STEPPED RESISTOR OPERATION
An example of a stepped resistor in operation is the blower motor circuit shown below. Notice the blower resistor is in series with the blower motor. Higher resistance in series will lower the current flow; thus, higher blower resistor resistance will result in lower blower speeds.
In low speed current flows through the entire resister from pin#3 though pin#1 to ground (shown in blue). The motor will be in low speed because less current is flowing. Moving the blower switch to medium 2 speed opens a path for current out pin #4 to ground. Current flows through pin#3 and out pin#4 to ground (shown in green). Current will flow through only part of the resistor. The motor will be in medium 2 speed causing the blower motor to spin faster compared to low speed because more current is flowing through the circuit.
Variable resistors provide an infinite amount of resistance values. Variable resistors are used by electrical circuits to provide information on temperature, position, or light source.
There are four types of variable resistors used:
Generic Variable Resistor
Rheostats at one time were used in the headlamp switch to dim or brighten dash panel lighting and pre-OBDII fuel gauge sending units. Rheostats have two connections, one to the fixed end of a resistor and the other to a sliding contact on the resistor. Turning the control moves the sliding contact away from or toward the fixed end, increasing or decreasing the resistance. Rheostats control resistance, thus controlling current flow.
Variable Resistor Symbol
As the wiper moves along the rheostat it exposes more or less of the resistor. Moving the wiper towards the high places a small portion of the resistor in series with the light, causing the light to glow bright. Moving the wiper toward the low, places a larger portion of the resistor in series with the lamp; this increased resistance causes less current to flow lowering the intensity of the light. Rheostats are not used on computer circuits because of temperature variations on the resistor when the wiper arm is moved.
Potentiometers are used to measure changes in position. Potentiometers have three connections or legs: the reference, signal, and ground. The reference is at one end of a resistor and the Ground is at the other end. Current flows from the Reference through the resistor to Ground creating a voltage drop across the resistor. The Signal is a sliding contact (movable wiper arm) that runs across the resistor. Unlike a rheostat, its main purpose is not to vary resistance but to vary the voltage in a circuit.
Variable Resistor Symbol
POTENTIOMETER OPERATIONPOTENTIOMETER APPLICATIONS
Remember a potentiometer has three legs, the reference (R), the signal (S) , and the ground (G) as shown below. 5 volts is supplied to the reference, current flows from the reference (R) through the entire resistor to ground (G). The Signal wiper slides across the resistor changing measure voltage as it moves. As the wiper moves towards the reference (R), the measured signal voltage at (S) will increase. As the wiper moves away from the Reference (R) towards ground (G), the measured signal voltage drops.
Since potentiometer are used to measure changes in position they naturally are used for throttle, EGR, AC blend door, and power seat position sensors. All potentiometers have three wires and are used to measure position changes.
Thermistors are resistors that change resistance as the temperature changes. These are ideal in electrical circuits where measuring temperature change is required. There are two types of thermistors:
NTC (Negative Temperature Coefficient)
NTC thermistors: as the temperature goes down, the resistance goes up, and as the temperature goes up, the resistance drops. This type is used today in automotive applications.
PTC (Positive Temperature Coefficient)
PTC thermistors: as the temperature go down, the resistance goes down also, and as the temperature goes up, the resistance increases.
Thermistors are used as Air, Coolant, EGR, and Automatic Air Temperature sensors. All thermistors have two wires and are used to measure temperature changes. When placed in series with a fixed (pull-up) resistor, thermistors create a variable voltage drop circuit, which is ideal for use by computer circuits. Engine coolant temperature sensor (ECT) is shown below.
In the example below, as the engine warms up, R2 (water temp sensor) increases in temperature; this lowers the resistance of R2. The drop in R2 resistance results in a decreased voltage drop across R2 (less voltage is used). This decreased voltage drop across R2 causes the voltage drop across R1 to increase. R1 now uses more voltage. This increased voltage drop of R1 results in a lowered measured voltage by the monitor circuit and is seen by the computer as an increase in engine temperature. The reverse is true as the water temp sensor cools.
A photoresistor, or photoconductive cell, is basically a light sensitive resistor whose resistance changes as light is exposed to it. Basically photoresistors change resistance as light intensity changes. The photoresistor has high resistance with no light exposed and decreases in resistance as light intensity increases. A common automotive use for photoresistors is the automatic headlamp circuit.
A photoresistor is placed in the automatic headlight circuit shown below. The photoresistor operates a normally closed headlight relay. During daylight driving light shines on the photoresistor, lowering its resistance, causing current to flow allowing the relay coil to energize, opening the relay contacts, thus preventing the headlights from operating. When the photoresistor is in darkness, its resistance increases preventing enough current flow through the relay coil to keep the relay contacts open. The closed relay contacts allow current to flow to the headlights causing them to turn on. Photoresistors respond slowly to changes in light intensity and may require several minutes to stabilize.
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