-----Original Message-----
From: REwald9535@aol.com <REwald9535@aol.com>
To: tboicey@brit.ca <tboicey@brit.ca>; mgs@autox.team.net
<mgs@autox.team.net>
Date: 14 December 1998 22:40
Subject: Re: Speedo Fun
>I gotta disagree with your conclusions here. There are two ways to wire up
a
>string of lights. In series or in parallel.
Wrong. What we have in our cars is a combination of series and parallel.
The lamps are in parallel with each other, and this parallel circuit is in
series with the various resistances - including the rheostat - that form
part of the circuitry back to the supply i.e. we have a network.
>You say that as a bulb fails there will be a little more voltage to each
bulb,
>and that LUCAS connectors might make that a lot more. There is no
connector
>made by LUCAS or anybody else that can increase the voltage over what the
>source voltage is. End of story. <G>
Trevor (?) is exactly right in what he says. n lamps in parallel will draw
a amps, which will drop v volts across the series resistances in the
circuitry. One lamp fails. The current drops. The voltage in the series
restances drops. Therefore with a given supply we now have more volts
across the remaining lamps than we did before. However, see below.
>So why did the bulbs fail all at about the same time? Simple, service
life.
>The bulb is designed for a set service life, say 1000 hours to pick a
number.
>The guys in the manufacturing plant then build a bulb that will last the
>design time of 1000 hours within a couple of hours or so. So when one goes
>out they are all on their last legs because they have reached the end of
their
>design life
The 'service life' effect (and the increase in voltage as each lamp blows)
is far outweighed by the fact that all the bulbs are being overdriven by
some 60% (nearly 100% if you consider the running voltage is nearly 14v),
it's hardly surprising that they failed. They are also drawing considerably
more current through the rheostat, not the most robust of electrical
components. Fitting 12v bulbs of a higher than standard wattage will
overcome the premature failure of the lamps, but will still overstress the
rheostat when in positions just short of full brightness.
PaulH.
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