Technician Handbook 623 Electrical Circuit Diagnosis Resistance Every electrical component or circuit has resistance. Resistance opposes electron flow. It does not slow down the flow of electrons, but rather reduces the number of electrons flowing. Resistance changes electrical energy into another form of energy, such a heat or light, etc. • Some materials have a low resistance to current flow. These are called conductors and include copper, aluminum, silver and gold . • Some materials have a high resistance to current flow. These are called insulators and include rubber, glass, paper, ceramics, plastics and air.
• Materials that can take on the properties of either a conductor or insulator are called semiconductors . These include carbon and silicon . Resistance is measured in ohms, abbreviated with the omega symbol – Ω. Some circuits have added resistance to control current in the circuit. This extra resistance in the circuit can be used to: • Protect sensitive devices (ECUs). Ignition primary resistors, also called ballast resistors, maintain and protect the electronic control unit (ECU) from excessive current. • Adjust brightness of lamps. A lamp rheostat adds or subtracts resistance to dim or brighten interior lamps.
• Control motor speeds, such as on some A/C and heating controls to adjust interior temperature by increasing or decreasing air volume and fan speed. Technical Training 11