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FIGURE 36-45
Mechanical variable resistors. A. Potentiometer. B. Rheostat.

Resistors are electrical components that resist a current running through them. Putting a resistor in a circuit causes a drop in voltage across the resistor. It also reduces the amperage in the circuit. Resistors are commonly used to control the voltage and amperage that reaches various components. It is important to remember that each electrical component has its own resistance. Most high-wattage resistors that can carry large amperage contain a coil of high-resistance wire wound around a ceramic form to dissipate heat.

Fixed Resistors

Fixed resistors are generally cylinders with connecting metal leads projecting along the axis of the cylinder at each end. Most axial resistors are marked with a series of colored stripes to indicate their resistance and tolerance levels. Fixed resistors can be manufactured as very tiny devices without leads and can be built into integrated circuits with many other miniaturized components.

Variable Resistors

Resistors found on circuit boards are normally fixed in value. Some resistors found in the motor vehicle are variable. Variable resistors can have their value altered by movement of a slide or by temperature change. The three types of variable resistors are rheostats, potentiometers, and thermistors. Variable resistors can be linear, meaning their resistance value varies proportionally with movement or temperature change, or nonlinear, meaning the resistance change is not proportional with movement.


A rheostat is a mechanical variable resistor with two connections. It consists of a resistance wire wrapped in a loose coil connected to the supply at one end only. A moveable wiper is connected to the other circuit connection and is made to move over the wire manually Figure 36-45. When the wiper is close to the beginning of the coil, the total resistance value is very small. As the wiper is positioned closer to the end of the coil, the resistance value increases. Rheostats are commonly used in dash light dimmer circuits and some fuel gauge sender units. They alter the current flow and voltage in a circuit.

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FIGURE 36-46
Thermistors. A. NTC thermistor resistance. B. PTC thermistor resistance.

Potentiometers are mechanical variable resistors with three connections, two fixed and one moveable. They act as voltage dividers and as such alter the voltage in a circuit. A resistance wire is wrapped between two fixed connections. One fixed connection is attached to the electrical supply, the other to a ground. The third moveable connection is moved across the coil by a wiper in a similar fashion to a rheostat. The variable voltage output is taken from this point. Throttle position sensors are potentiometers.


As mentioned earlier, thermistors are conductors in which resistance value is affected by temperature. There are two types: negative temperature coefficient (NTC) and positive temperature coefficient (PTC) Figure 36-46. NTC thermistors alter their resistance value inversely to temperature. As the temperature increases, resistance value decreases. PTC thermistors alter their resistance value proportionally to temperature. As temperature rises, so does the resistance value. NTC thermistors are the most common and are used in circuits for ECU inputs. They are the sensing elements of devices such as coolant and air temperature sensors.

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FIGURE 36-47
Resistor colors.
Resistor Ratings

Resistance is measured in ohms, represented by the Greek letter omega (Ω). Resistors are rated in ohms as well, to indicate how strongly they will oppose any current flowing through them. Because resistors work by converting some of the electrical energy passing through them into heat, they also have a power rating. Only the resistance value is marked. The resistor’s power rating is determined by its size.

Regardless of their power rating, resistors are small, so identification by numbers is impractical. To identify their value, many resistors are marked with four or five colored bands. Each color represents a numeric resistance value. The color bands are set close to each other and read from left to right. The last band, or tolerance band, is spaced farther apart. Figure 36-47 shows the resistor colors.