got from google.

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The first one is a huge .JPG that took forever to d/l, I wish the guy had saved it in a smaller size. He doesn't realize that most of us are still using dialup. Yeah, at work I have a 45 Mb connection to the rest of the world, but not at home. If I wanna d/l the latest Mozilla or other huge file, I do it at work. ;-)

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Watson A.Name - "Watt Sun

Thanks to all for the abundant and diverse information on #47 lamps.

Regarding Don Klipstein's ( snipped-for-privacy@misty.com) other replies to my questions, I'll take them in turn.

Thanks, I'll check this against the graph in Kloeffler's book and get back to this later. But how do you know this?

How do you compute the percentage that is radiated?

The electric field causes the electron to travel with a certain velocity before it collides with something and gives up some or all of its kinetic energy. Multiplying the electric field by the charge of the electron gives what I guess is the maximum kinetic energy of the electron. When it collides with something, that energy is converted to heat. That heat raises the temperature of the filament, changing the resistance. The amount of heat generated per second would be, I guess, the number of collisions per second times the average kinetic energy of the electrons. The number of collisions per second should be related to the mean free path of the electron in the metal at the given voltage (which will vary through the metal) and maybe also the number is proportional to the resistance. If so, that might give a conceptual way to compute the study the relation among voltage, heat generated and changes in resistance.

The electrons that are traveling fast enough have the possibility of escaping into the electron cloud and not colliding with anything while they are in it but maybe some energy is given up when they pass through the surface of the filament. Mostly they fall back in and are replaced by other electrons, so there is a certain percentage of the energy that is stored in the electron cloud, namely maybe the number of electrons per second that escape into the cloud minus the number per second that fall back in, all multiplied by the work function of the metal.

I don't care if it is insignificant. I just want to know how one computes it to prove that it is insignificant. I can speculate about methods for computing stuff like that, but I don't know if the methods are right.

That's why I'm asking. The best answer would be to relevant literature, preferably books that treat this kind of question. I've already mentioned a few that I know about: (1) Tolman's Principles of Statistical Mechanics (2) van Vleck's Electric and Magnetic Susceptibilities (3) Samsonov's A configurational model of matter. The latter reference shows that one can go even more deeply into it, by considering the actual electronic structure of the atoms involved and seems quite fascinating in the scope of its applications.

I'm going to cross-post this to sci.physics to see if they know more about this kind of question there.

Ignorantly, Allan Adler snipped-for-privacy@zurich.ai.mit.edu

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Reply to
Allan Adler

Examples to your question were posted in the newsgroup misc.consumers.frugal-living on 8 Oct 2003 entitled "Watts versus Lumens? Thoroughly confused". Notice the exponential relationship for incandescent lamps. Every bulb technology has a specific lumens per watt ratio when operated at its standard conditions. For example, incandescent bulbs typically have a 14 or maybe even as much as 18 lumens per watt efficiency.

As voltage > Thanks to all for the abundant and diverse information on #47 lamps.

Reply to
w_tom

actual_current = rated_current * ((applied_voltage / rated_voltage) ^ 0.55)

Reply to
Paul Cardinale

Apparently a lot of my questions about light bulbs and electron tubes are answered at the following website and the literature it cites.

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Ignorantly, Allan Adler snipped-for-privacy@zurich.ai.mit.edu

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  • metropolitan area. *
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Reply to
Allan Adler

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