- Atomic nucleus with electrons
- electron flow
- Current, voltage and resistance
Every car technician, from assistant to technical specialist, has to deal with electronics. In addition to the electronics of comfort and safety systems such as in lighting, the windscreen wiper motor and the ABS system, we find electronics in the regulation of the engine management system and in the form of communication networks (eg CAN bus). More and more vehicles are also getting an electric powertrain. Anyone who wants to understand electronics should start with the basics. In this section we start briefly with an explanation of the electrons that orbit an atom and we quickly move on to electrical diagrams where the basic concepts in vehicle electronics are explained in a practical way.
Atomic nucleus with electrons:
According to Bohr's atomic model, an atom consists of a nucleus, containing protons and neutrons, surrounded by orbiting electrons in several shells. The copper atom contains 29 protons and 35 neutrons in the nucleus.
The electrons are in four shells. The distribution of the 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 other shells for 32 electrons.
The electrons in the inner three shells are bound electrons. The electrons in the outer 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 move to another atom. In the case of copper wire, the outer shells overlap and the single electron can move across the shell of its adjacent atom.
The donation of the valence electron is important for this topic. Jumping the electron from one atom to another allows 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. This material is therefore also non-conductive.
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 may or may not flow through the circuit. The light blue rectangle represents the copper conductor with the copper atoms (yellow) and the overlapping valance electrons (green).
- Switch open: the electrons circle around the copper atom, but there is no electron flow through the consumer (the lamp) yet. The lamp is not lit;
- Switch closed: because the battery creates a voltage difference, an electron flow is created from minus to plus. The current flows through the lamp and burns due to the electron flow and the voltage difference.
The current moves from – (minus) to + (plus). This is the actual flow direction. It used to be thought that the current would move from plus to minus, but that is not correct. Yet for the sake of convenience we will stick to this theory, and call it the “technical flow direction”. In the following we will maintain this technical flow direction, assuming that the flow flows from plus to minus.
Current, voltage and resistance:
In this section we zoom in on the three concepts: current, voltage and resistance. We come across these concepts time and again in automotive technology. Current, voltage and resistance each have their own magnitude, unit and symbol.
- I = Current = Amps (A)
- U = Voltage = Volts (V)
- R = Resistance = Ohms (Ω)
Flow: In the previous section we saw the flow of electrons through a circuit. The amount of electrons that flows through a given cross-sectional area of an electrical conductor within one second is called the current intensity. The unit of current is ampere (A). A current of 1 A is achieved when 6,24 quintillions (6.240.000.000.000.000.000) electrons have flowed through a cross section within one second. The more electrons that flow within a given period of time, the higher the current.
To gain insight into how much power the electrical consumers in automotive technology require, here is a list where the current intensity is estimated at a charging voltage of 14 volts:
- Petrol engine starter: 40 – 80 A;
- Diesel engine starter: 100 – 300 A;
- Ignition coil: 3 to 6 A, depending on type;
- Petrol engine fuel injector: 4 – 6 A;
- Electric fuel pump: 4 – 12 A, depending on pressure and flow;
- Electric cooling fan: 10 – 50 A;
- 7 Watt H55 bulb (halogen dipped beam): 3,9 A;
- 35 Watt xenon bulb: 2,5 A;
- LED lamps (PWM controlled and not via a series resistor): 0,6 – 1 A;
- Rear window heating: 10 – 15 A;
- Seat heating: 3 – 5 A per seat;
- Standard car radio without on-board computer: ~5 A;
- Wiper motor: 2 -5 A, depending on the power;
- Interior fan motor: 2 – 30 A, depending on speed;
- Electric power steering: 2 – 40 A, depending on power.
Voltage: The voltage is the force that makes the electrons move. The voltage is a measurement of the difference in force between electrons at two points. The voltage is measured in volts, abbreviated to V. In automotive technology we work with a “nominal voltage” of 12 volts. That is, 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 always slightly lower, but especially often higher. In addition, the voltage with electric drive is many times higher. The consumers in a car consume voltage. Let's take the rear window heating as an example: it uses approximately a current of 10 Ampere at a voltage of 14 Volt. The flow becomes cannot consumed and goes back to the battery. The 14 volt voltage is used in the heated rear window to heat up. At the end (the ground side) there is still 0 volts left.
To gain insight into the possible voltage levels in a passenger car, here is a short list of voltages that we can encounter:
- Battery voltage: 11 – 14,8 V (battery low to maximum alternator charging voltage;
- Piezo injector opening voltage: momentary 60 – 200 volts;
- System voltage of an electric drive vehicle: 200 – 600 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. The resistor has the letter R and the unit Ohm. As a unit we use the omega sign from the Greek alphabet: Ω. We can in an electrical circuit a additional resistance add to limit the current.
The moment a short circuit is made, for example if a positive wire touches the body, there is a very low resistance. The current immediately rises high, until a fuse blows to prevent damage. In the following list we see how much resistance the components that we encounter in automotive technology have:
- Copper wire with a length of 2 meters and a cross-section of 1,25 mm²: 0,028 Ω;
- Bulb (21 Watt bulb): 1,25 ;
- Fuel injector petrol engine (the high-impedance variant): 16 Ω;
- Relay control current section: ~ 60 ;
- Relay main power section: < 0,1 .
The resistance of a component often depends on the temperature: for example, the resistance of the lamp is much higher when it is lit than during the measurement when it was cold, in which the current decreases as it gets warmer.
Summarized: 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 consumed in the electrical component, leaving 0 volts on the ground side. Power is not consumed, so it is just as high on the plus side as on the ground side.
To better understand the concepts, it is sometimes useful to take the example with the water barrel. The barrel is filled with water and closed at the bottom with a tap. The voltage and flow of the water through the tap, which allows a certain amount of water through, give a good idea of what happens to electricity at a consumer with an internal resistance.
When the vessel 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 will run away and there will no longer be any water pressure.
When we open the tap, the water 'flows' through the tap. The water flow can be compared to the concept of electricity with electricity.
The faucet regulates the resistance to the passage of the water flow. As the tap is opened further, the resistance decreases and the current increases.
The same goes for electricity. Thus, with more resistance in the electrical circuit, there is less current and vice versa. The resistance does not affect the voltage.