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Torch (flashlight) bulb voltage ratings



Jan 1, 1970
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

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

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)?

Thanks in advance, splin