What is a coil in a car

Electrotechnical basics in the car - types of voltage generation

Voltage requires a difference in charges, which consequently also means: If you want to generate voltage, you have to separate charges. And if charges are separated, energy must be used to overcome this force of attraction, which is stored in the separated charges. Because charges tend to equalize again, there is an effect of force, a tension, between them. We call all components that supply electrical voltage electrical voltage sources. There are various ways in which this can be implemented.

There are many ways in which voltage can be generated (Graphic: kfztech.de)

Electric voltage is created in a “natural” way, for example through friction, during thunderstorms and during redox reactions. For technical use, voltages are mostly generated by electromagnetic induction and electrochemistry. This is also the case in automobiles. No modern vehicle would run today without the interaction of generator and battery.

Table 1 provides an overview of this.

Generation by ComponentVoltage range automotive

Electromagnetic induction

  • Induction of movement
  • Induction of calm
  • Magnetic fields
  • Generator, dynamo, induction generator
  • transformer
  • Hall generator
  • up to 500 V
  • several kV
  • in the mV range
Electrochemical processes / chemical conversionBattery, accumulator (galvanic element), fuel cell, lambda probeup to 24 V.
Pressure or bending / crystal deformation (piezo effect)Knock sensor
Pressure sensors
in the mV range
Light (photoelectric effect, photoelectric effect) Photo element, solar cellmV to V
Warming (Seebeck effect)ThermocouplemV range
Friction / voltage generation through electrostatic dischargeIgnition of fuel by electrostatic dischargeup to kV

Electromagnetic induction

The cause of the voltage that a generator supplies isinduction. This means the generation of electrical voltage by changing a magnetic field (magnetic flux) in a coil or conductor loop. For example, if you move a magnet back and forth in a coil (or vice versa), an alternating voltage is generated in the coil. This process is also called induction of movement. The voltage is only generated as long as the magnetic flux in the coil changes. Magnetic flux is the total number of magnetic field lines that are encompassed by the coil. The level of the induced voltage depends on the number of turns of the coil and the rate of change of the magnetic flux.

In addition to the generator, the induction generator in a motor vehicle generates voltage in this way to serve as a sensor for the speed of the engine or the wheels.

The power plant in the automobile, the three-phase generator - behind it, dismantled into its main components, front part of housing with rotor, stator, rear of housing with diode panel, regulator, plastic cover (Image: Bosch)

The magnetic flux can be changed in other ways than by moving the coil or magnet. If you change the amperage periodically in onetransformer or switches the current in a coil on or off, a voltage is also induced. This is also referred to as the induction of rest, as no movement is required.

Examples of use in an automobile are transformers such as the ignition coil, in which the primary current is switched off at the point of ignition, or the switching on and off of the excitation current in the generator to regulate the excitation voltage. A transformer consists of two coils that are located on a common iron core. The input winding (primary winding) absorbs energy, generates a magnetic field and passes it on to the iron core and the output winding (secondary winding). The alternating magnetic field, through alternating voltage or periodic switching on and off, induces a voltage in the secondary winding. The primary voltage is related to the secondary voltage like the number of turns of the primary winding to the secondary winding.

A Hall sensor has a Hall generator as its main component. A supply current flows through its semiconductor layer. If there is a magnetic field perpendicular to the semiconductor layer, the free electrons of the semiconductor are pushed to one side, creating the Hall voltage. Hall sensors are used in motor vehicles for speed measurement and as reference mark transmitters.

Electrochemical processes

Since the voltage of a generator is only available when it is running, you also need a way to store voltage. The solution here is the battery, or rather the accumulator.

The battery provides the necessary energy when the engine is not running

A battery in its original form is aGalvanic element. If you immerse two different metals in an electrically conductive liquid (electrolyte), a direct voltage is present between the two poles (electrodes). The level of the voltage in a galvanic element depends on the position of the material pairing of the electrodes in the Electrochemical series dependent (Table 2). When electricity is drawn, a chemical conversion takes place inside, the electrolyte is decomposed and the electrode material is decomposed or converted. Galvanic elements, in which the chemical conversion process can be reversed by charging, are called accumulators. The best example is the starter battery.

Elements (selection)tension
Gold (Au)+ 1.50V
Platinum (Pt)+ 0.86V
Silver (Ag)+ 0.80V
Coal (C)+ 0.74 V.
Copper (Cu) + 0.34V
Hydrogen (H2)0 V
Lead (Pb)- 0.13V
Tin (Sn)- 0.14V
Nickel (Ni)- 0.23V
Cadmium (Cd)- 0.40V
Iron (Fe)- 0.44V
Zinc (Zn)- 0.76V
Aluminum (Al)- 1.67V
Magnesium (Mg)- 2.40V
Lithium (Li)- 2.96V

But even with the lambda probe, the oxygen sensor in the exhaust gas flow, a small voltage between 0.1 and 0.9 V is generated chemically through charge separation. And the fuel cell, announced for years as a future energy supplier, is also a small chemical power plant specializes in the decomposition of water and provides free electrons as a by-product.

Crystal deformation

If you exert pressure on a crystal such as silicon dioxide, a voltage is created because the charge separation takes place on the surface of the quartz crystal. Such piezo elements are ideally suited as sensors in which rapidly changing pressures have to be recorded.

If you exert pressure on a quartz crystal, it deforms and a charge separation occurs on the surface (graphic: kfztech.de)

In automobiles, they are therefore used as knock sensors in combustion engines. In the event of a knocking combustion, the ignition point can be set in the early direction in order to get as much power as possible and at the same time protect the engine from wear.


The knock sensor delivers the upper signal in the event of knocking combustion and the lower signal in normal combustion (graphic: Bosch)


When an unoxidized metal surface or a semiconductor is irradiated with short-wave light, a charge separation takes place. Thisphotoelectric effect (photo effect) is technically implemented today with semiconductors such as selenium and silicon in photo elements and solar cells. In automobiles these are calledPhotodiode or Phototransistor Used for light-dependent controls, e.g. as a dimming control for interior mirrors.Solar cells are used today, for example, on motorhome roofs as an additional voltage source.


Charge separation takes place in the semiconductor due to the incidence of light, the solar cell supplies voltage (Graphic: kfztech.de)


If you connect two wires made of different metal materials and then heat the connection point, a voltage is created between the two wire ends. The voltage level depends on the wire pairing and the temperature. This effect is calledSeebeck effect designated. Such thermocouples are used, for example, to measure temperature or to control electrical cooler fans. Thermocouples are also calledThermoelectric Generators (TEG)when they provide electrical energy. BMW had tried in a project to generate electricity from the exhaust gas heat with the help of a TEG. Otherwise, thermocouples do not play a major role in vehicles.

Scheme of a thermocouple (Wikipedia Arne Hückelheim)


Frictional electricity is the oldest form of electricity, because it was first observed. It is based on the transfer of electrons between two touching substances as soon as they have different charges. Electrons are transferred to the material with less tension until the charges have equalized. By rubbing the fabrics against each other, a sufficient approximation of the fabrics is temporarily achieved.

Everyone knows from their own experience the crackling of the sweater when taking it off. A spark is often visible in the dark. The electrostatic attraction between the plastic ruler and scraps of blotting paper may have been tried out in the classroom. The automotive mechatronics technician is probably also familiar with other examples of static electricity from everyday life or school lessons.

In many a car workshop, a "tiny" sparkover on the fuel filler flap or when the petrol is drained has already led to serious fires. The reason: the fuel wasstatically charged and suddenly discharged again via sparks (striving for balance!). That is why tank trucks have to go through before loading and unloadingGrounding be protected from sparking. And every automotive mechatronics technician knows or should have learned that before he doesControl units must first ground itself in order to prevent possible destruction of the sensitive electronics.

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Source for text and image: Wikipedia, Electronics Compendium, Bosch, Europa Fachkundebuch, Horst Weinkauf

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