Anything which occupies space and has mass is called matter.Matter is composed of small particles called atoms. Atoms combine to form molecules. One molecule of water contains two hydrogen atoms and one oxygen atom; this combination is indicated by the chemical symbol H2O. In 1808 John Dalton noted that matter consists of individual atoms which are identical and unchangeable.Humphry Davy and Michael Faraday proved that electricity and matter are closely related.Their research into electrolysis helped to establish that electricity is atomic in character and that the atoms of electricity are a part of atoms of matter. Examination of a hydrogen atom under a powerful microscope shows that it consists of a positively charged proton at the nucleus (centre) and one electron, which orbits around the proton at high speed. The negatively charged electron is the lightest particle known at this time: its mass is only about 0.0005 of the mass of the hydrogen atom. The complete atom is uncharged because the charge on the nucleus is balanced by the charge on the electron. Ch03-H6955.qxd 6/9/06 3:47 PM Page 9596 Automotive Air-conditioning and Climate Control Systems
Other materials have different combinations of electrons and protons.A copper atom has 29 electrons; these move in four different orbits around the atom’s nucleus.The central region consists of 29 protons and 29 neutrons.The neutrons have no electrical charge.
If an electron is removed from the atom, then the atom and its material become positively charged. If an electron is added to the atom then it and the material will become negatively charged.
The difference in electric charge creates a potential difference or voltage. The size of the voltage is directly proportional to the charge difference.The charge on a electron is tiny, so to create a useful voltage a very large number of charges have to be separated.
The magnitude of the force produced between two electrically charged bodies was studied by the French scientist Coulomb in 1775.To honour his work, the SI unit of electrical charge is called the coulomb (C) and 1C is the charge equivalent to about 6 10–18 electrons. That is 6 million million million electrons (6 000 000 000 000 000 000). The charge on a single electron is about 1.6 10–19 C, or 0.00000000000000000016C.
The actual voltage created by a particular charge separation depends on the area over which the separated charges are spread, the separation distance and the electrical properties of the material between the charges.
Moving charges apart to create a potential difference requires energy because each negative electron is being pulled away from a positive nucleus against the force of attraction.This is similar to the way that energy is needed to lift an object, like a bottle of water, up in the air against the attractive force of gravity (which you can think of as a mass separation).We know that energy can’t be destroyed – so what happens to the energy used in creating the charge separation? The answer is that it is transformed into electrical potential energy. Again this is very much like the way that the energy used to lift the water upwards against the pull of gravity is transformed into gravitational potential energy; if the water is poured out the potential energy is transformed into kinetic energy (energy of movement) as the water accelerates downwards.
This kinetic energy can, in turn, be harnessed to do useful work, for example in a hydroelectric power station.The amount of energy released depends on the difference in height between the start and end of the drop and the amount of water. In a similar way the potential energy of separated electrons can be released to do useful work. If the electrons are allowed to return to the positively charged atoms from which they were separated, their potential energy will be released and can be harnessed.To do this requires an electric circuit made of a conductor that allows the electrons to return.To summarise, separated charges create an electric potential difference (a voltage). The size of this potential difference is measured in volts.When measuring the size of an electric potential difference you are measuring the difference in electric potential
between two points. Separating charges requires energy.When separated charges are reunited,
the energy is released and can be harnessed for useful work.
There are a number of ways that charges can be separated:
1. Electrostatics.
2. Electromagnetism.
3. Chemistry.
4. Photoelectric effect.
5. Piezoelectric effect.
Note – for more information on electromagnetism and the photoelectric and piezoelec-
tric effects see section 3.2.
Electrostatics
When different types of materials rub against each other, electrons can be moved from one material to another. This causes a charge separation, but one that isn’t generally useful; the amounts of separated charge are generally too small to do anything practical with. However, engineers have recently been investigating whether the rubbing of your clothes as you walk could be used to generate enough usable electricity to charge a mobile phone.
For example: when a glass rod is rubbed with a silk cloth the surfaces of both the glass rod and the silk cloth become charged with electricity. The charges within the glass rod and silk cloth do not move unless the glass rod and silk cloth are brought closer together or are connected by a conducting substance. If two glass rods are hung on threads and both rubbed with a silk cloth and both rods moved closer to one another they will repel each other. If one rod is brought near to the silk cloth then the rod and cloth will attract each other.
Electromagnetism
When an electron is moved through a magnetic field a force is created that pushes the electron. An electron in a conducting wire is pushed to one end of the wire, so creating a voltage along the wire. If you can arrange for the wire to keep moving in the magnetic field then you will have a continuous force on the electrons; as you release the potential energy by allowing electrons to flow back through a circuit and do useful work,more electrons will take their place and you will have a permanent potential difference.An electrical generator is, put simply, a device that keeps wires moving through a magnetic field to produce a permanent and usable potential difference.
Chemistry
Some chemical reactions can be used to separate charges.A chemical cell (often called a battery though usually a battery is a collection of cells) contains chemicals that react together; as the potential difference caused by the reaction rises it suppresses the reaction, for example in common batteries the reaction stops occurring at a voltage of 1.5V. When the battery is used the voltage drops slightly and the reaction starts up to maintain the voltage – until all of the chemicals have reacted and the battery is ‘dead’. Fuel cells use a rather different chemical technology to generate charge separation. In a basic fuel cell, hydrogen and oxygen are combined to create water; this is a chemical reaction that releases energy and the cell is designed to use this energy to separate charges creating a voltage of about 0.7V. The main difference between a fuel cell and a battery is that,when the hydrogen in a fuel cell runs out, the cell can be refuelled by adding more hydrogen (the oxygen used comes from the air).
Photoelectric
Certain materials release electrons when light falls on them. This is called the photoelectric effect and can be used to separate charges.Although first explained by Albert Einstein a hundred years ago, it is only recently that the cost of ‘solar cells’ based on the closely related photovoltaic effect has become low enough to make them a realistic cost-effective way of generating electricity. See section 3.2 for examples of photoelectric sensors used to measure light intensity (sun load sensor).
Piezoelectric
When you apply pressure to some crystalline materials you get a charge separation within the crystal.The resulting voltage across the crystal can be very high.This effect is commonly used to create a spark to light a flame in hobs and gas fires and also in ‘crystal’ microphones. In most situations it would be impractical to generate a constant source of separated charges using this effect as this would mean constant rapid pressurising of the crystal. However, as with charge separation through rubbing, engineers have recently been investigating whether the constantly changing pressure in your shoes as you walk could be used to generate enough usable electricity to charge the battery in a mobile phone or MP3 player. See section 3.2 for examples of piezoelectric sensors used for refrigerant pressure sensing.
Conductors and conductivity
Copper is a very good conductor so this metal is used as a material for cabling.The electrons are not tightly bonded to their nucleus and drift at random from one atom to another.Materials that have a number of free electrons make good conductors of electricity because little effort is needed to persuade charge separation.
Conductivity takes a somewhat different form when it comes to semiconductor material. For electronic applications, semiconductor materials are grown into crystalline structures which are given conductive properties by virtue of the impurities (or dopants) which are added. In their purest form (i.e. without dopants), the base semiconductor materials form crystalline lattices which become very stable by sharing electrons among the constituent atoms. In this pure state, the material is not very conductive.
Insulators
Insulation materials have no loosely bound electrons, so movement of electrons from one atom to the next is very difficult.
Semiconductors
A semiconductor is a material having an electrical resistance higher than conductors like copper but lower than that of insulators like glass or rubber.A semiconductor has a range of properties which can be used within the electronics industry.The two most common semiconducting materials are germanium and silicon. In their pure state they are not suitable for practical use as semiconductors. For this reason they must be doped,which is achieved through adding impurities to enhance their effectiveness.
N type
An N type semiconductor consists of a silicon (Si) or germanium (Ge) base or ‘substrate’ which has been doped with a slight amount of arsenic (As), antimony (Sb) or phosphorus (P) Ch03-H6955.qxd 6/9/06 3:47 PM Page 98Air-conditioning electrical and electronic control 99 in order to provide it with many free electrons (i.e. electrons which can easily move through the silicon or germanium to provide electrical current).
P type
A P type semiconductor, on the other hand, consists of a silicon or germanium substrate that has been doped with gallium (Ga), indium (In), or aluminium (Al) to provide ‘holes’, which can be thought of as ‘missing’ electrons, and hence as positive charges flowing in a direction opposite that of free electrons.
Movement of electrons
All electrons have a potential energy, given a suitable medium in which to exist they move freely from one energy level to another. This movement from one energy level to another is called current flow. Using conventional flow ( to ), electrical energy is considered to move from a point of high potential to a point of lower potential. Unfortunately it was found that electron flow is the opposite direction since the negatively charged electron is attracted to the positive potential.This text uses the conventional current flow of positive to negative to aid the understanding of electronic principles.A battery and generator are both capable of producing a difference in potential between two points.The electrical force that gives this increase in potential difference at the source is called the electromotive force.The terminals of a battery and generator are called positive ( ) and negative ( ) and these relate to the higher potential and lower potential respectively. Moving charges apart to create a potential difference requires energy because each negative electron is being pulled away from a positive nucleus against the force of attraction. Separating a coulomb of charge to create 1V requires 1 joule of energy. This is a small amount of energy when compared to kW/h 3.6 million joules of energy which is the measurement used in domestic homes.
The unit of potential is the volt, named after the Italian scientist Volta:
1 volt 1 joule per coulomb
Summary of potential difference (volt)
Separated charges create an electric potential difference (a voltage).The size of the potential difference is measured in volts.When measuring the size of an electric potential difference you are measuring the difference in electrical potential of two points.
Electromotive force (emf)
An emf is the driving influence which causes the current to flow.The emf is not actually a force but is related to the energy expended during the passing of a unit charge through the source. The emf is related to energy conversion. As a charge passes through a source of electrical energy, work is done on them. The emf of the source is the work done per coulomb on the charges.A typical car battery has an emf of 12 volts which means that 12 joules of work is done on each coulomb which passes through the battery.
Ampere (A)
The ampere is the unit of electric flow and is the rate of electron movement along a conductor. A coulomb is the quantity of electrons so when one coulomb passes a given point in one second the current is one ampere:
1 ampere 1 coulomb per second
Various standards have been used in the past to define the ampere; nowadays it is defined in terms of the force between conductors. If two parallel conductors are placed a given distance apart,when current is passed through the conductors, a force is set up which is proportional to the current.
Watt (W)
The watt is a unit of power and applies to all branches of science. It is equivalent to work done at the rate of one joule per second. (One joule is the product of the force, in newtons, and the distance, in metres, 1 J 1Nm.) A power of 1W is developed when a current of 1A flows under the ‘pressure’ or potential difference (pd) of 1V:
Voltage drop
This represents the energy used by free electrons while engaged in current flow. Passive devices such as resistors create voltage drops in a circuit because they absorb some of the energy and dissipate it in the form of heat.The volt drop across a series circuit is equal to the emf of the circuit.
Ohm’s law
In 1826 Ohm discovered that the length of wire in a circuit affected the flow of current. He found that as the length was increased, the current flow decreased and from these findings he concluded that:
Under constant temperature conditions, the current in a conductor is directly proportional to the pd between its ends.
This statement is known as Ohm’s law.
Resistance
From Ohm’s law the relationship between potential difference (V) and current (I) is expressed:
In this case, R is a constant which changes only when the length, cross-sectional area or temperature of the conductor is altered. Evidence shows that the value of the constant is related to the conductor’s opposition to current flow, so R is called the resistance and given the unit name of ohm:
The ohm is the resistance of a conductor through which a current of one ampere flows when a potential difference of one volt is across it.
Rearranging the expression V/I R is often called Ohm’s law. Rearranged it gives:
V = IR or volts = amperes x ohms
If two of these values are known, the third can be calculated, so this expression has a number of practical uses. The resistance of a conductor is affected by its material, temperature and dimensions.The SI unit of resistivity of a material is the ohm-metre. The rule can be expressed using the following equation:
R = resistivity in ohms of material
L = length of the current path through the conductor in metres
A = cross-sectional area of the conductor in metres squared
p = constant of proportionality between R and I/A
Most metals increase their resistivity when the temperature is raised, so these metals are said to have a positive temperature coefficient. Conversely if the resistivity decreases with an increase in temperature then the material has a negative temperature coefficient.
Circuit resistors
Motor vehicle circuits normally consist of a number of resistors/electrical consumers controlled by switches and connected to an electrical supply. Consumers can take many forms; they can be a lamp,motor, solenoid or be a part of some other energy-consuming device.
A basic understanding of circuit behaviour may be helped if the effect of resistors on voltage and current flow is considered.A temperature sensing circuit will be discussed to aid the explanation of series resistors/consumers. The temperature sensing circuit is used to sense cabin temperature; the temperature of the air inside the vehicle. This is measured by an air temperature sensor and monitored by an onboard computer (climate electronic control unit). The information is used to control cooling rate by adjusting air distribution and blower speed.
Electrical consumers may be connected in series, in parallel, or a combination of both.The diagram in Figure 3.2 shows the temperature sensing circuit in series.The circuit is a typical temperature sensing circuit used by most manufacturers. Figure 3.3 shows the circuit modelled using Crocodile Clips software.
Circuit operation (Fig. 3.3)
The Semi Automatic Climate Control (SATC) module will have a power supply from the battery and the ignition switch.The SATC A58 has internal circuitry that can reduce the power supply from 12 volts (14V when engine is running) to 5 volts. Using DIN standards this changes the wire code from 30 (battery feed) to 8 (sensor signal).This allows differentiation between a battery supply of 12/14V and a fixed 5V supply from a computer. The current then flows to the fixed resistor situated inside the SATC. The resistor causes a potential difference to occur across itself.This potential difference depends on the current flowing through the resistor and the number of consumers it is in series with. Because electrical circuits are dynamic the other resistor in the circuit will affect the current flow. The current will reduce through the circuit and two potential differences will be created across both consumers – fixed resistor inside the SATC module and the temperature sensitive resistor called a thermistor.Although the thermistor changes with temperature, when calculating the current and pd across each resistor the value is fixed as illustrated 25°C is 500 . For a full explanation on the operation of a thermistor see section 3.2.
Resistors in series
Placing two resistors in series (Figure 3.3) means that the full current must pass through each resistor in turn.When they are connected in this end-to-end manner, the resistance of the two resistors is the sum of their values, so:
Assuming the resistance of the cables is negligible, by applying Ohm’s law the current flow can be calculated:
In this case, a current of 0.005 amps will pass through each resistor and around the whole circuit. Inserting an additional resistor of 500 in series with the other two would give a total resistance of:
And a current flow of:
Voltage distribution (resistors in series)
Figure 3.4 shows two resistors in series with two ammeters and two voltmeters, positioned to measure the current and pd.
It will be seen that the ammeter is fitted in series with the resistors.This means that all current flowing in the circuit must pass through the ammeter, no matter where the meter is inserted in the circuit.
Energy is expended driving the current through a resistor so this causes the potential to drop. The voltage drop V1 and V2 (decrease in pd) when the current passes through R1 and R2 can be found by applying Ohm’s law:
A voltmeter connected to R1 will measure a volt drop. In this case it will register 2.5 volts so the pd across R2 will also be 2.5 volts as well because the total voltage is 5V.
This is because V1 and V2 will always add up to the voltage supply in a series circuit, 5 volts in this case.
By moving the voltmeter around the circuit, the voltage distribution can be determined:
Rules of a series circuit
In a series circuit the resistance is added together to calculate the total resistance; this is due to the constant current flow through the resistors, unlike parallel circuits where the current flow splits due to branches.The volt drop across resistors in a series circuit will always add up to the supply voltage.The current flow in a series circuit is constant throughout the whole circuit.
What happens if the temperature of the thermistor changes? Figure 3.5 shows the thermistor has now changed its resistance due to a change in its temperature from 25°C to 35°C, this could be due to the vehicle being stationary (parked) for a period and has picked up heat due to solar radiation (UV through the glass area).This increase in temperature has caused the resistance to reduce from 500 to 333 .This means that the material of the sensor is NTC – a negative temperature coefficient, as the temperature increases the resistance reduces.
It will be seen that the ammeter fitted shows an increase in current flow due to the overall reduction in the total resistance of the circuit.
Ohm’s law
And a total current flow of:
Voltage drop across R1:
A voltmeter (2) connected as shown will measure a volt drop. In this case it will register 3.0 volts.The pd across R2 will be 2.0 volts because the total voltage is 5V:
This is because V1 and V2 will always add up to the voltage supply in a series circuit, 5 volts in this case.
By moving the voltmeter around the circuit, the voltage distribution can be determined:
As the temperature acting on the thermistor changes the component’s internal resistance changes and so does the potential difference across it.This causes the pd across the fixed resistor to change also.The SATC senses this change and can alter the fan speed and flap position to maintain a certain cabin temperature.
Resistors in parallel
Connecting resistors in parallel ensures that the pd applied to each resistor is the same. Current flowing through the ammeter is shared between the two resistors and the amount of current flowing through each resistor will depend on the resistance rating of each resistor. To aid the explanation of parallel resistance an A/C heater control module circuit will be used.
Lighting circuit for the A/C display
Figure 3.6 shows the heater control module A128. C41 is the connector code which is referenced so you can obtain a pictorial representation of the connector and its wiring connections. As with all DIN standard diagrams the power enters the top of the diagram and the grounds (earth) are based at the bottom. Standard symbols are used for bulbs, LEDs (Light Emitting Diodes) etc. The heater control module is a part of the facia panel and includes switches for the A/C system and heater control. These switches must be illuminated so they can be seen in the event of poor light conditions, at night time and to tell the operator the A/C has been switched on.
C41 pin 4 has the code 29S-LA28 .35 OG/GN. Using DIN standards, 29 indicates voltage supplied at all times overload protected. S indicates ‘switched’, this generally means that voltage will be supplied all the time once it has been switched on and it will have a fuse in the circuit. LA is the code for the ‘Switches and Instruments’. .35 is the cross-sectional area, which is comparatively low, so it is a low current carrying wire. OG is orange, which is the functional base colour for 29 voltage at all times and GN is green, which is the tracer colour for identification purposes only.This means that the wire will be live when the headlight/sidelights of the vehicle are switched on (Switches and Instruments). It is overload protected (fused) and supplies the heater control module with power for illumination via bulbs and LEDs. For more information on wiring identification, see section 3.7.
Only the lamps will be included in this explanation due to their being positioned in parallel with each other. Lamps generally have a hot and cold resistance value. The lamps when cold will have a low resistance value, for example 3 .When the filament gets hot inside the bulb its resistance value increases to approximately 100 . This is known as a PTC (Positive Temperature Coefficient), this is because the resistance increases with an increase in temperature.To calculate the resistance of the circuit we will use the hot resistance value of the lamp and not the cold. If the hot resistance of the lamp is unknown then it can be calculated using Ohm’s law by knowing the voltage applied to the lamp and the current flowing through it.
Resistors/consumers in parallel (Figs 3.7 and 3.8)
Applying Ohm’s law to find the current flow in each lamp:
When calculated in this way, the current through each branch circuit can be found easily.Also
it is possible to find the total resistance of the circuit.Applying Ohm’s law:
The equivalent resistance of a number of resistors R1, R2, R3 can also be found by applying
either formula 3.1 or 3.2:
Formula 3.1 works with any amount of resistors in parallel including resistors in compound circuits.
Formula 3.2 works with two resistors in parallel including resistors in a compound circuit.
Rules of a parallel circuit
The potential difference (voltage drop) across each consumer is the same value as the supply voltage, or example if the supply is 12V then every consumer in the circuit will have a potential difference of 2V (theoretically). The total current is equal to the sum of all the branch currents, i.e. if the current splits into two branches then the total current of the circuit is the sum of the two branch currents.
Compound circuit (series/parallel circuit)
This circuit uses resistors, or consumer devices, connected so that some parts are in series and other parts are in parallel (Figure 3.9).
When calculating the current flow in these circuits, it is imagined that the parallel resistors are replaced by a single resistor of equivalent value so as to produce a series circuit.
Not all resistance values are known so they must be calculated. The resistance of the diode and LED is calculated by knowing the voltage drop across it and the current flowing through it:
So Figure 3.9 has changed to Figure 3.10.
Important note – because R2 and R3,R4 and R5 are in series they must be added together before they can be considered to be placed in parallel together.
The total resistance of the circuit is found by:
(3.3)
201 ohm is the resistance of the parallel circuit so you must now add the series part of the circuit
for the total resistance RT:
Total current flow in the circuit is calculated by
We must remember that amps split when going through a parallel circuit, 0.056 amps is the
total amp flow I =I1+I2.
Consideration of the current flow through R1 and R2 shows that 0.056A is the total current
flowing through both resistors. This current divides according to the resistor values – the
higher the value, the smaller the current:
This result may be verified by adding the two branches together I =I1+I2.
Another method to calculate the current flow through the branches I1 I2 is to calculate the volt drop across each resistor starting with the resistors in parallel.Once the volt drop and the resistance value are known the current can be calculated.
Resistance calculation using circuit equivalents
If a network of resistors ends with just two terminals as shown in Figure 3.12 then an equivalence resistor can be placed in the circuit to represent a combination of two previous resistance values. Figure 3.12: Because the two 400ohm resistors are in parallel the current passing through the resistors will divide and because the values are equal the current will divide equally.The equivalence resistance therefore is half the single value due to half the current flow, i.e. 200ohm .
Figure 3.13: You can see that a 200ohm resistor has replaced the two 400ohm resistors.The 300ohm
and 200ohm resistors are in series so the resistance can be added.
Figure 3.14: The two resistors have now been added and form two 500ohm resistors in parallel.This means that the current will split and form two paths. The equivalent resistance is half due to the equal split.
Figure 3.15: Finally, the 250ohm resistor represents the equivalent resistance of the whole circuit replacing the 500ohm , 300ohm , 400ohm and 400ohm resistors.
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