Wiring 101
 
by John Meister  © 2000 

 Ohm's Law

The best place to start a discussion about wiring is with Ohm's Law.

It is possible that scientists and engineers find the concepts of electricity so 
easy that they think anyone could figure it out.  So I have a theory that they purposely 
set out to complicate things by describing it with mathematics and odd terms.  I think
they do this to protect their jobs and make themselves look smart.  

I refer to this phenomenon as "pseudo-intellectualizing".

Large words, complex diagrams and formulas are used to hide behind.  
However, when you get into the subject you find that the language of 
mathematics is useful to describe what is happening.  That doesn't explain
the odd terms... at least not until you study the history of how things
came about.  That, of course, is not a subject we'll be able to pursue.

So, at the risk of looking smart, and those that know me won't so falsely accuse me,
I'll attempt to describe the basics in such a way that you too will be able to appear
smart to your dog and kids, and maybe even your friends, provided of course they don't
read this article too.  (quick, hide it before they see it!)

Ohm's Law is probably the most useful way to explain what happens when you connect
a power source to a load.  Basically there are three elements of a basic circuit.  


 Voltage (the power source, aka, the battery),  
 Current (the "juice flowing through the wires"), 
 and the Load (the resistance to the juice, e.g. a light bulb).


Using our high brow mathematical type terms, let's refer to the same three elements:


 E - for electromotive force.  E is measured in VOLTS
 I - for current.  I is measured in Amperes, or AMPS  
 R - for resistance.  R is measured in Ohms.


Without going into a lot of detail, let's identify some very simple arrangements
of the mathematical relationship and then provide a few simple examples to follow.

There is a relationship associated with the elements.  The following formulas
define those relationships:


 E = I * R
 I = E / R
 R = E / I


In the first equation we see that VOLTAGE is equal to CURRENT times the RESISTANCE.
Ok, so what the heck does that mean?  Simply that if you apply 12 volts to a circuit
that has a specific resistance it'll allow only so much current flow.  That current
flow multiplied times the value of the resistance will equal the total voltage applied.

In the second equation we see that CURRENT is equal to VOLTAGE divided by the RESISTANCE.
That's just another way of saying what we just said, right?  If you apply 12 volts to
a circuit you will restrict the current by that resistance.  The formula tells you by 
how much.

In the third equation we see that RESISTANCE is equal to the VOLTAGE divided by the CURRENT.


In all three equations there is a relationship among the elements.   Let's show a simple example:

  E = 12 Volts
  I = ??
  R = 100 Ohms
  
What would our current be?   We have two of the three elements.  Ok, we could do the math
and find it... but some of us have  trouble transposing even the simplest mathematic formulas.
The formula we need solves for I, so we should use I=E/R, therefore I=12/100 or 0.12 AMPS.
(this may also be refered to as 120 milliamps)  In the grand scheme of things that isn't
a lot of current.  But we'll get into more of this in a little bit.

What we need is a simple tool to show us the correct formula we need to solve for the
missing element.  Anyone with any training in electronics or electricity, or has looked
ahead, knows what's coming next...


 The "magic circle" 
 

So know we can quickly "see" the formula we need by covering the missing element.
Because we were looking for I, or the current, we simply cover the "I" and we can
see that we need to divide E by R to find I.  Simple.  

Ok, so with that out of the way, let's progress a bit more in our understanding
of electricity.  We saw that we had 0.12 Amps flowing in a circuit with 100 Ohms of
resistance when 12 Volts was applied.  What does this mean anyway?  What's voltage
and what's current and what's resistance???


Ok, simply put, VOLTAGE is a POTENTIAL.  It's a stored up charge if you will.  It's
energy just sitting there. It's a bunch of electrons sitting on one side of the
battery trying to get to the other side of the battery. Why?  Because one side of
the battery will have an EXCESS of ELECTRONS, the other an ABSENCE.  Electrons have
a NEGATIVE charge.  The holes, or absence of electrons represents a POSITIVE charge.
Current flows from NEGATIVE to POSITIVE.   Before you get all excited, you need to
understand that this is the view of the electrons.  If you were to look at the
holes and watch them "flow", you'd say exactly the opposite, that electricity flows
from positive to negative.  Well, you'd also be right.  It's a matter of perspective.

ELECTRON flow dominates much of electronics.  However, following the holes helps
one understand semiconductors.  So, not wanting to get into a prolonged discussion
on atomic theory, we're gonna say that an excess of ELECTRONS represents a NEGATIVE
CHARGE, and an absence of electrons, or an excess of PROTONS represents a POSITIVE
charge.  

A battery has two terminals.  One POSITIVE, the other NEGATIVE.  Are ya still with me?
Basic physics states that things seek to equalize.  So the electrons want to fill
the holes.  But they can't.  There is something in their way.  A "dielectric" if you will.
Some insulation or barrier.  They need a path, a conductor in order to equalize the
charge and seek their balance in nature.  (We don't want them to seek their balance
unless we have a set of jumper cables and a friend with an imbalanced battery...)

When you place a conductor across those terminals you have a rush of electrons 
seeking to fill the holes, but they meet opposition, RESISTANCE if you will.  The
resistance restricts the CURRENT FLOW.    

 a quick note on SAFETY 

When dealing with large supplies of electrons you want to make sure that 
YOU are not the resistance to those electrons!  Direct Current, mostly used 
in automotive circuits, will not typically harm you in a serious way.  Alternating 
current, as found in your home and in your alternator, will do bad things to your body.  

The greatest danger is associated with your cardio-respiratory system.  

NEVER allow your body to complete a circuit where that circuit crosses your chest.  
Your muscles can't react as fast as Alternating Current and you will go into cardiac 
or respiratory arrest, or sustain permanent muscle damage.   In other words, either
your heart quits beating or you stop breathing, neither event is a great deal of fun
even if you don't meet up with Jesus.

Our goal is NOT to stop breathing or our heart beats.  Fortunately
when dealing with automotive electricity we're not in serious danger.  Of course
if you've ever latched on to a spark plug cable you might think otherwise.

For what it's worth, as little as 21 volts at 9 milliamps can kill you.  
Of course you'd have to have the electrodes placed directly across your temples or 
heart to do it... but the point is it's a risk.  Getting an electrical shock is NOT fun.
(Although when it happens to someone else, and they survive, it can be funny.)


 a slight problem with the math on resistance...

Ok, so we can determine the resistance in a circuit if we know the voltage and
the current... But you just looked at your headlights and they're rated at 60 WATTS. 
They don't have a resistance value.  So how can you tell how much current they'll draw?

That's a good question, you can't based on what we've been dealing with so far.  A headlight,
or a lamp, does not have a resistance that we can easily measure because it would be
very low.  A lamp has a resistance that increases as the temperature goes up.  We say
it has a positive temperature coefficient.  (there I go, pseudo-intellectualizing!)  In
other words, as it gets hotter it has more resistance.  When you first apply power to
a light bulb it has very low resistance, and LOTS of current flows through it... the
current flow through it causes it to heat up.  Because of the properties of the metal
involved the resistance goes up, reducing the current flow, but keeping the filament
hot enough to glow white hot!  The problem is we can't really measure that resistance
accurately as it's quite low and varies with temperature.  So we use a different
means to determine what the load on our battery will be.

 the POWER factor 

The use of a battery represents a DC circuit, DC stands for Direct Current.  Meaning
that electrons are flowing one way to the holes on the other side of the battery
through our circuit.  Power is what is expended across the RESISTANCE when you apply
the VOLTAGE.  The CURRENT going through the RESISTANCE typically creates heat, or some
other release of energy.  This release of energy is measure in WATTS.  So, when we
say a headlamp is rated at 60 Watts, we know that it is expending energy when it
has 12 volts applied to it.  The electrons flowing through the filament meet resistance,
expend energy in the form of heat that is in the visible light spectrum and voila,
we can see the trail, the road or the tree we're about to hit...  

So now we need to determine how to calculate POWER.   Wait, why do we need to do this?
I'm glad you asked.  So we can determine the amount of CURRENT going through the
wires to the lights that are on our front bumper so we can determine how big
the wire should be and how big our alternator should be, that's why!


 another "magic circle"
 

Simply put POWER, measured in Watts, is equal to VOLTAGE times CURRENT.

We can also manipulate the basic formula in the same way we did with Ohm's law.
So, if we take our 60 WATT lamp and divide it by 12 VOLTS we see that we
have 5 AMPS flowing!   Hey, NOW we're getting somewhere.  Now we've got
a simple way of understanding what's happening and can total up all the
current flow and determine what we need for our alternator.  


  left headlight - 60 Watts
  right headlight - 60 Watts
  dash lights, 8 * 3 Watts = 24 Watts
  tail lights  2 * 15 Watts = 30 Watts
  side markers 4 * 3 watts = 12 Watts
  stereo ...
  ignition ...
  defroster fan...


It all adds up... We've got 186 Watts with just lights in the example...  
That's 15 AMPS.  If your lights are all running through that one little
headlight switch on the dash and if there is ANY corrosion on the connectors,
that puppy is gonna get warm.  Corrosion is resistance.  Resistance results in
heat.  Heat results in an unpleasant physical stimulation if you touch it,
and if it gets hot enough it results in an unpleasant roadside BBQ, or the
meltdown of wiring and another unpleasant situation known as darkness.

Before I go any further, here are the formulas for calculating Power:


  P= EI
  P= (I*I) * R  (that's I squared times R)
  P= (E*E) / R  (that's E squared divided by R)


 the summary, and the next step

To summarize we discussed the very basics of Ohm's law.  We identified
that the charge on the battery represents VOLTAGE, the wires carry the current, 
and things like Headlights represent resistance.  We also discussed that resistance
dissipates Power, measured in Watts.  Current flows through our circuit when
we complete it.

The next step is WIRING 101 - PART 2, where we will discuss series and parallel circuits
and begin calculating specific loads and currents that we will see in typical
automotive circuits.   We'll also begin looking at wire sizes and examine the
role of the relay.

In Wiring 101 - PART 3 we will discuss specific design concepts and troubleshooting  
techniques.  We will also discuss fire prevention by good wiring practices. 
(# ed note: maybe... haven't read this since 2000... jm 8/11/2021)


This is part 1:
http://johnmeister.com/jeep/SJ/tech/Wiring/101-wiring-PART-1.html 

 http://johnmeister.com/jeep/SJ/tech/Wiring/101-wiring-PART-2.html

 http://johnmeister.com/jeep/SJ/tech/Wiring/101-wiring-PART-3.html






 








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