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 that their resistance value varies proportionally with movement or temperature change, or non-linear where the resistance change is not proportional with movement.
A rheostat is a mechanical variable resistor with two connections. They consist 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. 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, the resistance value increases. Rheostats are commonly used in dash light dimmer circuits and some fuel gauge sender units. They alter the current flow in a circuit.
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 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.
Thermistors are conductors whose resistance value is affected by temperature. There are two types: NTC and PTC. Negative Temperature Co-efficient thermistors alter their resistance value inversely to temperature. As the temperature increases their resistance value decreases. Positive Temperature Co-efficient 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 inverted circuits for ECU inputs. They are the sensing elements of devices such as coolant and air temperature sensors.