The electric circuitry of the average car.

The electric circuitry of the average car

By Danielle Atlas

When the manufacturing of cars had just begun, electricity was used only to ignite the fuel inside the engine. By the late 1920's, the electric starter replaced the hand crank, electric headlights made acetylene lamps outdated and the louder electric horn replaced the squeak of the hand-squeezed air horn. Today, a car requires an elaborate electrical system of circuits just to produce, store, and distribute all the electricity it requires simply for everyday operation.

In the average car, electric circuits are to perform a variety of tasks – starting the car, recharging the battery and turning on headlights are only a few. The electrical circuits of the car are also used for radios, electric door locks, window defrosters, electric windows and other accessories. Many of these tasks can be performed using the same circuit.

In order to ensure that the circuits therefore, are not overloaded, each circuit has a fuse, to ensure that if the circuit does surge or overload, the car will not be severely damaged.

Almost everything that is powered electrically in an average car is ‘grounded’ to the frame or the body of the car. These are called "grounding points" and complete electrical circuits around the car. Therefore, an earth symbol depicted on a car’s electric circuit is not literally sending the electricity to the ground but rather sending the current through the conductive metal of the car’s frame.

The Battery

The major component in the electrical system of the car is the battery. The battery is used to store power for starting, and for running accessories such as clocks, radios and alarms when the engine is off. The battery allows every component of its electrical circuits able to utilise 12 volts or electricity.

A car's battery is designed to provide a very large amount of current for a short period of time. This large amount of current is needed to turn the engine during starting. Once the engine starts, the alternator can provide the power that the car needs. Therefore, a car battery can last a number of years. As shown in the diagram on the next page, a 12-volt car battery consists of layers of positively and negatively charged lead plates. The cells are filled with an electrolyte that is two-thirds water and one-third sulfuric acid. The interaction of the plates and the electrolyte produces chemical energy. This becomes electricity when a circuit is formed between the negative and positive battery terminals.

With this and other basic components, the car maintains its supply of electricity. A device called the voltage regulator keeps the power level stabilized, and the fuse box keeps problems from disabling or damaging the entire car.

Starting the car

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Starting the car

To start the car, you insert the key into the ignition and turn. This activates a circuit, which directs electricity that is chemically stored in the battery to the starter motor. The starter motor is responsible for turning the engine over. In order to do this, it must consume a large amount of energy from the battery. The starter motor, utilising a solenoid, spins the engine over so that the process of combustion may start. The solenoid is capable of directing hundreds of amps of electricity needed to spin the engine. This is necessary because the starter motor needs enough energy to overcome the following -

Internal friction
Pressure
Energy needed to open and close surrounding valves
All of the other components directly attached to the engine, like the water pump, oil pump and alternator.

Once the engine is started, the starter motor’s role is complete. At this point the alternator is activated (which will be discussed later in this report).

At the same time, the ignition system of the car is working. The ignition system on the car has to work perfectly with the rest of the engine. The fuel must be pumped to the engine at exactly the right time so that the expanding gases can do the maximum amount of work.

The electrical current travels through an ignition coil. The coil is generates the high voltages required to create a spark. It is essentially a high-voltage transformer made up of two coils of wire. One coil of wire is called the primary coil. The secondary coil is wrapped around it. The secondary coil normally has hundreds of times more circles of wire than the primary coil. Current flows from the battery through the primary coil. The ignition key is then removed, breaking the circuit. This causes the magnetic field around the primary coil – as it is an electromagnet - to collapse. The secondary coil is then surrounded by a powerful magnetic field. This field induces a current in the coils because of the number of coils in the secondary winding, and this current has an extremely high voltage – about 20,000V. The secondary coil distributes this voltage to the distributor.

The distributor must distribute the high voltage from the coils to the spark plugs in the cylinders of the engine. This is done by the cap and rotor, which are contained in the distributor. The coil is connected to the rotor, which spins inside the cap. This creates an electric pulse, which arcs across the small gap between the rotor and the contact wire (they don't actually touch). The pulse then continues down the wire of the circuit to the spark plug on the appropriate cylinder.

The spark plug forces electricity in the circuit to traverse a gap. The electricity must be at a very high voltage in order to travel across the gap and create a good spark. Voltage at the spark plug can be anywhere from 40,000 to 100,000 volts. The spark plug must have an insulated passage for the electricity. The plug also has to withstand the extreme heat and pressure inside the cylinder. Spark plugs use a ceramic insert which insulates to ensure that the spark happens at the tip of the electrode and not anywhere else on the spark plug. It is important to ensure that the spark plug is the correct type for each car – if the car should have a hot spark plug or cold one. The difference between the two is the ceramic tip, and the heat retention of the plug. If the spark plug gets too hot, it could ignite the fuel before the spark fires, which is extremely dangerous. The spark, as it bridges the gap between the wire and the spark plug allows the fuel in the engine to ignite, thereby creating motion.

Recharging the battery

Like all batteries, a car's 12-volt battery would eventually go dead if it were not recharged, so an average car has a built-in recharging system. This charging system includes the alternator, voltage regulator and the battery. The charging system's job is to generate enough current to keep the battery fully charged, and to satisfy the demands of the ignition and electrical systems. The voltage regulator controls alternator output – as the voltage transmitted by the alternator must be 13.5 to 14.5 volts to keep the circuit stable and long-lasting.

The alternator is the essential element of the charging system, and is activated when the engine starts. It is an alternating-current generator. The alternator develops alternating current, which is changed to direct current, as only direct current can be used to charge a battery.

As you can see in the above diagram, the alternator is directly attached to the battery so that it can supply electricity to it. Recharging is achieved by the four main parts of the alternator - the rotor, stator, diode pack and voltage regulator, as well as an ammeter or indicator light on the dashboard to inform the driver of any problems.

The rotor is basically a magnet that rotates inside thousands of loops of copper wire wound around a core of iron. This wire wrapped iron core is the stator. As the rotor spins inside the stator the magnetic force cuts through the copper wire, inducing voltage. This is the voltage that will go to the battery.

The only problem is that this produces AC current and your car runs on DC current. The diode is used to convert AC current to DC. A diode is an electrical one-way check valve that will let current flow in only one direction. A typical diode pack of an average car uses four diodes to accomplish this. AC current is feed in on one side of the diode pack and DC current comes out the other side. The diode pack also works the dashboard ammeter or indicator light.

The voltage regulator then controls the current produced. It regulates the voltage going to the battery by turning the flow of electricity of the alternator on and off. If the battery voltage goes below 13.5 volts, the voltage regulator allows the alternator to start charging. Current will then flow into the battery and bring it up to full charge. If the voltage goes above 14.5 volts, the voltage regulator shuts off the current, and keeps the battery from overcharging itself. This is how the voltage regulator controls the alternator’s output.

Turning on the headlights

The lighting circuit of a car includes the wiring harness, all the lights – including the headlights, and the various switches that control the use of these lights. The different lights are connected in parallel, meaning that each set of lights can run off the same battery and receive the same amount of voltage, but can be switched on and off at different times. If the wires were to be connected in series, then either all the lights of the car would have to be on, or no lights at all.

There are a variety of lighting systems set up in the average car to suit each different set of lights. In some types of lighting systems, one switch controls the connection to a fuse box, while a selector switch determines which of two circuits is energized. The headlights, which have upper and lower beams, are an example of this type of switch.

The diagram on the following page shows the basic lighting system of a car. The path of electricity from the 12-volt battery to the headlights is identified.

The main lighting switch (sometimes called the headlight switch) is the centre of the lighting system and identified as switch 1 in the diagram. It controls the headlights, parking lights, side marker lights, tail lights, license plate light, instrument panel lights, and interior lights.

The various switches allow the headlights to be used in different ways. When the main lighting switch completes the circuit to the headlamps, the low beam lights the way for city driving and for use when meeting oncoming traffic on the highway. When a dimmer switch is activated, the single filament headlamps go "on," along with the high beam of the two filament headlamps. The next activation of the dimmer switch returns the head lighting system to low beams only on the two filament lamps.

The electrical circuits of an average car enable the driver to perform many tasks easily. Processes such as ignition and starting the engine, recharging the battery and turning on the headlights of the car have improved as time has passed, and will continue to be improved and modernised in the future.

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