Torch (flashlight) bulb voltage ratings

In article <[email hidden]>, [email hidden] says...

Quoted message said:

I expect this question comes up from time to time, but can someone explain the voltage ratings of
small bulbs?

I want to build a voltage regulator for a cycle lamp to provide a reasonable compromise between
output and lifetime (say 25 hours?). I understand the relationship between the light output and
filament lifetime as the applied voltage varies above and below its rated value. However in the UK
(at least) bulbs for 2 cell torches (flashlights) or cycle lamps are commonly available marked as
2.2, 2.3, 2.4 2.5, 2.7V and even
2.8V. The most common seem to be 2.4 and 2.7V

I think the main difference is the chemistry of the batteries they are designed to run with. Typical
NiMH batteries run about 1.2V, while alkalines will be more like 1.4V to 1.5V when new. Of course
your comment below about de-rating them to get different brightnesses and lifetimes is probably also
part of the determination.

Quoted message said:

Since these are all intended for use with a nominal battery voltage of 3V (for new batteries),
what are we to make of these ratings? Is there a 'proper' standard for the voltage ratings or is
this mostly historical and related to the size (and hence the internal resistance) of the
zinc-chloride cells which used to power them before low impedance alkaline and rechargeable cells
became commonly availabile?

The 2.5V bulbs are .8A Philips halogen bulbs (GH-90 I think). What were they designed to be driven
by? I'm guessing NI-Cd cells, but then they don't seem to be any brighter than 2.7V/.7A Krypton
bulbs when also driven by 2 Ni-cd cells..

Or are they all pretty much equivalent and will all have similar lifetimes when operated from the
same type of batteries?

Or are the bulbs designed and rated relative to the size of batteries they are typically used with
- eg. the 2.2V prefocused bulbs seem to be typically used in lamps driven by 2 AA cells, which
will have a rather higher internal resistance than a pair of D cells and thus will be exposed to a
lower average voltage. My experience of the 2.2V bulbs is that they do have a rather short
lifetime - a few hours at most - and thus probably are substantially overdriven.

Searching on the Web it seems to rather hard to get proper lumen and lifetime specifications for
these small bulbs. So without having to do a lot of experimentation/life-testing can anyone help
me out? Incidentally I'm not restricted to using 2 cells but I am curious as to why so many
different voltage bulbs are available.

And finally, would a 6V/3W bulb be more or less efficient than a 2.4V/3W bulb (for the same
lifetime)?

Well, to get 3W at a lower voltage, it will need more current, so your resistance losses in the
battery and wiring will probably be higher, causing a drop in overall system efficiency. I doubt the
bulb itself will have much difference in efficiency though, at least not due directly to the
difference in voltage rating. The lower voltage bulb will probably take much more abuse and
vibration, giving you a longer service life before failing however, because the filament will be
thicker and heavier.

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