On Sun, 16 Jun 1996, Glen Eldridge wrote:
> Mark Jurras wrote:
>
> >
> > Sorry to burst everyone's bubble but it is not good Lucas bulb design
> > but poor Lucas electrical design that keep you bulb burning. Bulbs are
> > designed to have a certain life at a certain wattage. The wattage of
> > the bulb is obtained by applying the correct voltage to the bulb.
> >
> > Let's look at an example I am familiar with. A certain 12V 100W quartz
> > halogen projector bulb life is rated at 25 hours. Not to long. But if
> > the voltage is reduced, the life goes up exponentially. Running this
> > bulb at 8 volts the calculated life is >20,000 hours.
> >
> > Now with all the Lucas connectors and grounds that corrode the full 12V
> > never gets to the head lights thus extending the life dramatically.
> >
> > - -Mark "My bulbs last longer than my connections" Jurras
>
> Mark et al., From the depths of my rusty store of electrical formulae,
> I seem to remember that watts divided by volts equals amps. Given that
> is so, if the voltage drops for a given wattage of lamp, that SHOULD
> mean that the amperage goes up. I don't know how much current that
> headlight circuit is fused for, but a drop of 4 volts in a 12 volt
> circuit means an increase of 50% in the amperage flowing through it.
> Somewhere, something has got to give; let's hope it doesn't require a
> fire extinguisher! :-)
>
> Regards,
>
> Glen Eldridge
>
While your rusty store is correct, the assumption that the
wattage of the bulb remains constant is at fault. The wattage rating is
at some particular voltage - 12V in this case. What does remain constant
is resistance of the bulb, and the formula you want is V=iR, where i is
current in amps. Thus, if the voltage decreases, the current does also,
and the power in watts (and brightness) decreases quadratically.
Len Bugel '51 TD
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