Electrical fundamentals in the car – the ohmic resistance

An apprentice automotive mechatronics technician from the 1. the first year student should find out why the dipped headlights did not light up. 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 wrong in the process. The solution follows later.

The resistance of the lamp is measured here. But what is wrong with this measurement? (picture: kfztech.De)

Inseparably linked with the physical quantities of voltage and current is the electrical resistance.

Inhibition of cargo transport

if you connect a voltage source such as a battery through a conductor, the voltage causes a directed electron movement, 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 their flow is disturbed.

When charge carriers move in a conductor, they bounce off the atomic trunks again and again. This inhibits their movement. (picture: kfztech.De)

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

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

A simple conductor, however, has far too low a 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 only a wire. But this is so thin that only a relatively small current can flow. In electronics, resistors play a very important role as components. In addition to these classic resistors, every electrical component as a voltage consumer also has a resistance value that influences voltages and currents in circuits.

Every conductor and every consumer opposes the electric current with a resistor. The greater 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 hulls are amplified, the obstruction of the charge carriers becomes greater. the resistance consequently increases and the conductor heats up and expands. All metal conductors behave like this, they conduct better when cold. That is why they are also called cold conductor denoted.

Physical quantity ohmic resistance

The formula symbol of the electrical resistivity is R. It stands for the english term resistor. The unit of measurement for electrical resistance is ohm with the abbreviation Ω (omega) from the Greek alphabet. by the way, the unit of measurement ohm was derived from georg simon ohm, a german physicist. Common measurement parameters in automotive applications can be found in table 1. Resistors as circuit symbols are always drawn in circuit diagrams in the ratio 1:3.

General circuit symbol of a resistor

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

Conductivity and resistivity

Different materials conduct the current differently well. This depends on the distance between 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 resistivity and vice versa. The conductance is therefore the reciprocal value of the resistor. 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 conductive surface, 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 a conductor cross-section of 1 mm 2, in relation to a material constant and a reference temperature of 20°C. The formula for calculating the conductor resistance is: R = ϱ l / A. Table 3 shows the resistivity and conductivity for different 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, the more free electrons there are, the better the electrical conductivity of the conductor material. Conductivity is divided into three groups:

Conductors, semiconductors and non-conductors

Conductors have a large number of free electrons. The good conductivity of metals is based on the many free electrons that are present due to the metal bonding. Even with little energy, enough electrons are released from the atoms to achieve conductivity.

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

The best conductors are gold and silver, which are relatively rarely used due to their high cost. However, their advantage is also that, as precious metals, they do not give corrosion a chance. Gold is u.A. Used in the contacting of the chips and at the connection contacts of the control units. Copper and aluminum are very good and affordable conductors. Copper is the only cable material used in automobiles. Electrically conducting 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 do not have any free charge carriers in the form of electrons or ions. Therefore, electrons cannot be moved through them. Usually insulators or insulating materials are used to electrically isolate electrical conductors from one another. Non-conductive materials are found in non-metals. The most suitable non-conductors are plastics, rubber and ceramics. Cable insulation is made of PE and PVC; rubber is also used for insulation.

Semiconductors have few free electrons. These are solids whose conductivity lies between that of conductors and non-conductors. In the normal state, they behave like non-conductors. Under certain conditions such as e.g.B. Pressure, temperature, exposure or magnetism allow current to flow through them. In contrast to metals, the 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. The main materials used for semiconductors are silicon, germanium and selenium. 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 the resistance for the work

Resistors play an important role in the daily work of automotive mechatronics engineers. 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 should be checked: 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 in order. This measurement does not apply to electronic components.

One type of resistor plays an important, albeit unintentional, role in everyday workshop life: the contact resistance. It usually "hides" at contacts of switches, connectors and components, or even in broken stranded wires. The reason for these contact and contact 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 a 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.

The automotive mechatronics technician does not get to the bottom of this voltage loss with resistance measurements, but with specific 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, resistors are not noticeable: "where there is no current, there is no ohm".

First, the automotive mechatronics technician measures the battery voltage and then the voltage at the consumer. The difference is the voltage drop. If an impermissible voltage drop is detected, it must be narrowed down further. Next, check the voltage drop on the positive side and the voltage drop on the negative side.

To measure the voltage drop, first measure the battery voltage (top picture) and then the voltage at the load (middle picture). If a voltage drop is detected, it must be narrowed down further (see figure below). – (pictures kfztech.De)

According to DIN 72551, only 0.3V voltage drop is permitted for headlight leads. In the charging line of the generator there is 0.4V approved voltage drop, and the starter main line may have 0.5V voltage drop.

But every lead and every plug contact has its own resistance, even in a "good" state, which also causes a voltage drop. But the longer the cables that are laid in the vehicle, the greater this drop becomes. This could, of course, be compensated for by thicker lines. But copper is 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 is not a problem in itself, as long as retrofitting components does not place additional strain on the existing cables. The automotive mechatronics technician must therefore also take this into account when retrofitting electrical systems.

Resistance measurement

The usual procedure to measure a resistance is the direct resistance measurement with an ohmmeter. Indirect resistance measurement is a simultaneous measurement of current and voltage with subsequent calculation and is rather unusual in the automotive workshop. For direct measurement, the resistance value is either read off (analog meter) or displayed (digital meter). To avoid reading errors and inaccuracies, the measurement is best taken 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 in parallel, the measurement result is distorted because the current flows along different paths.

When measuring the resistance, no voltage may be applied to the component (picture: kfztech).De)

By the way: when measuring the resistance, in reality the resistance is not measured at all! Rather, 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 meter). In order for a current to flow, a voltage source must be present at this series connection of current meter, known and unknown resistor. The meter itself applies a small voltage to the resistor through a battery in the ohmmeter for this purpose. 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 select the correct 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 sensible measured value is displayed.

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

Before measuring, it is also useful to hold the measuring tips together briefly to see what the basic value of the meter is without a connected component. In the case of 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 has to be resolved what the automotive mechatronics trainee did wrong. The measurement on the HB4 lamp shown was carried out with the connector plugged in. The measurement could, as already explained, be distorted by the connected wires. Therefore, the automotive mechatronics technician should carry out the measurement in the unplugged condition. Another mistake: ordinary test probes should not be used to pierce the connector either. Corrosion can spread through the small punctures, a loss of voltage would be the consequence. The correct handling is shown in the picture.

If possible, a resistance should always be measured in the unplugged state (picture: kfztech).De)

Summary

The electrical resistance is the inhibition of the electrical current in a conductive material. Each conductor and each consumer offers a resistance to the electric current. The greater the resistance, the lower the current. The formula for electric resistance is R. The unit of measurement for electrical resistance is ohm with the abbreviation Ω (omega). The longer the length of a conductor, the higher its resistivity and the smaller its surface area, the greater its resistance. The contact resistance "hides" mostly 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 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 stom density of conductors depends mainly on the cooling capability of the conductor surface. Thin wires have a larger surface area relative to the conductor cross-section than thick wires, and thus can conduct more current per mm 2 of conductor cross-section.

Maximum load-bearing capacity of copper pipelines:

Table 1 common measurands in the car

Table 2 other circuit symbols for resistors

resistor, changeable

temperature dependent resistance (NTC. Hot conductor)

light-dependent resistor (LDR)

coil, inductance

incandescent lamp, also symbol for a general voltage consumer

Table 3 conductivity and resistivity of some conductors

sources: elektronik-kompendium by patrick schnabel, basic quantities of current, voltage and resistance by horst weinkauf, wikipedia, europa verlag,