wiring 101 part two - by John Meister © 2000

In the last issue (of FSJ magazine, Vol #2, 2000) we left Biff and 
Billy Bob stuck on the side of the interstate with a fried electrical system.  
Biff was just going to explain the magic circle when headlights appeared.

Biff looked up and watched the headlights of a nicely lifted 1977 Cherokee 
close in on him.  The lights were bright, crisp and white, and getting closer by the
second.  He stood there like a deer in the headlights, a large deer.

The 401 loped to an idle and the pilot jumped to the ground.  It was Sally
Sue Jane McWheeler, PhD.  Her petite frame contrasted greatly with Billy
Bob's, who was now looking down at his sister.  Words were not needed, he
knew that she knew that he knew that he really screwed up, again.  He just
got the tow strap out and hooked it up.

Back at the house Biff grabbed a napkin and drew the magic circle for 
Ohm's Law.   (insert circle here)

Biff described in great detail the inverse relationship of current to resistance.

Billy Bob said "Hey, I got it!  If I add more lights to the system, 
that adds more resistance, right?".  Biff and Sally looked at each other 
and smiled, said that there was hope.  Then Billy Bob continued, 
"So, if the current goes down with more resistance then the more lights I add, 
the less current I need, right?"  Biff and Sally's head hit the table about the same time.

Sally raised her head first, she said, "Hey, at least you're thinking, as scary 
as that might be, but there is something we haven't discussed yet, the circuit
configuration. The way we hook up lights to a battery actually reduces the
resistance and increases the total current flow."

Biff nodded and said, "There are different circuit configurations:
    SERIES, PARALLEL and SERIES-PARALLEL.  

In a SERIES circuit, you add up all the resistance in one leg.  Then determine
POWER by taking the total current and dividing by the resistance.  

In other words," Biff explained, "if you put two lights in series they
would basically see HALF the total voltage and produce HALF the normal light.  
You could say they'd be dim bulbs."

Sally Sue suppressed a snicker and Billy Bob just ignored both the comment and reaction.

Biff continued, "In a PARALLEL circuit, the POWER of each element is 
added.  Using two identical lights, they'd have the same voltage, use the 
same current and produce the same amount of light, BUT you'd be using 
TWICE the CURRENT as when they were in SERIES.

In a SERIES circuit you add the total resistance, but in a PARALLEL circuit 
(the one used in your FSJ), your total resistance is less than your least 
individual resistance.  We won't show you this formula because we like you 
Billy Bob.

Anyway, there is MORE CURRENT FLOW when MORE ITEMS ARE ATTACHED to the battery."

Billy Bob nodded his head in agreement, both Biff and Sally Sue could see that
the lights were on and someone was home.  They smiled at each other in a way
that disturbed Billy Bob.

Biff continued, "Let's look at the POWER factor."

Biff drew the magic circle for POWER.  
(insert power circle here) 

Biff said, "It's simple really.   We're dealing with 12 volts, 
we're not worried about resistance, so that leaves only TWO THINGS
to deal with Billy Bob:  WATTS and CURRENT

Billy Bob looked at Sally Sue, smiled and said, "Just two things?  
I think I can handle that much. There is hope."  They all laughed.

Biff said, "Well, there is a lot more involved, but let's see if we 
can't boil this stuff down to a simple plug and play, you know, 
like the latest Linux install."  

Billy Bob looked puzzled and spoke up, "Don't you mean like MS Windows?".

Biff laughed and said, "No, Billy Bob, that's plug and PRAY!"

Biff and Sally chuckled and Biff continued, "Look at what I've got on this napkin, 
a virtual Rosetta Stone, if I don't say so myself.  Simple stuff.  
Of course it assumes that each light you decide to add is a standard 
H1 bulb rated at 55 Watts, and of course the values are approximate."

The napkin was laid out in a nice table format:


# of lights	P 	I 	wire size
---------------------------------------------
1      		55	4.6	16 ga
2      		110	9.2	14 ga
3      		165	13.8	14 ga
4      		220	18.3	12 ga
5      		275	22.9	10 ga
6      		330	27.5	10 ga
7      		385	32.1	8 ga
8      		440	36.7	8 ga

(P - in Watts, I - in Amps, E = 12 volts)


Biff, trying to sound like a radio announcer attorney for a used car lot commercial,
said, "Of course, design for worst case conditions, highest temperature,
lowest resistance, use stranded wire, use relays, always use a fuse in the
circuit close to the power source, keep wire lengths as short as possible, 
when in doubt use a thicker wire (lower gauge number), secure wiring so
it doesn't rub against sharp or hot objects, avoid sharp bends, make sure you 
make solid splices or crimp connectors, make sure you have a solid ground from the
battery to the body and one solid ground connection to the dash, and don't 
worry about using too much electrical tape, of course, your mileage may vary."

Sally Sue applauded and said,"All true, but don't forget to figure in all 
the normal electrical loads on your rig, like the wipers, tail lights, dash 
lights, heater fan, radio, and so on when sizing your new alternator." 

Then Sally Sue said, "Let's get Billy Bob's Wagoneer back on the road,
I just upgraded my alternator to a new on board welder type and I've got my old 100 
Amp alternator in the back of the Cherokee, let's drop it in right now!"

Biff looked at his watch, it was almost two in the morning.  He was thinking
that Billy Bob's sister wasn't the tyrant that he had made her out to be.  

What's in store for our circle of power loving FSJ owners?  Will they find
power, and friendship under the hood of Billy Bob's Wagoneer?  Will Biff get
to ride in Sally Sue's Cherokee?  Will Biff ever give up his AMC Pacer for
a Full Size JEEP?  


The continuing saga continues:

http://johnmeister.com/jeep/sj/FSJ/FSJ-Magazine-ARTICLES/BillyBob-3.html