Auto electrics
First in a 10-part beginners’ guide.
Although most motorists consider the engine to be the heart of a motor vehicle, of equal importance is the electrical system. This series of articles will take us from the fundamentals of automotive electricity through to circuit design and the function and operation of some common components. As you gain an understanding of voltage, resistance, current and the relationship between them, your ability to interpret wiring diagrams and decipher more complex circuits will grow. Throughout the series, the theory will be linked to real-world fault-finding and diagnostics.
The basics
Electricity is a form of energy. In a vehicle, chemical energy from a battery is converted to electrical energy when it is provided with a closed circuit through which to flow.
Current, which is measured in amps, is the flow of an electrical charge through a circuit. When a circuit is complete, current flows from positive to negative – this is known as ‘conventional flow’. However, the electrons are negatively-charged, and actually they move from negative to positive – this is known as ‘electron flow’. For practical purposes, any circuits or testing described in this series will use the theory of ‘conventional flow’.
Voltage, which is measured in volts, is the force that pushes an electrical charge around a circuit. The greater the voltage applied, the greater the current that will flow. In a vehicle, the voltage is provided by the battery or the alternator and should be roughly between 12-14 volts for most cars. Some trucks use 24 volts and some electric vehicles use more than 400 volts.
Resistance, measured in ohms, opposes the flow of current. In a circuit, this is usually provided by the ‘consumer’ – for example a bulb, motor or heating element. The consumer in a circuit is what is doing the ‘work’. A small amount of resistance is created by wiring, connections and switches.
For a circuit to work, you need a conductor, usually copper wire. Typically, metals are good conductors, causing little resistance, while rubber and plastic are good insulators.
There is an analogy often used to describe voltage, current and resistance. Think of a circuit as a garden hose with water flowing through it. The pressure from the tap pushing the water through the hose can be likened to voltage. The amount of water flowing could be considered the current. A kink in the hose, restricting flow, is the resistance.