electrical basics in the car – ohmic resistance

A trainee automotive mechatronics technician from the 1. Apprentice year was to determine why the low beam did not burn. He wanted to try out his newly acquired digital multimeter and measure the resistance of the HB4 lamp . But he seems to have done something not quite correct. The resolution follows later.

Measure lamp resistance (wrong)

Here the resistance of the lamp is measured. But what is wrong with this measurement? (Image: kfztech.en)

Inextricably linked to the physical quantities of voltage and current is the Electrical resistance.

Inhibition of charge transport

If you connect a voltage source such as a battery through a conductor, a directed electron movement takes place through the voltage, current flows until the charges are balanced (battery is empty) or the current flow is interrupted. The movement of free charge carriers (electrons) inside the conductor causes them to collide with atoms of the conductor material and to be disturbed in their flow.

Resistance (diagram)

When charge carriers move in a conductor, they bounce off the atomic trunks again and again. As a result, they are inhibited in their movement. (Image: kfztech.en)

This inhibiting effect is called electrical resistance or ohmic resistance. However, the term resistance can also refer to the electrical component resistor.

Electrical resistance is the inhibition of charge transport of electrical charges in a conductive material.

However, a simple conductor has a much too low resistance, the current flow would be unacceptably high. The direct connection of both poles with only one conductor is therefore called a short circuit. Therefore, every circuit contains a resistor as a component that inhibits the flow of current, such as.B. an incandescent lamp. The filament in the lamp is also just a wire. But this resistor is so thin that only a relatively small current can flow. Resistors play a very important role as components in electronics. In addition to these classic resistors, every electrical component also has a resistance value as a voltage consumer that influences voltages and currents in circuits.

Every conductor and every consumer opposes the electric current with a resistance. The higher the resistance, the lower the current

Due to the described inhibiting effect, a resistor therefore has a current limiting property. When the electrons collide with the atoms on their way through the conductor, they transfer energy to the atoms. The natural oscillations of the atomic trunks increase, the obstruction of the charge carriers becomes larger. The resistance increases consequently and the conductor heats up and expands. All conductors made of metal behave like this, they conduct better in cold state. Therefore they are also called PTC thermistors denotes.

Physical quantity Ohmic resistance

The formula symbol of electrical resistance is R. It stands for the English term resistor (=resistance). The unit of measurement for electrical resistance is the ohm with the abbreviation Ω (omega) from the Greek alphabet. Incidentally, the unit of measurement Ohm was derived from Georg Simon Ohm, a German physicist. Common measured variables in motor vehicles can be found in Table 1. Resistors as circuit symbols are always drawn in circuit diagrams in the ratio 1:3.

General circuit diagram of a resistor

Other circuit symbols of resistors are, for example, variable resistance, coil, temperature-dependent or light-dependent resistance. A lamp is sometimes also used as a symbol for a general voltage consumer (table 2).

Conductivity and resistivity

Different materials conduct current differently. This depends on the distance of the atoms and on the number of free electrons. Physically, this is expressed by the electrical conductivity σ (Greek sigma). Sometimes one also reads about gamma (γ) or kappa (κ). The unit of conductivity σ is m/Ω mm2. The better the conductivity, the lower the resistance value and vice versa. The conductance is therefore the reciprocal of the resistance. Correctly expressed: the specific electrical resistance ϱ (gr. letter rho). The unit of resistivity ϱ is Ω mm2/ m.

Factors that influence the conductivity and resistance of a conductor are the conductor length, the conductor cross-section, the conductor material and the temperature. The longer the conductor, the more often the electrons collide with each other and therefore the greater the resistance. And the smaller the conductor area, the more difficult it is for the electrons to “force their way through”. Mathematically, we refer to a standardized conductor length of 1 m, with 1 mm 2 conductor cross-section, in relation to a material constant and a reference temperature of 20°C. The formula for the calculation of the conductor resistance is: R = ϱ l / A. Table 3 shows the resistivity and conductivity for various conductor materials.

The longer the length of a conductor, the higher its resistivity and the smaller its surface area, the greater its resistance.

As far as the conductor material is concerned, it can be stated that the more free electrons there are, the better the electrical conductivity of the conductor material is. Conductivity is divided into three groups:

Conductors, semiconductors and non-conductors

Conductors have a lot of free electrons. The good conductivity of metals is based on the many free electrons that are present due to the metal bond. Already with little energy enough electrons are released from the atoms to achieve conductivity.

Conductivity Resistivity

The conductivity of a material depends on the number of free electrons. In conductors (top) there are very many, in non-conductors there are none (middle) and in semiconductors there are very few (bottom) – (Image: kfztech.de)

The best conductors are gold and silver, they are used relatively rarely due to the high cost. But their advantage also lies in the fact that, as precious metals, they do not give corrosion a chance to develop. Gold becomes u.a. used in the contacting of the chips and at the connecting contacts of the control units. Copper and aluminum are very good and affordable conductors. Copper is the only cable material used in automobiles. Electrically conductive liquids are called electrolytes. The charge carriers are both positive and negative ions.

Substances that cannot conduct electric current are called non-conductors or insulators. Non-conductors have no free charge carriers in the form of electrons or ions. Therefore, no electrons can be moved through them. Insulators or insulating materials are usually used to electrically separate electrical conductors from one another, i.e. to insulate them. Non-conductive materials are found in non-metals. Non-conductors are mainly plastics, rubber and ceramics. Cable insulation is made of PE and PVC; rubber is also used for insulation.

Semiconductors have few free electrons. They are solids whose conductivity lies between that of conductors and non-conductors. In the normal state they behave like non-conductors. Under certain conditions like z.B. Pressure, temperature, exposure or magnetism allow them to carry a current. In contrast to metals, conductivity increases with rising temperature up to a certain level. To specifically influence the conductivity of semiconductors, foreign atoms are incorporated into the lattice structure. Silicon, germanium or selenium are mainly used as materials for semiconductors. Components made of semiconductor materials are the basis for electronic components and circuits. Automotive mechatronics engineers are familiar with diodes, transistors and NTCs.

Significance of resistance for work

For the daily work of the automotive mechatronics technician, resistance has an important meaning. For electrical components can be tested for function with the ohmmeter. Thus, continuity testing of components is often very helpful in evaluating many electrical components. If an incandescent lamp or a coil like z.B. a solenoid valve or inductive sensor are to be tested: With an ohmmeter at hand, it is at least possible to determine whether an interruption is present. If there is a measurable resistance, i.e. continuity, the component is usually OK. However, this measurement does not apply to electronic components.

One type of resistance plays an important, albeit unintentional, role in everyday workshop work: contact resistance. It is usually “hidden” at contacts of switches, plugs and components or in wire strand breaks. The reason for these contact and transition resistances is poorly loosened connectors, crimped cables and corrosion. Here, the inhibiting effect of the resistor consequently results in a voltage loss, which is referred to as voltage drop. This leads to lower terminal voltage at the components and thus to power losses up to total failure. A voltage drop of 10% at the headlight, for example, results in about 30% less light.

However, the automotive mechatronics engineer does not get to the bottom of this voltage loss with resistance measurements, but with targeted voltage measurements. Resistance measurements in a closed circuit are not allowed and in an open circuit the full battery voltage is always applied to the switch. Since no current flows, resistances are not noticeable: “Where there is no current, there is no ohm”.

So first the automotive mechatronics technician measures the battery voltage and then the voltage at the load. The difference is the voltage drop. If an impermissible voltage drop is detected, it must be narrowed down more closely. The next step is to check the voltage drop on the positive side and the voltage drop on the negative side.

Voltage drop across the battery

To measure the voltage loss, first the battery voltage (top picture) and then the voltage at the load (middle picture) is measured. If a voltage loss is detected, it must be narrowed down even further (picture below). – (Pictures kfztech.de)

Voltage drop at the consumer

Voltage drop on the positive side

According to DIN 72551 only 0.3V voltage drop is allowed for headlight leads. In the generator charging line, it is 0.4V approved voltage drop, and the starter main line is allowed 0.5V voltage drop.

But every wire and every plug contact, even in “good” condition, has its own, albeit low, resistance, which also brings a voltage drop with it. However, the longer the cables that are laid in the vehicle, the greater this becomes. This could, of course, be compensated for by appropriately thicker lines. But copper is now very expensive and the space for cables is limited. This means that the designer of the wiring harness in the vehicle always chooses the smallest possible cable cross-section, taking into account the current density (Info1). This in itself is not a problem, as long as the existing cables are not additionally loaded by retrofitting components. The automotive mechatronics technician must therefore also take this into consideration when retrofitting electrical systems.

Resistance measurement

The usual way to measure a resistor is to measure it directly with an ohmmeter. Indirect resistance measurement is a simultaneous measurement of current and voltage with subsequent calculation and is rather uncommon in the automotive workshop. In direct measurement, the resistance value is either read (analog meter) or displayed (digital meter). To avoid reading errors and inaccuracies, the measurement is best determined with a digital multimeter.

When measuring with a resistance meter, the component to be measured must not be connected to a voltage source during the measurement. The component to be measured must either be removed or disconnected from a circuit on at least one side. If other components are connected in series or parallel, the measurement result is distorted because the current flows different ways.

when measuring resistance, no voltage must be applied to the component (Image: kfztech.en)

By the way: When measuring the resistance, in reality the resistance is not measured at all! Instead, you measure the current flowing through a series circuit consisting of the unknown resistor you are looking for and a known resistor (measuring resistor in the measuring device). For a current to flow, a voltage source must be applied to this series circuit of ammeter, known resistor, and unknown resistor. The measuring device itself applies a small voltage to the resistor via a battery in the ohmmeter. This now also explains why no on-board voltage may be present during the resistance measurement.

When measuring a resistor, it is also important to choose the right measuring range. The rule for unknown resistance is to first set the range selector switch to the largest measuring range and then to switch down until a meaningful measured value is displayed.

Measuring range large

If the resistance is unknown, select the largest measuring range (above). If the meter shows infinity, gradually switch down until an optimal reading is displayed (below)

Measuring range low

Before measuring, it is also useful to hold the measuring tips together briefly to see what basic value the measuring device shows without a connected component. With an analog meter, the measuring range must be set so that the pointer deflection can be read in the last third if possible. This is the only way to display the measured value as accurately as possible.

Now it remains to be resolved what the automotive mechatronics apprentice had done wrong. The measurement on the HB4 lamp shown was carried out with the plug connected. The measurement could be falsified by the connected lines, as has already been explained. Therefore, the automotive mechatronics technician should perform the measurement in the unplugged condition. Another mistake: You should not poke into the plug with ordinary test probes either. Corrosion can spread via the small punctures, a voltage loss would be the consequence. The correct handling shows the picture.

Measure lamp resistance

If possible, a resistor should always be measured in the unplugged state (Image: kfztech.en)


The electrical resistance is the inhibition of the electric current in a conducting material. Every conductor and every consumer opposes the electric current with a resistance. The greater the resistance, the lower the current. The formula sign of the electric resistance is R. The unit of measurement for electrical resistance is the ohm with the abbreviation Ω (omega). The resistance of a conductor increases the longer its length, the higher its resistivity, and the smaller its conductor area. The contact resistance is usually “hidden” at contacts of switches, plugs and components or also in wire strand breaks. Here the inhibiting effect of the resistor results in a voltage loss. When measuring with a resistance meter, the component to be measured must not be connected to a voltage source during the measurement. The component to be measured must either be removed or disconnected from a circuit on at least one side.

Info 1 Current density

The permissible strom density of conductors depends mainly on the cooling possibility of the conductor surface. Thin wires have a larger surface area in relation to the conductor cross-section than thick wires and can therefore conduct more current per mm 2 of conductor cross-section.

Maximum load capacity of copper conductors:

Table 1 Usual measured variables in the motor vehicle

Megaohm 1MΩ 1.000.000Ω 10 6 Ω
Kiloohm 1kΩ 1.00Ω0 10 3 Ω
Ohm 10 0 Ω
Milliohm 1mΩ 0,001Ω 10 -3 Ω

Table 2 Other circuit symbols for resistors

Resistance, changeable

Temperature dependent resistor (NTC. thermistor)

Light-dependent resistance (LDR)
Coil, inductance

Incandescent lamp, also symbol of a general voltage consumer

Table 3 Conductivity and resistivity of some conductors

Material Spec. Resistance ϱ
in Ω mm 2 / m
Conductivity σ
in m/Ω mm 2
Aluminum (Al) 0,0278 36
Lead (Pb) 0,208 4,8
Iron (Fe) 0,13 7,7
Gold (Au) 0,022 45,45
Carbon (C) 40-60 0,02
Copper (Cu) 0,0178 56
Nickel 0,095 10,5
Platinum (Pt) 0,098 10,2
Silver (Ag) 0,0167 60
Tungsten 0,055 18,2

Sources: Elektronik-Kompendium by Patrick Schnabel, Basic quantities of current, voltage and resistance by Horst Weinkauf, Wikipedia, Europa Verlag,

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