Wiring 101
by John Meister © 2000
Smoke on the shoulder
Billy Bob Joe McWheeler and Biff Buford were tooling along down the
Interstate in Billy Bob Joe's lifted 66 Wagoneer. Billy Bob Joe decided
to turn on some auxillary lights... all of them... And that's when
it happened... everything went dark and all they could see and smell
was smoke... Billy Bob Joe wrestled the Full Size Jeep to the shoulder
and both of the beef brothers rolled out and headed for the hood. Biff,
the lightweight one of the pair at only 275 pounds made it there first. He
popped the hood and toxic smoke wafted up and slapped him upside the inside
of his nostrils. He reeled back coughing and hacking like he'd just taken
his first cigarette. Billy Bob Joe came up a bit slower, being about 325
pounds he was a few seconds behind Biff, which allowed the smoke to clear
so he could see what he saw. And what he saw was a glowing melted line of
burnt plastic and the remnants of copper running from the top of his
battery across the fender and into the opening on the firewall. The top
of the battery had nearly melted through, lucky for him the wire melted
away before the battery case did.
Billy Bob Joe could also see the remnants of what was once the wire between the
alternator and the battery. The only reason the fire only lasted a few
seconds was because he had replaced the fusible link with
some speaker wire just a few days ago... The last time he tried the lights.
Billy Bob Joe looked at Biff and made some disparaging remarks about how
they didn't make wire like they used. Biff looked at him and said,"My
dear comrade. How could you have possibly overlooked the fact that you
have significantly overloaded your electrical system. Why just look
at that poor little 35 amp alterntor. The only thing that saved it
from total destruction was your use of 24 gauge speaker wire.
Why didn't you use a real fuse? Why didn't you upgrade your alternator to
accomodate those 8 - 130 Watt Driving lights that you just tried to
turn on! Do you realize that you tried to extract about 87 amps from a 34 year
old 35 amp alternator?"
Billy Bob Joe just stood there with one of those looks that makes for a great
gag greeting card. When he found some words he opened his mouth and the
following words fell out, "Well, I guess I shoulda bought that Heavy Duty
Alternator at the junk yard last week."
Biff rolled his eyes and said, "Billy Bob, that was a 40 Amp alternator. Not
exactly what I meant by an upgrade. You should consider having that electrical
shop in town rebuild your alternator to produce about 105 Amps. Will only cost you
about $100 if they do it. Or you can buy the kit in one of those mail order
catalogs you keep behind the throne in your mobile home for about $36.95, plus
shipping, of course. But, that isn't gonna solve your fundamental problem.
You need to learn a few things about wiring."
Ohm's Law
Biff continued,
"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 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 real smart. You know
what I mean Billy Bob, kind of like that kid sister of yours that teaches Physics
at the U. (Biff wondered how the same family could produce both Dr. Sally and
Billy Bob from the same gene pool.)
Anyway, I refer to this phenomenon as "pseudo-intellectualizing".
Large words, complex diagrams and formulas are used to hide behind.
However, I've found that once 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, mainly
because we're standing by the side of the road, in the dark and your Wagoneer
ain't going anywhere until we get the electrons back on the job.
But, since we're gonna be standing here for a while waiting for someone to stop
and give us a jump, we've got some time to look at the wiring situation.
So, at the risk of looking smart, and since you know me I know you won't
so falsely accuse me."
Billy Bob looks at Biff with one of those grins that explains all that's going
on between those two crusty ears... Biff looks up and says, "Don't say nuthin'
Billy Bob... just nod your head, ok?"
Biff grins back at Billy Bob and continues to explain,
"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 kid sister... well... that might
be stretching it a bit...
Ok, so the basics go something like this, 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 some 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
Biff takes a breath and looks up at Billy Bob, who appears to be following the
discussion.
So Biff asks, "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. It's a whole lot less than what you let out of your rig tonight...
Anyway, 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 used something called a "magic circle", maybe even your kid
sister."
Biff draws a circle in the soft sand on the side of the shoulder... the light
of the street light near the off ramp lights it up nicely.
The "magic circle"
Biff continues, "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???"
Billy Bob Joe is nodding his head in agreement. Biff notices that he's following
along and is even interested.
Biff continues, "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, I'm 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
Billy Bob?"
Billy Bob looks at the battery and nods.
Biff realizes he still has Billy Bob's attention and continues, "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, otherwise you'll end up on the
side of the road listening to someone explain basic electricity...
Anyway, 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. In our case when you flipped the switch
on those driving lights the resistance couldn't handle the current and burned up."
(sidebar)
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.)
(end of sidebar)
a slight problem with the math on resistance...
Biff continues, "Ok, so we can determine the resistance in a circuit if we know
the voltage and the current... But just look at your headlights, 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.
Biff laughs, and says, "There I go, pseudo-intellectualizing! I sound like your
kid sister... 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. Just like
what happened when you threw that switch. The current went through the wire, but
it offered up resistance and heated up to the point of melting... or probably
vaporizing. You do realize we're lucky you didn't burn this rig to the ground!"
Biff continued with the lesson, "Because of the properties of the metal
involved in the lamp, 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
Biff took a breath and said, "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... Unlike right
now where we see nothing because your battery dumped all it's electrons into the
holes, melted your wires and nearly turned your FSJ into a road side BBQ pit."
Biff looked at Billy Bob to see if he was still clinging to the concepts,
"So now we need to determine how to calculate POWER. We can determine the
amount of CURRENT going through the wires to the lights that are on your
front bumper so we can determine how big the wire should be and how big your
alternator should be."
Billy Bob took an interest in what Biff was saying now. He was expecting to
find the answer he was looking for and was having trouble staying awake as
Biff droned on. Biff drew ANOTHER circle on the side of the road and continued.
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
It all adds up... We've got 186 Watts with just lights in the example...
That's 15 AMPS. That leaves about 20 Amps for your ignition, fan,
wipers and extra lights. 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.
The darkness we are experiencing right now. What amazes me is why your wiring
didn't fail before now.
Ok, Before I go any further, here are some other 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)
Biff gets to the summary
Biff stands up and says with a professorial air,
"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 begin calculating specific loads and currents
so we can fix your Waggy. We'll also look at wire sizes and how we can use
some relays. If you're gonna be running all that current out to those lights
on the front bumper you're gonna need them."
Billy Bob Joe looks up and sees a friend pulling up behind his rig and heads
toward the headlamps like a moth drawn to a flame... He's moving as fast
as he can to get away from Biff before his brain gets hurt any more than
it already is... sheesh, it's not like he hasn't heard all this stuff before.
After all his kid sister told him the same thing just last weekend when he
burned up the stock wiring and they had to push his Wagoneer back into town.
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