Electrical Circuits
CIRCUITS
Batteries and AC generators (alternators) apply electrical pressure and cause an electric current to move through a circuit from the positive side to the negative side. But there must be a complete circuit or electrical path made up of wires or other conductors or a current will not flow. If a wire is broken or disconnected, current stops. Circuits are not all alike and electricity does not behave exactly the same in different types of circuits. Therefore, it is important for you to understand the basic kinds of circuits found in an automobile and the way the volts, amps, and ohms behave in these circuits.
An electrical circuit is a continuous loop made of conductors (wire), a power supply, and a component, such as a lamp, which is the load. The two basic types of circuit - parallel and series - have quite different characteristics.
SERIES CIRCUITS
The series circuit is the most simple, basic type of circuit. The current follows a single continuous path through the load devices. If there is an open in any part of a series circuit or load device, the current flow stops throughout the entire circuit.
Because all current passes through each of the resistance units, the current flow (measured in amperes) is the same everywhere in the circuit, no matter how many resistance units are connected in series.
Voltage in a series circuit drops across each of the load devices. This voltage drop can be calculated using Ohm's Law. Voltage drop across the load device equals the resistance of the load device times the current flow.
The sum of all voltage drops in a series circuit is equal to the battery voltage. For example, two equal-wattage lamps are wired in series. The supply voltage will therefore drop one-half over each lamp.
To calculate the total resistance in a series circuit, simply add up all the resistance values:
R1 + R2 + R3 = Total Resistance
For example, in this circuit we have a 12-volt battery in series with three 0.5 light bulbs. The total resistance of the circuit can be obtained by adding together the value of each bulb:
0.5 ohm + 0.5 ohm + 0.5 ohm = 1.5 ohm Total Resistance
We can now calculate the current flow and voltage drop in this circuit by using Ohm's Law as follows:
Voltage drop across each bulb equals the resistance of the bulb times the current flow. The equation is shown as:
Voltage Drop = R x I R = 0.5 ohm I = 8 amps
0.5 ohm x 8 amps = 4 amps = 4 volt drop across each bulb
Therefore, the total volume drop in this circuit is 12 volts, which equals the battery voltage
SINGLE CONTINUOUS CURRENT PATH
PARALLEL CIRCUITS
Most circuits in a car are parallel. Power feed is directly from the battery (positive) through the load, and back to the battery (negative) through the chassis ground.
A parallel circuit provides several paths or branches for the current to flow through. Should any of the branches open, current will still flow through the other branches of the circuit operating the remaining load devices.
Components on parallel circuits therefore all get the same voltage since they all connect directly to the battery positive and negative posts.
Unlike a series circuit, current flow (amperage) through a parallel circuit may not be the same in all of its branches. The flow through each branch depends on the resistance of the branch. Since more current can flow through two or more branches than through one, the total current flow in a parallel circuit equals the sum of the flow through each branch. The total resistance of a parallel circuit is less than the resistance value of any single branch. Because of this lower total resistance, parallel circuits will have a greater amount of current flow if compared to a similar series circuit.
The following formula is used to calculate the total resistance of a parallel circuit:
Parallel Circuit
For example, the parallel circuit as shown contains three light bulbs rated at two (2) ohms each
By applying these values to the above formula, we have:
Using Ohm's Law we can calculate the current movement through the circuit:
Since each branch of the circuit in our example has two (2) ohms of resistance, the current flow through each branch will be the same.
To check your arithmetic, add the current flow of each branch. The sum should be equal to the flow through the entire circuit. For example:
SERIES-PARALLEL CIRCUITS
Series-parallel circuits are very common in automobiles. Many parallel circuits in an automobile use a switch that is in series with the circuit to control current flow. They are also used to put a load device in series to drop the voltage of the load devices in parallel. A good example of this type of series-parallel circuit would be the instrument cluster illumination lamps and the dashboard rheostat (variable resistor). The rheostat or dimmer switch varies the voltage to the lights.
An open in the series portion of the circuit will stop all current flow, but an open in one of the parallel branches will stop current flow in that branch but not the circuit.
To calculate the total resistance of a series-parallel circuit, first calculate the resistance of the parallel branches. Then add that value to the resistance of the load(s) in the series portion of the circuit.
OPEN CIRCUITS
An open circuit means that there is a break or open in the circuit and there is not a complete continuous path for the current to flow through. The open may be caused by a break in a wire, loose connection, defective switch, defective load device, or incomplete ground. The open can be located on either side of the load.
The effect of an open in a parallel circuit depends on the location of the open. If a break is located in the circuit feed or control switch (series portion of circuit), current flow will stop in all of the circuit branches. However, an open in an individual branch will stop the current flow in that branch only. Current flow in other branches will not be affected by the open.
SHORT CIRCUITS
When a circuit provides a continuous path for current to flow but does not allow it to flow through all of the load and control devices, it is referred to as a short circuit. Shorts can occur between the circuit and ground, or one circuit can short into another one. It is also possible for a load or control device to short internally.
SHORT CIRCUIT TO GROUND
A short circuit to ground in which the load device is bypassed will generally result in the formation of a new circuit with extremely low resistance compared to the original circuit. Since electrical current always flows through the path of least resistance, a very large amount of current will flow from the power source to the points of the short. If the circuit is protected, the fuse will open. In unprotected circuits, the amount of current flow will become too great for the conductor, causing the wires to overheat and melt.
TYPES OF SHORTS
CONDUCTOR SHORTED AHEAD OF LOAD AND CONTROLS
1. Insulation melted to point of short
2. Wire burnt to point of short
3. Controls and load do not operate
CONDUCTOR SHORTED BETWEEN CONTROL AND LOAD
1. Wire burnt (control may also be burnt), if control is ahead of load
2. Load and control are inoperative
3. Load operates at full capacity if control is alter the load
SHORTS INTO ANOTHER CIRCUIT
A short into another circuit is usually the result of two bare wires touching or a damaged wiring harness.
Often this situation means an additional branch of a parallel circuit. Since current follows the path of least resistance, the majority of the current will flow in the branch of least resistance. With the short into another circuit, additional circuits are being energized when a switch is closed in one of these circuits.
If the circuit was a series circuit with the short into another circuit it has now become a parallel circuit. Since the parallel circuit has less resistance than a series circuit, additional current will flow which may cause the fuse to blow.
TYPES OF GROUNDS
DESCRIPTION
The return "ground" for a component in a circuit may be shown one of two ways on a wiring diagram. It can be shown either at the case of the component or in the wiring. The symbol used to denote a "ground" is a horizontal line with two or three succeeding smaller lines underneath the first one.
CASE GROUNDS
Case grounds are used where the component is securely attached to a well grounded part of the vehicle. The case of the component also should be made of metal to make the ground circuit complete. However, a tab or ground strap can be used in some instances.
REMOTE GROUNDS
Remote grounds are used where the component itself is not grounded, or where remote ground is used to control the component. This is the case of reversible motors, where the current must flow in both directions. This also occurs in some instrument panel warning lights, where the circuit is grounded through the wiring harness.