
Car Audio  Electrical Theory
Ohm's Law
Voltage (volts) is the force that moves electrons, forcing a
current. Voltage can be compared to a tank of water elevated at a certain height
(potential). If the tank is placed low (low voltage), water will not flow very
quickly (low current). If the tank is raised to a higher location (higher voltage),
the water will flow rapidly (high current).
Current (Amperes) is, in simple terms a measurement of how many
electrons flow through a device. In the water tank analogy, current would be water
flow rate.
Resistance (Ohms) slows down current flow. The higher the
resistance of a circuit, the lower the current will be. Resistance would be
equivalent to pipe size. If you have the water tank at a high level, but the pipe
is very small in diameter (high resistance), not much water will flow. If you use a
big pipe (low resistance), then the water flow rate will be larger.
Knowing the
relationships between voltage, current and resistance brings us to ohm's law:
"Current is proportional to Voltage divided by Resistance". This equation
can be manipulated to obtain any value knowing the other two. For example, by
measuring the voltage and current of a circuit, resistance can be calculated by dividing
voltage by current. When a circuit is open (disconnected), the resistance is
infinite (zero current). The formula also shows what happens when a circuit is
shorted (resistance = 0): The battery will put out as much current as it can
instantaneously (not a good sight).
DC vs. AC Circuits
On a DC circuit, current flows in one direction only. Voltage
can remain at a level or change, but it always has the same polarity. A car's
battery produces DC voltage.
AC circuits are a bit more complicated to understand. The
voltage supply reverses its polarity switching from positive to negative. The
current produced goes in one direction while the voltage is positive and then flows in the
opposite direction when voltage is reversed. AC circuits have a frequency associated
with them. The frequency (Hertz or Hz) is how many times per second (cycles) the
current (and voltage) switch from positive to negative and back. The higher the
frequency, the faster the circuit will switch polarity. AC voltage in the car is
produced by the alternator, which is converted to DC voltage to charge the battery (even
though some of the AC energy from the alternator remains in the electrical system, this is
what causes alternator whine when the car is running). The audio signal that comes
from the head unit, gets amplified and drives the speakers is also an AC signal.
A Typical DC Circuit
Electrons
flow in a circuit from the negative side of the battery to the positive side of the
battery (that is why physicists will argue with the direction of the current in the
circuit). Engineers represent current in the opposite direction of electron flow, as
in the diagram. It does not matter what convention you follow for current
direction. The important thing to keep in mind is how much current flows through the
circuit, and that you stick to only one of the models when analyzing a circuit.
For a circuit to have current, there has to be a path (i. e. wire) and a battery. A
circuit also has a resistance, which slows down flow of electrons.
If the path is broken, current can not flow. The battery supplies the voltage.
The top portion of the circuit in a car is represented by the positive battery cable going
to the fuse box and to all the accessories (radio, wipers, lights, etc). Each
accessory has a resistance. As more accessories are added, the resistance drops, and
more current flows through the circuit. To save money, car manufacturers use the car
metal for the bottom part of the circuit, instead of running a ground wire to every
device.
Common Engineering Notations
To represent very high or low values, zeros or decimal points are
represented by letters. These are the most common used in car audio:
Symbol: 
Value: 
Used mainly For: 
Example: 
µ (micro) 
millionth 
Capacitors, which are measured in Farads 
0.000001F = 1µF 
m (mili) 
thousandth 
Capacitors (F), inductors (Henries), voltage (V),
current (A) 
0.001Volts = 1mV 
k (kilo) 
thousand 
Resistance (Ohms), frequency (Hertz), power (Watts) 
1000W = 1kW 
M (mega) 
million 
Frequency (Hz), resistance(Ohms) 
1,000,000 Hz = 1MHz 







