TEMPERATURE SENSORSAT 261 - Chapter 6 (B) A-Tech Trainer Boards - Electronic Engine Control Systems. (Part 1)Name: _________________________________________________Date: _______________________________________________________TEMPERATURE SENSORSOne of the most common sensing devices found in automotive applications is the temperature sensor.Temperature sensor circuits are used in electronic systems to monitor the temperature of various components, fluids, and even the air. Engine Coolant Temperature (ECT), Intake Air Temperature (IAT), and Transmission Fluid Temperature (TFT) are all examples of this type of system. The circuit operation is basically the same for all three systems.The circuit consists of a control module, temperature sensor, wiring and connectors. The control module contains a voltage regulator, current limiting resistor and a signal processing area that acts like a voltmeter.The voltage regulator supplies a constant voltage level to the circuit. The control module interprets any voltage fluctuations a sensor change. The supply voltage must be regulated for the system to function properly.The current limiting resistor is a fixed resistor that protects the circuit from an amperage overload. The resistor limits the amount of current flow if a short-to-ground condition exists between the control module and the temperature sensor.The voltmeter portion of the control module measures the voltage level at point M. This voltage level depends on the resistance value of the temperature sensor.The temperature sensor is a variable resistor in which the resistance values change as the temperature of the monitored medium changes. This type of sensor increases in resistance as the temperature decreases, and decreases as temperature increases. This sensor is called a thermistor.The temperature sensor circuit is a type of voltage divider circuit. In this circuit a limiting resistor is in series with a variable resistor. This configuration creates a voltage drop across the thermistor that is directly proportional to the thermistor’s percentage of the total circuit resistance.During normal operation, as the temperature beingsensed increases, the resistance of thetemperature sensor decreases and the voltagelevel M decreases. The reverse is true, as thesensed temperature decreases, temperaturesensor resistance increases and voltage at point Mincreases. The control module uses the voltagevalue of M as an input to determine what type ofchanges should be made in the system. This circuitproduces an analog voltage signal ranging fromzero to five volts.NORMAL OPERATION: Resistance Measurement - The Thermistor on the board. It is labeled Rt.How many terminals does the thermistor have? ________________ The thermistor is presently at room temperature (ambient). Measure the resistance of the thermistor.What resistance do you measure at ambient? ______________________________ With the Ohmmeter still connected, slowly warm up the thermistor with your finger.1What happens to the resistance of the thermistor as it is heated? __________________________________ Now allow the thermistor to cool.What happens to the resistance of the thermistor as it cools? ___________________________________Therefore as temperature ____________, resistance ___________________, and as temperature___________________, resistance __________________The operation of this thermistor is similar to the ones used in the automobile. Most thermistors operate in this way. This is known as a negative coefficient thermistor.Voltage Measurements build the circuit shown above. A circuit breaker is included in the circuit to prevent possible damage to the components. Measure the voltage at the input to Voltage Regulator.What is the input voltage? __________________________ Measure the voltage at the output of the Voltage Regulator.What is the output voltage? ________________________The electronic control system in an automobile operates at 5 Volts. The job of the voltage regulator is to provide a CONSTANT (Stable) 5 volts that does not fluctuate. The regulator also provides a 5 Volt REFERENCE VOLTAGE to many ofthe sensors in the automobile, including the thermistor. Measure the thermistor voltage at point M in the circuit.The voltmeter symbol indicates the point where the computer “MONITORS” the thermistor signal.2What is the thermistor voltage at ambient temperature?_________________________________ Heat up the thermistor and measure the voltage again.What happens to the thermistor voltage as the thermistor is heated? _____________________________What happens to the voltage as the device cools down? ____________________________Complete the following statement:As temperature __________________, the thermistor voltage ____________________, and as temperature____________________, this voltage ___________________________.Does the thermistor produce an ANALOG signal (varying BETWEEN 0 and 5 volts) or aDIGITAL signal (switched, 0 OR 5 volts)?_____________________________________________________________________________________ABNORMAL OPERATION:These exercises introduce faults into the circuit in order to observe the change in normal operation and to identify the fault.Open Ground: Monitor the voltage at point M in the circuit.What voltage do you measure at ambient? _____________________________ REMOVE the ground to the thermistor at point 1.What voltage do you measure at point M? _________________________________ REPLACE the ground at point 1.Open Signal: REMOVE the signal wire at point 2, in the circuit.What voltage do you measure at point M? _____________________________ REPLACE the signal at point 2.Short Signal: SHORT point 2 to ground with an additional wire.What voltage do you measure at point M? ________________________________REMOVE the short at point 2. Complete the following table to summarize your results:Fault Voltage at MOpen Ground ____________________Open Signal ____________________Short Signal __________________3During normal operation, the thermistor voltage should be between approximately 0.5 and 4.5 volts. Signals out of this rangeindicate a fault.During abnormal circuit conditions, such as an open or short, the circuit control cannot provide an accurate representation of the temperature that it is designed to sense. Any resistance value that exceeds the circuit design will affect the voltage level at point M, giving the control module inaccurate input. An open between