There are 2 main theories for explaining which way DC electricity flows. Electron theory states that electricity flows from negative to positive. This is because electrons are negatively charged and flow from negative to positive. The other theory is conventional theory, which states that electricity flows from positive to negative. From what you just learned, this may sound wrong, but it isn’t completely wrong. As an electron moves from one atom to the next, a “hole” or “void” is created in the previous atom, which will be taken up by the next electron flowing in the circuit. As the electrons flow one way, the holes or voids move the opposite way. In automotive we use conventional theory, there is a reason for this. To save wiring, automotive engineers use a “frame ground” which means that the frame or structure of the car can be used like a wire to complete an electrical circuit. The positive terminal of the battery will be connected to fuses, controls and loads, while the negative terminal will be connected straight to the frame. After the electricity has gone through the load and done some work for us, it will be directed to the frame ground which will take it back to the battery. Some older (mostly British) cars ran their systems backwards, these systems are called “positive earth” (electron theory). The problem with having some cars positive earth (conventional theory) and some cars negative earth is because if cars of different types got into a major collision, their frames could touch causing sparks until both batteries discharged or exploded. So the need for standardisation was necessary. Positive earth cars tend to rust faster than negative earth cars so conventional theory won and is what all new cars use today.
This is a simple circuit. It is a series circuit because there is only one path for electrical flow, if there were multiple paths it would be considered a parallel circuit.
The source in automotive circuits is the battery (or the alternator). It makes the push or electrical pressure to move electrons through the circuits resistance/loads. Any electrical current that leaves the source, must return to the source.
The fuse is an intended weak point in the electrical system. If amp flow (not volts) in a circuit gets too high, the $0.50 fuse will blow rather than say a $1000 PCM that needs to be programmed to the rest of the modules.
The switch is the control device that turns the circuit on. Some devices are turned on when the operator turns the key to the “run” position and some things need to be turned on manually by a button or toggle switch. One switch can turn on multiple circuits and circuits can have multiple switches.
The load is the device that uses the electrical current to do some kind of work. This could be a light bulb, the horn, the stereo or anything else that requires electrical flow to run.
Conductors are used to transport electrons from one component to the next. In automotive we use stranded copper wires, wrapped in rubber or plastic to insulate the circuit from the frame or other circuits.
Voltage, Current, and Resistance
Voltage is simply the electrical potential between two points. It is the “push” that the circuit needs to move electrons and create voltage. Voltage is measured in volts (V) and indicates how much work “could” be done. Voltage can be referred to as EMF (electromotive force). Automotive batteries are rated 12.6V which is used for starting the engine.
Current is the actual “flow” of electrons and is measured in amps (A). Current describes “how much” electricity is flowing through the circuit or part of a circuit. It takes millions of electrons moving to do most kinds of work. 1 amp is equal to 6.28 billion electrons flowing past a given point in 1 second. Other than the starter, most automotive electrical systems are fused under 40A.
Resistance is the opposition to electrical flow and is measured in ohms (Ω). Even good conductors have “some” resistance in them, this gets more noticeable as a conductor gets longer. Excessive resistance can be caused by rust, corrosion or an unclean contact. Electrical flow through a resistance produces heat.
Ohms laws states that it takes 1 volt to push 1 amp through 1 ohm of resistance. We can use this in an equation to solve for an unknown value.
- To figure out volts we can use V=AxR, which means Volts = Amps x Resistance. (many people who are more experienced with electronics will use E=IxR, but I find that V=AxR makes more sense when first learning)
- To figure out amps we can use the same formula, only slightly modified. A=V/R which means that Amps = Volts ÷ Resistance.
- To figure out resistance we can use the same formula again. R=V/A which means that Resistance = Volts ÷ Amps.
For instance, when using a multi-meter to check amps, most multi-meters are fused at 10 amps. To check a circuit that we know flows more than 10 amps we can use ohms law to calculate amps instead. Consider the following circuit.
In this example we need to figure out the amps flowing through the circuit, so we will use A=V/R. Since we are working with a 12V battery the V=12, and the resistance of the load is 1Ω and assuming there is no other significant resistance in the circuit, this is our total resistance. So our equation will be A=12÷1. Since 12÷1=12 that means that 12 amps would flow through this circuit, this also means that this would have blown the fuse in our multi-meter.
This rule can come in handy any time you need to know a value that you can’t measure, such as in our example or finding the resistance of something that you can’t get your meter to.
Electricity and Magnetism
Electricity and magnetism are closely related. Electricity can be used to create a magnet and a magnet can be used to create voltage. We do both in automobiles every time we start the engine.
Magnets can occur naturally, they can be man made or they can be an electromagnet. The first two are full time magnets that can’t be switched off. However, an electromagnet can be switched on and off, polarity can be reversed, and intensity can be changed with electric current. Electromagnets can also be much stronger than the other two kinds of magnets if needed. Magnets create an invisible magnetic field around them which attracts “ferromagnetic” materials such as iron, they can also attract or repel another magnets poles. Magnets have a north and a south pole, a north pole will attract another magnets south pole but repel its north pole and vice versa. Repulsion is caused by the two magnets lines of force repelling each other. As the two like poles are forced closer to each other, the magnetic lines of force bend. The lines of force want to return to its natural shape, this is what causes repulsion. This is the basic theory behind an electric motor, such as a starter motor. Additionally, by cutting these magnetic force with a conductor, we can induce a voltage into the conductor. This is the basic theory behind a generator or alternator.
Electrical current can be used to create a an electromagnet. As current passes through a conductor, a small magnetic field is created around the conductor, if that conductor is wrapped into a coil shape, the effect increases drastically. The strength of the electromagnet depends on current flow through the conductor, the number of windings, the size of the windings and the size/shape and material of a core (usually iron). An electromagnet will work without a core but nowhere near as well as an electromagnet with a core. Electromagnets care used everywhere in cars. Fuel injectors, starter solenoids and the starter motor itself, drive-by-wire throttle bodies as well as many other devices around the car that create movement electronically require an electromagnet.
A magnet (or an electromagnet) can also be used to generate electricity. To do this, we need a magnetic field, a conductor and movement. As a conductor passes through a magnetic field, a voltage is induced on the conductor. One end of the conductor will become more positive and the other will become more negative. The magnetic field pushes all the electrons over to one side. Which side is positive or negative is determined by the north and south poles of the magnet as well as which way the conductor goes through the magnetic field. The amount of voltage induced depends on the strength of the magnetic field, the speed of the conductor and the angle the conductor cuts through the magnetic field. A conductor must cut through the magnetic field at a 90° angle to generate the most voltage possible. This is the basic theory behind an alternator/generator and a PM generator (various sensors).
Relays are used in automotive applications to allow a small current (not voltage) circuit, to turn on a large current circuit. If it wasn’t for relays, every switch on the car would have to be able to handle the current of the circuit it turns on. Most of the switches on the car are near the driver so that would mean a designing a ton of heavy duty switches around the driver, making an interior look more like a cockpit. It would also mean many heavy duty wires would have to run to these switches, increasing the risk of a fire in the cabin.
A relay contains a coil of very fine wire, which when energised becomes an electromagnet. The terminal 30 contact is held open by a fairly weak spring. When the switch closes and the coil receives current, the electromagnet pulls the terminal 30 contact down, connecting terminal 30 to terminal 87 completing the circuit. The coil of wire is essentially one very long thin wire, because of this the coil has a very high resistance, which means 12V can only push a small amount of current through that circuit. This means that the switch that controls the coil doesn’t see high current and can be made much cheaper.
In an automobile we need a battery to store electricity so we can use it next time we want the engine to start. Inside the battery, a chemical reaction pushes electrons to the negative terminal of the battery. This creates voltage between the positive and negative terminals.
A battery cell has positive plates made of lead peroxide (Pb02), and negative plates made of sponge lead (Pb). These plates are arranged positive, negative, positive, negative with separators in between to keep the plates from touching each other. These plates are submerged in submerged in electrolyte which is a mixture of 65% water (H2O) and 35% sulphuric acid (H2SO4). A battery is made up of 6 individual cells connected in series with the battery terminals on either end. Each cell produces about 2.1V resulting in a total voltage of 12.6V in a fully charged battery. When a battery discharges as current starts flowing, the SO4 in the electrolyte combines with the lead (Pb) in both plates to form lead sulphate (PbSO4) which cannot produce a voltage. Also, the oxygen (O2) from the positive plate combines with the left over hydrogen from the electrolyte to form water (H2O). So the electrolyte becomes plain water. When the alternator or generator forces a charge into the battery, oxygen (O2) is returned to the positive plate and the SO4 is returned to the electrolyte and recombines with the hydrogen (H2).