Power (Source or Feed) and Ground

Power and ground are terms to describe the beginning and end of a circuit. Power, source, and feed signify the supply side (beginning) of the circuit, where the electricity originates. Ground signifies the return side of the circuit Figure 36-8. In conventional theory, the supply side is the positive side of the circuit, and the return side is the negative side of the circuit, which we will be using throughout this text. In a vehicle, the positive battery post is considered the source. The power or feed side of a circuit refers to the wires and components that originate at the positive post of the battery and end at either a switch or a load, whichever occurs first.

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Power and ground.

Ground is a term used by technicians to indicate the portion of the circuit that returns current flow to the negative side of the battery. The ground side of the circuit starts at the negative post of the battery and ends at either a load or a switch, whichever occurs first. The term ground is also used to mean a direct connection with the negative side of the circuit, as in “the switch is grounded.” Also, if you are told to ground something, that means it needs to be connected to the negative side of the circuit. Many vehicles connect the chassis and body to the negative battery terminal, which means most of the metal components on the vehicle are grounded. As a result, many manufacturers use the chassis as the return path to the negative battery terminal, as this cuts down on the amount of wire needed in the vehicle. At the same time, many computer circuits use dedicated ground wires from their sensors back to the computer so that the electrical signal is accurate and not affected by any stray electrical signals on the ground circuit.

AM-48: Algebraic Expressions: The technician can use Ohm’s law and the power law to determine circuit parameters that are out of tolerance.
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The power law, or Watt’s law, is a mathematical equation describing the relationship of voltage, current, and power in electrical circuits. Power may be defined as the amount of work done in a given period of time. Understanding this equation is useful in calculating characteristics within circuits. The power law is written as

Power (watts) = Voltage (volts) × Current (amps).

An unknown parameter can be calculated from any two known parameters. Like Ohm’s law, the power law is commonly represented in a triangle diagram, as shown here:

For example, we have an electrical circuit driving a power window motor in a passenger car. The vehicle operates on a 12-volt electrical system. The wattage of the window motor is 22 watts. To calculate the current flow in the circuit, we divide the power (or the motor’s wattage) by the voltage, or P/V. In this case the amperage would be 22/12, or 1.8 amps.

AS-60: Ground: The technician can demonstrate an understanding of the problems associated with having an electrical circuit inadequately grounded.

Ground problems can present some of the most challenging issues encountered in automotive diagnostics. Let’s illustrate why ground points are so important on motor vehicles. In a simple circuit, we might have a battery, a fuse, and a load, all connected by wire. In automotive applications, manufacturers take advantage of the conductivity of metal car and truck bodies by connecting the ground side of electrical components to the vehicle’s body or chassis. The negative side of the battery is also connected to the body, so instead of electrical current returning to ground via wiring, it is conducted through the body.

If we have a component with no ground connection, we have an open circuit and no current flow. Loose or dirty ground connections create a point of high resistance in the circuit, resulting in lower current flow and excessive voltage drop. This results in lamps illuminating dimly, electric motors not working or not working at full speed, and running problems in computer-controlled systems. In some circuits, issues may be encountered with electricity finding alternative paths to ground. You may sometimes see vehicles with brake lights flashing along with, or opposite to, turn signal lights. This is commonly caused by a poor ground in the taillight circuit.