Introduction:
Every automotive technician, from assistant to technical specialist, deals with electronics. In addition to the electronics of comfort and safety systems such as in lighting, the wiper motor, and the ABS system, we find electronics in the control of the engine management system and in the form of communication networks (including CAN-bus). Also, more and more vehicles are equipped with an electric drivetrain. Anyone who wants to understand electronics must start at the basics. In this section, we begin briefly with an explanation of the electrons orbiting an atom and quickly move on to wiring diagrams where the basic concepts in vehicle electronics are practically explained.
Atomic Nucleus with Electrons:
According to Bohr’s atomic model, an atom consists of a nucleus containing protons and neutrons, with electrons circling around it in multiple shells. The copper atom contains 29 protons and 35 neutrons in its nucleus.
The electrons are located in four shells. The distribution of electrons over these shells is called the electron configuration. Each shell has a maximum number of places for electrons. The first shell (K) has room for two electrons, the second shell (L) for eight, the third shell (M) for eighteen, and the remaining shells for 32 electrons.
The electrons in the innermost three shells are bound electrons. The electrons in the outermost shell participate in chemical bonds and reactions and are also called the “valence electrons.” The copper atom contains one valence electron. These electrons can move freely and transfer to another atom. In the case of copper wire, the outer shells overlap, and the single electron can move over the shell of its adjacent atom.
The donation of the valence electron is important for this subject. The electron jumping from one atom to another enables the material to conduct. Materials such as copper, gold, and aluminum have a valence electron in the outer shell. In contrast, insulators such as plastic, glass, and air do not have a valence electron. These materials are therefore not conductive.
Electron Flow:
In the following image, we see a battery, a lamp, the conductor (copper wire), and a switch. Depending on the position of the switch, current either flows through or does not flow through the circuit. The light blue rectangle represents the copper conductor with the copper atoms (yellow) and the jumping valence electrons (green).
- Switch open: the electrons circle around the copper atom, but there is no electron flow through the consumer (the lamp). The lamp does not light;
- Switch closed: the battery creates a voltage difference, causing an electron flow from negative to positive. The current flows through the lamp and ignites due to the electron flow and the voltage difference.
The current moves from – (negative) to + (positive). This is the actual current direction. Previously, it was believed that the current moved from positive to negative, but this is incorrect. For convenience, we adhere to this theory and call it the “technical current direction.” From now on, we will maintain this technical current direction, assuming that the current runs from positive to negative.
Current, Voltage, and Resistance:
In this section, we will focus on the three concepts: current, voltage, and resistance. These concepts are constantly encountered in automotive technology. Current, voltage, and resistance each have their own quantity, unit, and symbol.
- I = Current = Ampere (A)
- U = Voltage = Volt (V)
- R = Resistance = Ohm (a9)
Current: In the previous section, we saw electron flow through a circuit. The amount of electrons flowing through a specific cross-sectional area of an electrical conductor in one second is called the current strength. The unit of current is ampere (A). A current strength of 1 A is reached when 6.24 quintillion (6,240,000,000,000,000,000) electrons have flowed through a cross-section within one second. The more electrons flow within a certain period, the higher the current strength.
To gain insight into how much current the electrical consumers in automotive technology demand, here is a list estimating the current strength at a charging voltage of 14 volts:
- Starter motor gasoline engine: 40 – 80 A;
- Starter motor diesel engine: 100 – 300 A;
- Coil: 3 to 6 A, depending on the type;
- Fuel injector gasoline engine: 4 – 6 A;
- Electric fuel pump: 4 – 12 A, depending on pressure and flow;
- Electric cooling fan: 10 – 50 A;
- H7 lamp (halogen low beam) of 55 Watt: 3.9 A;
- Xenon lamp of 35 Watt: 2.5 A;
- LED lights (PWM controlled and not via a ballast resistor): 0.6 – 1 A;
- Rear window defroster: 10 – 15 A;
- Seat heating: 3 – 5 A per seat;
- Standard car radio without onboard computer: ~5 A;
- Wiper motor: 2 – 5 A, depending on the force;
- Interieur fan motor: 2 – 30 A, depending on the speed;
- Electric power steering: 2 – 40 A, depending on the force.
Voltage: The voltage is the force that makes the electrons move. Voltage is a measure of the difference in force between electrons at two points. Voltage is measured in volts, abbreviated as V. In automotive technology, we work with a “nominal voltage” of 12 volts. This means the battery and all electrical consumers are based on 12 volts. However, in practice, we see that the voltage is never exactly 12 volts, but is always slightly lower, and especially often higher. Additionally, the voltage with electric drive is much higher. The consumers in a car use voltage. For example, the rear window defroster uses about 10 amperes at a voltage of 14 volts. The current is not consumed and returns to the battery. The 14-volt voltage is used in the rear window defroster to generate heat. At the end (the ground side), there is 0 volts remaining.
To gain insight into the possible voltage levels in a passenger car, here is a brief summary of voltages we may encounter:
- Battery voltage: 11 – 14.8 V (almost empty battery to maximum charging voltage of alternator);
- Opening voltage piezo injector: briefly 60 – 200 volts;
- System voltage of a vehicle with electric drive (hybrid or BEV): 200 – 800 volts.
Resistance: Every electrical component has an internal resistance. This resistance value determines how much current will flow. The higher the resistance, the lower the current strength. Resistance is denoted by the letter R and the unit Ohm. For the unit, we use the omega symbol from the Greek alphabet: a9. In an electrical circuit, we can add an additional resistor to limit the current.
When a short circuit occurs, for example, if a positive wire contacts the body, there is very low resistance. The current then increases rapidly until a fuse blows to prevent damage. In the following list, we see the resistance of components encountered in automotive technology:
- Copper wire 2 meters long with a cross-section of 1.25 mmb2: 0.028 a9;
- Lamp (21 Watt incandescent bulb): 1.25 a9;
- Fuel injector gasoline engine (the high-ohmic variant): 16 a9;
- Relay control circuit: ~ 60 a9;
- Relay main circuit: < 0.1 a9.
The resistance of a component often depends on the temperature: for example, the resistance of the bulb is much higher when lit than during the measurement when it was cold, where the current strength decreases as it becomes warmer.
In summary: the resistance of an electrical component determines how much current will flow. Low resistance means a lot of current will flow. The supplied voltage (usually around 12 volts) is used up in the electrical component, resulting in 0 volts on the ground side. Current is not consumed, so it is just as high on the positive side as on the ground side.
To better understand the concepts, it’s sometimes helpful to use the water tank analogy. The tank is filled with water and closed off at the bottom with a tap. The voltage and the flow of water through the tap allowing a certain amount of water to pass provide a good illustration of what happens with electricity in a consumer with internal resistance.
Voltage:
When the tank is filled with water, the water pressure at the tap increases. The water pressure can be compared to the concept of voltage in electricity. The system must be closed; otherwise, the water escapes and there is no longer any water pressure.
Current:
When we open the tap, the water starts to ‘flow’ through the tap. The water flow can be compared to the concept of current in electricity.
Resistance:
The tap regulates the resistance to the flow of water. As the tap is opened further, resistance decreases and flow increases.
The same holds true for electricity. In an electrical circuit with more resistance, less current flows, and vice versa. Resistance does not affect voltage.
