BONNET 7N9477 - Caterpillar

In order for a circuit to function properly, the resistors must have the correct wattage rating as well as the correct resistance rating. The resistors and other components could be damaged by additional current flow and heat if the resistance or wattage ratings are incorrect.You can identify the wattage of a carbon-composition resistor by the size. The most common ratings are 1/10 watt, 1/4 watt, 1/2 watt, 1 watt, and 2 watts.Variable Resistors
Illustration 26 g01072325
The kinds of resistors that have been discussed so far are fixed. This means that the resistors rating cannot be adjusted. Other resistors are variable (Illustration 26). This means that the resistor resistance can be changed by adjusting a control. The control moves a contact over the surface of a resistance. As current flows through a greater length of resistor material, the current decreases. As current flows through less resistor material, current increases.The amount of variance and the number of resistance positions depend on how the variable resistor is constructed. Some variable resistors have only two different resistance values. Other variable resistors have an infinite range between the minimum values and their maximum values.Variable resistors can be linear or non-linear. The resistance of a linear resistor increases evenly. When the control is set at one fourth of the travel, resistance increases to one fourth of the maximum. When the control is set to half of travel, resistance increases to half of its maximum.There are many kinds of variable resistors. Some variable resistors are called rheostats, potentiometers or thermistors. Illustration 27 shows a schematic symbol for a rheostat.
Illustration 27 g01072326
A rheostat typically has two terminals. A rheostat allows current flow in one path. On Caterpillar machines, a rheostat is used to control the brightness of the instrument lights.Another type of variable resistor is the potentiometer. The potentiometer allows two paths for current flow. The potentiometer can be controlled manually or controlled mechanically. Illustration 28 shows a potentiometer that is from a fuel system. The fuel sender measures a specific system resistance value which corresponds to a specific system condition. The output resistance is measured at the main display module and the value corresponds to the depth of fuel in the tank.
Illustration 28 g01072327
A potentiometer (pot) has three terminals and works by dividing the voltage between two of the terminals. Potentiometers can also be designed to work as rheostats.Thermistors
Thermistors (thermal resistors) are a type of variable resistor that operate without human control. A thermistor is made of carbon. The resistance of carbon decreases instead of increasing at higher temperatures. This property can be useful in certain electrical circuits. Thermistor elements are used extensively in sensors on Caterpillar machines for measuring system temperatures.Failed Resistors
Fixed resistors either work or do not work.. Fixed resistors work by passing the proper amount of current. A fixed resistor that has failed, will not allow current to pass or will allow too much current to pass.Variable resistors, on the other hand, can exhibit a flat spot where the moving parts brush against one another and