Thermistors are commonly used to measure the temperature of liquids and ambient air. A thermistor is a solid-state variable resistor made from a semiconductor material, such as metal oxides, that have very reproducible resistance verses temperature properties.
FIGURE. A thermistor is used to measure temperature. The sensing unit measures the resistance change and translates the data into temperature values.
By monitoring the thermistor’s resistance value, the computer is capable of observing very small changes in temperature. The computer sends a reference voltage to the thermistor (usually 5 volts) through a fixed resistor. As the current flows through the thermistor resistance to ground, a voltage sensing circuit measures the voltage after the fixed resistor. The voltage dropped over the fixed resistor will change as the resistance of the thermistor changes. Using its programmed values, the computer is able to translate the voltage drop into a temperature value.
There are two types of thermistors: negative temperature coefficient (NTC) thermistors and positive temperature coefficient (PTC) thermistors. NTC thermistors reduce their resistance as the temperature increases, while PTC thermistors increase their resistance as the temperature increases. The NTC is the most commonly used.
FIGURE. Chart of temperature and voltage correlation.
Using the circuit shown in Figure, if the value of the fixed resistor is 10K ohms and the value of the thermistor is also 10K ohms, the voltage sensing circuit will read a voltage value of 2.5 volts. If the thermistor is a NTC, as the ambient temperature increases its resistance decreases. If the resistance of the NTC is now 8K ohms, the voltage reading by the voltage sensing circuit will now be 2.22 volts. As ambient temperature increases and the NTC value continues to decrease, the voltage sensing circuit will measure a voltage decrease. If the thermistor was a PTC, the opposite would be true and the voltage sensing circuit would measure an increase in voltage as the ambient temperature increases.
FIGURE. Dual ramp temperature sensor circuit.
Some temperature sensing circuits are designed as dual range circuits. This circuit provides for a switch in the resistance values to allow the microprocessor to more accurately measure temperatures. When the voltage sensing circuit records a calibrated voltage value (1.25 volts for example), the microprocessor will turn on the transistor which places the IK resistor in parallel with the 10K resistor. The circuit will operate the same as described until the 1.25 volts is recorded. With the IK resistor now involved in the circuit the resistance of the fixed resistor portion of the circuit is now 909 ohms. At this occurrence, the voltage sensing circuit will record the sharp increase of voltage and a second range of values can be used. Depending of which side of the switch the voltage is sensed, it can represent two totally different temperatures.
FIGURE. The same voltage value can represent different temperatures.