In a message dated 12/14/98 12:10:41 AM Pacific Standard Time, tboicey@brit.ca
writes:
> > Uhhh...Skye, did I mention that since I put all mine in at the same
> > time, they all blew within a week of one another? 8-) I guess
> > that's the price you pay for progress!
>
> While they are all running, the current draw through
> the "feeder" line is highest, and so is the voltage drop
> across the feed wire. So, the voltage at each bulb is
> lowest.
>
> Once they start to go, the current draw for the circuit
> falls, and so does the voltage drops along the way. So the
> bulbs start to get a little more voltage. Actually, with
> the state of a lot of LUCAS connectors out there, the
> "little more" might actually be more than a little.
>
> So, it really might be8j more than bad luck causing them
> all to go.
I gotta disagree with your conclusions here. There are two ways to wire up a
string of lights. In series or in parallel. Series is how a cheap set of
Christmas lights are set up, you know if one goes out they all go out. In a
series circuit each bulb receives it's share of the source voltage. For
example a 12 V circuit and three identical bulbs each bulb gets 4 V to
operate. If you were to rewire a three bulb circuit to a two bulb circuit
then each bulb would see 6 V. Under this scenario yes the remaining bulbs
would come to a quicker end, however there are two problems here. First when
the first bulb blew, there would be no more lights. Second to get lights back
you would have to either rewire the circuit or replace the bad bulb. Not even
the Prince of Darkness himself would wire an MG that way.
In the other type of circuit, parallel, each bulb has a separate feed, and
ground. This way if one bulb goes out the rest stay lit. Wire in good
condition generally has little to no voltage drop. I would expect you to be
able to wire in taillight bulbs and not see any voltage drop through the wire,
on most test equipment. YMMV. I teach automotive electronics to technicians
and I do not recall ever seeing over one-one-hundredth of a volt drop in any
wiring harness EXCLUDING connectors, switches, or starter circuit.
Well, what about connectors and switches you ask. Well, I'm glad you asked.
Automotive industry standard has always been to allow no more than one-tenth
of a volt drop in any connector or switch. I am going to assume here that all
of the members of this list have wiring harness that conform too, at the very
least, accepted automotive standards. <VBG>
But wait, you say, there are several connectors in line to each bulb and each
one has a voltage drop and voltage drops add up. Yup they sure do, so let's
do some math shall we?
Lets assume that the circuit has six bulbs, and each bulb has four connectors
either on the power side or on the ground side. Each connector has a one-
tenth voltage drop. Let's further assume that the switch has a voltage drop
on tenth of a volt.
So our total drop to each bulb is 4/10 + 1/10 for a total of 5/10 or 1/2 volt.
Now what happens if a bulb blows? Basically not much. The voltage drop
through the switch might go down a smige (1/6 of 1/10) and the current that is
not going to the one bulb will be split among the remaining five. The net
effect is that each bulb might and I repeat might see an extra 1/10 volt. Big
Deal. <G>.
> Once they start to go, the current draw for the circuit
> falls, and so does the voltage drops along the way. So the
> bulbs start to get a little more voltage. Actually, with
> the state of a lot of LUCAS connectors out there, the
> "little more" might actually be more than a little.
>
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>
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. I see this on the newer cars that have day time running lights
one will fail with in a week of the other.
If you still have doubts replace all your dash bulbs with these Radio Shack
ones. In eight months when the first fails, replace it post haste and keep on
driving. Then report back to the list as to the lasting ability of the rest
of the bulbs.
I will await your experimental results. <G>
Rick Ewald
67 B roadster
99 Volvo S 80 T-6
Why have a complex explanation when a simple one will fit?
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