This article was first published in 2008.

Last week in
How to Electronically
Modify Your Car, Part 2 we looked at circuits – including series circuits,
parallel circuits, short circuits and fuses. This week it’s time to examine some
of what is happening in a circuit.
Voltage
This circuit, that we looked at last
week, consists of only a battery and a light. The battery is marked as being
‘12V’ or 12 volts. But what does this mean?
Like many electrical terms, it’s
easiest to understand if an analogy is used  the voltage of electricity is a
bit like pressure, for example, the pressure of fuel in a
fuelline. A fuel pump in an EFI system pressurises petrol, pushing it through
the fuel line to the injectors. A battery produces an electrical pressure,
causing an electric current to flow through a circuit.
The higher the voltage, the greater
the distance that an electrical spark will jump. The ignition system produces a
voltage of more than 20,000 volts, and this high voltage allows the spark to
jump across the plug’s electrodes.
Electrical pressure is measured in
Volts.
Current
Current is the amount of
electricity flowing past a point. Using the fuel line example, it’s like
measuring how many litres per second are passing along the pipe.
Current is measured in Amps.
Wires that need to take a lot of
current (like the one to the starter motor) are thick. In contrast, a fuse uses very thin
wire inside it – so thin that if it is required to flow a current greater than
its rating, the wire burns out.
A starter motor might take 100 amps while an interior light might draw only half an amp.
Resistance
Resistance is a measurement of
how hard or easy it is for a current to flow through a substance.
Something with a really high
resistance is called an insulator  it lets almost no current through it. On the
other hand, anything which allows current to flow very easily is called a
conductor.
The normal wires within a car loom
are good conductors, while the plastic covering around them is a good insulator
 stopping the current from going where it’s not intended to.
As the resistance goes up, the flow
of electricity is reduced (more on this relationship in a moment). There are
lots of graduations between good conductors and good insulators, and the exact
value of the resistance posed to the flow of electricity is measured in
Ohms.
Many engine management sensors work by varying
their resistance. For example, this intake air temp sensor is a resistor that
varies in electrical resistance with temperature.
Resistors can also be thought of as
being a bit like a restrictor on a pipe. We mentioned above that you can think
of voltage as being rather like fuel pressure, and current flow as being like
fuel flow along the pipe. If you put a restrictor in a pipe, there will be a
pressure drop across the restrictor. In the same way, if a resistor is placed in
a circuit, there will be a voltage drop across the resistor. And, again
like the flow of fuel along a pipe, the greater the current flow, the greater
the voltage drop across the resistor.
Relationships
There is a strict mathematical relationship
between voltage, current and resistance. (It’s called Ohms law.) For example, if
you know the voltage drop across a known value of resistor, you can work out how
much current must be passing through the resistor.
The equation is:
This can be rearranged to be:
and
I don’t think it is worth memorising these (or any
other equations we cover – just look them up as needed). What is
important to remember is that there is a relationship between volts,
amps and ohms. Therefore, change one variable and the others will change.
There is also another relationship with which you
should be familiar.
It’s this:
‘Watts’ in electrical terms has exactly the same
meaning as ‘watts’ (or kilowatts) applied to car engines – it’s the rate at
which work is being done.
This equation can also be expressed as:
and
Again, unless it’s easy for you, don’t bother
memorising all these – just remember that there is a relationship between
volts, amps and watts.
Real World Stuff
OK, let’s look at an example involving watts.
You decide you want to put some driving lights on
the front of your car. They’re rated at 50 watts each, and because you are using
two, you know you’ll need to supply enough juice to run 100 watts of extra
lighting. You go off to chase some wire but you find that automotive wire isn’t
rated in watts, it’s rated in amps. So how many amps will need to flow in this
new wire?
Watts divided by volts = amps. We know the wattage
is 100 watts. We know that car systems run on 12 volts. So what is 100 watts
divided by 12 volts? It’s 8.3 amps. Use 10 amp cable and you’ll be fine.
Exactly the same idea applies to more powerful
systems. A hybrid petrol/electric car might have a 30 kilowatt electric motor
and a battery pack that provides 288 volts. So how much current does the wiring
need to handle? Watts divided by volts = amps, so that works out to 30,000 watts
(ie 30kW) divided by 288, which is over 104 amps! No wonder the cables are so
thick...
Now what about a trickier example?
We’ll assume here that you know how to use
multimeter (although we won’t cover that until next week!) so if you’re unsure
of how a multimeter is used, that’s fine – at this stage just pretend you do.
Let’s say that someone has told you that your
trailer brake lights are very dim. You check and they’re working – but the
person was right, they are hard to see. You get out the multimeter and
probe the trailer socket on your car. When someone puts their foot on the brake
pedal, there’s a measured 12V at the socket – so that’s OK.
Or is it?
Here’s the circuit  or at least the bit of it
that matters. The multimeter is plugged into the trailer socket brake light
connections, and is showing 12V. And if the meter reads 12V, then it must be 12V
that gets to the trailer brake lights, right? Now you might be thinking: “Not if
the wires that connect the trailer plug to the trailer lights are bad,” but
let’s state that all the bits on the trailer (eg the plug, wiring, bulbs,
reflectors, etc) are fine.
So why on earth would the trailer brake lights be
dim?
Here’s a clue: remember we said above that a
resistance in a circuit is a bit like a restriction in a fuel pipe? If the
current flow (ie amps) is very low, the voltage drop across the resistance is
also very low. A multimeter takes only a tiny amount of current from the circuit
it is measuring, so if there is any resistance in the circuit, measuring voltage
won’t show it. You need to have lots of current flowing to see what really
happens.
So what we’ve done here is to plug in the trailer
brake lights (they draw lots of current) and then again measure the voltage at
the socket with the brake pedal pressed. As can be seen, the voltage has dropped
to 8V, explaining the dim trailer brake lights. Therefore, there must be a
resistance (ie lots of ohms) in the circuit between the car brake lights and the
socket.
So without the correct current (amps) flow, the
voltage drop across the resistance was not visible  there’s another example of
that relationship between volts, amps and ohms.
Example
Car Modification  Dashboard Monitoring LED
LEDs
are often used as dashboard indicators – to show when an intercooler water spray
pump is working, or even when an Exhaust Gas Recirculation (EGR) valve is
switched on. (Running lots of EGR is good for partthrottle fuel economy, so it’s useful
to know when an electronically controlled EGR valve is activated.)
But
LEDs operate on very small currents – they are devices that die if too much
current flows through them. Connect a normal LED across 12V and the LED will
immediately be destroyed.
The
easy way of using LEDs with 12V car systems is to place a series resistor
in the circuit. The series resistor limits the current flow through the LED.
Resistors
are electronic components that are very cheap (a few cents each) and are
available in an enormous range of types. They have two key specs – their
resistance (measured in ohms) and the power that they can
dissipate (measured in watts).
So
what type of resistor do we need to allow the LED to be run off 12V?
In
addition to colour, intensity and package size, LEDs have two other important
specs. One is what is called the “forward voltage drop” and the other is the
LED’s “maximum current”. With these two bits of information (that are available
from where you buy the LED), the required resistor can be calculated.
A
bright orange LED might have these specs: voltage drop of 2.2V and a current of
75 milliamps (ie 0.075 amps).
From
Ohms Law above, ohms = voltage drop (across the resistor) divided by amps
If
we are supply 12V, and we only want 2.2V at the LED, we want the resistor to
drop (12V – 2.2V =) 9.8V.
So,
ohms of the required resistor = 9.8V divided by 0.075 amps (the
required current flow through the LED)
9.8
divided by 0.075 = 131 ohms.
Therefore
a 131 ohm resistor will limit the current flow to 0.075 amps through the LED.
The
other spec of a resistor is its required power dissipation in watts.
Watts
= amps x volts, so that’s 0.075 amps x the voltage drop across the resistor,
which is 9.8V.
Watts
= 0.075 x 9.8V = 0.7 watts.
So
we’ve worked out we need a resistor with 131 ohms of resistance and a power
handling of 0.7 watts. The nearest offtheshelf design to this is a spec of 120
ohms and 1 watt.
(However, with a running car voltage that is higher than the nominal 12V, and with the fact that you probably don't want a dashboard monitoring LED to be superbright, a 620 ohm, 1 watt resistor will drop LED current and still result in a LED bright enough to be easily seen.)

Conclusion
Now if you recoil from maths, you might be looking
at the above breakout box and seeing just mumbo jumbo.
But don’t worry about it!
Take just this information from it: we decreased
the current through the LED by putting a resistor in series with the LED. The
greater the ohms value of the resistor, the less current that will flow through
the LED (and, incidentally, the dimmer it will be).
As I have kept stressing in this article, it’s the
idea that there is a strict relationship between volts, ohms and amps
that is the critical thing to remember. If one is changed in a circuit, then the
others are changed as well. The same applies for watts, amps and volts: if one
is changed, then the others must change too.
If you simply remember (volts, ohms, amps) as one
bunch of interrelating variables, and (watts, amps, volts) as another bunch of
interrelating variables, you’ll be streets ahead of where you were at the
beginning of this article.
Next week, we’ll use a multimeter to directly
measure these things.
The parts in this series:
Part 1  background and tools
Part 2  understanding electrical circuits.
Part 3  volts, amps and ohms
Part 4  using a multimeter
Part 5  modifying car systems with resistors and pots
Part 6  shifting input signals using pots
Part 7  using relays
Part 8  using prebuilt electronic modules
Part 9  building electronic kits
Part 10  understanding analog and digital signals
Part 11  measuring analog and digital signals
Part 12  intercepting analog and digital signals
Part 13  the best approaches to modifying car electronics and the series conclusion
