What bulb voltage?

The battery label voltage spec is the unloaded voltage. The bulb is chosen to match the battery actual voltage under that load. All batteries have an effective internal resistance that uses up some of the available voltage as current passes through the battery. This is why using a bulb specified for a 2 AA cell flashlight on a 2 D cell battery will have a short life, but be very bright. The D cells have a much lower internal resistance, but the same no load voltage.

I can't find the reference, but as I recall, there is a twelfth power somewhere in that equation...dramatically indeed.

Design is a tradeoff. Most people opt for more light output and shorter battery and bulb life in their flahslights. We know that because that's where the market has settled. Unless you're an emergency worker, you probably don't use your flashlight very much. I remember my grandfather used to put a 9V bulb in his 12V hunting light. When you've invested $2K in the hunt, short bulb life ain't an issue. mike

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Richard wrote: (snip)

I think most small, low voltage lamps were designed with a particular cell type and number in mind. As cell chemistries have improved, new types have been added to work well with these lower internal resistance cells. However I have not seen a list of cell and lamp combination compatibility. But I'll bet it is available, somewhere.

Most data is in the this form:

If you look at page 193 of this catalog:

and look through the voltage and current ratings for the PR8 through CM395X, (all lamps designed for two 1.5 volt cells, I think) you will see a general trend that the lower the lamp current, the closer the rated voltage is to 3 volts. I think this implies that they expect lower internal cell resistance drop for lower lamp current. You could probably go to the cell manufacturer's voltage versus current curves and deduce what cell chemistry and size would produce these voltages at these currents. All these lamp designs were initially requested by some customer who needed a lamp for a specific application.

I would try to find a similar sized lamp with both a slightly higher voltage rating (to increase life) and also a slightly higher current rating to produce the same light output power at a lower filament temperature).

Richard wrote: (snip)

That process would specify a lamp that would run at rated filament temperature and that would consume the same power from the battery. It would also produce less light than the bulb that was running well above its ratings.

Electrical power to light power conversion efficiency goes up dramatically as filament temperature rises. But life expectancy goes down dramatically, also.

(snip)

(snip) The graph of these relationships is on page 20 of:

sure

battery

bulb. Is

bulb

Thanks. Yea, when you look at a bulb out of some battery equipment, you quite often fail to see the bulb voltage match the nominal battery voltage. I think what this tends to mean is that in most cases the bulb voltage tends to be lower than the nominal battery voltage.

And of course, the main consideration is light output, and I suppose not drawing a current higher than the bulb is rated. Voltage of the bulb then then does not seem always to be of much concern, certainly it seems to me that you seldom find the lower voltage bulbs in battery equipment matching the nominal battery voltage.

I suppose things are slightly different when one is dealing with non battery source of power.

Point noted about different internal resistances of batteries.

One wonders whether the bulb or battery industry ever produced a table having suggested voltage ratings of bulbs for the various batteries.

You know, when one starts out with an interest in electronics first thing is messing about with bulbs and battery's. I did not carry the interest on professionally, however, I never realised there was so much to say about selecting bulbs given a certain set of battery conditions.:c)

I bought this halloween pumpkin with a lamp in it. It's run off two Duracell AA bateries. Actually I bought two, and both bulb have gone already. I think they are underated, I mean they must be drawing an overated current. But when they were lit the light output was about right.

Okay, this is what is says on the bulbs: 2.5v 0.2A. Okay so the problem is, how do you go about getting a bulb that gives the same light output, but is not going to be current overated? Rich.

Duracell

think

is,

is

When I think about it, I think I would have to measure potential diference across the bulb and current to get watts.

Then seek a bulb that was of the wattage measured.

And even then I don't suppose the task is an easy one.

Actually isn't that the right answer.

If when the overated lamp is burning I read 2.4v, 0.3A, then surely I'd need a lamp with those specs?

I used to put a diode in series with the bulb on cordless soldering irons to make the bulb last longer. They went from a few hours to a few years service when the diode was added.

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Michael A. Terrell
Central Florida

But

problem

but

diference

need

Actually, I think you would be reading a voltage higher than the rated bulb voltage. If you did not, then you would not be getting the overrated current.

Anyway, say the measured voltage was 2.7v, 0.3A. That's overvoltage of 0.2v and overcurrent of 0.1A. I ought to be able to approximate the increase in light output now given some formula.

"Light, Life, and Voltage For any particular lamp, the light output and life depend upon the voltage at which a lamp is operated. For instance, as approximations, the light output varies as the 3.6 power of the voltage and the life varies inversely as the 12th power of the voltage."

The normal light output would be 2.5v x 0.2A = 0.5W

The output when the bulb is being overated would be: 3.6 power of (2.7/2.5) = 1.319 times 0.5W = 0.659W

My math could be wrong here.

Yes, but how would I proceed from here. I just know that I'm seeking a lamp that's going to give out 0.7W

I've now got to match that with a battery voltage/current characteristic I think. But how. This is the tricky bit.

All this is presuming that I'm seeking a bulb working in it's ratings, which I might not if I were to feel efficiency is important.

Also the bulb is often rated at 2.4V, a 3V bulb will last longer.

--

Bye.
   Jasen

I

bulb

0.2v

in

varies

Really, all I know is that the battery is supplying 0.5W. I've no idea what the light output is, unless I could get to know that from a bulb spec.

(2.7/2.5)

Well, probably wrong. The *light output* will be 1.319 times whatever it was in the initial conditions of 2.5v

lamp

All I know is that if the bulb was working at 2.7v rather than the bulbs design voltage of 2.5v, the light output was 1.319 times the rated bulb light output, i.e, if 2.5v were across it.

I'm not sure how you would go about selecting a bulb that gave the same light output of the over-rated bulb, yet the new bulb within it's maximum ratings. Except by a bit of trial and error.

One ought to be able to calculate these things of course.

which

Life expectancy varies inversely proportionally with voltage to the 12th power.

Light output varies proportionately with voltage to the 3.2-3.5 power.

Current is usually close to proportional to square root of voltage.

This "rule" for tungsten incandescent lamps, however, is a "1-size-fits-all" and is only approximate, and holds up better for "reasonable" applied voltages.

Now for one more bit on design voltage of incandescent lamps to be used with batteries: It is common to have the design voltage only around

1.2-1.25 volts per cell. Not only is resistance of the cells a factor, but also the open circuit voltage of a cell that is halfway used up is less than 1.5 volts.

Keep in mind that flashlight lamps usually have a design life expectancy of only 15-30 hours or so at design voltage.

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