To reach maximum efficiency, present day automobiles require many control methods. For example, temperature control of engine parts and fluids is essential. However, accurate electronic temperature measurements are not simple. Of the variety of thermocouples, resistance sensors, and thermistors available, the positive-temperature-coefficient silicon thermistor is an excellent choice for this application. Planar technology using the spreading-resistance principle allows this integrated circuit to be built. The TSP102 has a positive resistance temperature coefficient of 0.7%/ °C and has very close resistance tracking from unit to unit. Nominally it is about 1 k Ω resistance at 25°C and changes from 500 Ω at -40°C to 1900 Ω at 120°C.
The example circuit (Figure 3-11) is used to indicate two different water temperature trip points by turning on LEDs when the temperatures are reached. The circuit is constructed around the LM2904 dual operational amplifier which was designed mainly for the automotive industry.
The circuit is powered from the 12-V auto system. The 1N5239 zener diode supplies a regulated 9.1 V to operate the circuit. The thermistor is in series with a l0-kΩ resistor from ground to the positive 9.1V point. The top of the thermistor is tied to both non inverting inputs of the LM2904. The voltage at these inputs will change as the thermistor resistance changes with temperature. Each inverting input on the LM2904 has a reference, or threshold trip point, set by a l0-kΩ resistor and a 2-kΩ potentiometer in series across the 9.1- V regulated voltage. When this threshold is exceeded on the noninverting input of LM2904, the TIL220 LED
lights. In this circuit, the FAN ON trip point was set at 70°C. This occurs with approximately 1.3 V on the inverting input of the top section of the LM2904. The OVERHEAT trip point was set at 95°C. This condition exists when the bottom section of the LM2904 has approximately 1.44 V on the inverting input. The two trip points can be recalibrated or set to trip at different temperatures by adjusting the 2-kΩ potentiometer in each section. In addition to being used as warning lights as shown here, circuits can be added to turn on the fan motor or activate a relay.
Other types of thermistors and temperature sensors manufactured by other companies can be used with this type of circuit.