24-08-2011, 11:29 AM
INTRODUCTION
There is a variety of temperature sensors on the market all of which meet specific application needs. The most common sensors that are used to solve these application problems include the thermocouple, Resistive Temperature Detector (RTD) thermistor, and silicon- based sensors. For an overview and comparison of these sensors, refer to Microchip’s AN679, “Temperature Sensing Technologies”. This application note focuses on circuit solutions that use Negative Temperature Coefficient (NTC) thermistors in the design. The Thermistor has a non-linear resistance change-over temperature. The degree of this non-linearity will be discussed in the “Hardware Linearization Solutions” section of this application note. From this discussion, various linearization resistor networks will be shown with error analysis included. Finally, the signal conditioning path for the thermistor system will be covered with complete application circuits from sensor or microprocessor.
THERMISTOR OVERVIEW
The term “thermistor” originated from the descriptor THERMally Sensitive ResISTOR. The two basic types of thermistors are the Negative Temperature Coefficient (NTC) and Positive Temperature Coefficient (PTC). The NTC thermistor is best suited for precision temperature measurement. The PTC is best suited for switching applications. This application note will only discuss NTC applications. The NTC thermistor is used in three different modes of operation which services a variety of applications. One of the modes exploits the resistance-versus-temperature characteristics of the thermistor. The other two modes take advantage of the voltage-versus-current and current-over-time characteristics of the thermistor. Voltage-Versus-Current Mode Voltage-versus-current applications use one or more thermistors that are operated in a self-heated, steady-state condition. An application example for an NTC thermistor in this state of operation would be using a flow meter. In this type of circuit, the thermistor would be in an ambient self-heated condition. The thermistor’s resistance is changed by the amount of heat generated by the power dissipated by the element. Any change in the flow of the liquid or gas across the device changes the power dissipation factor of the thermistor element. In this manner, the resistance of the thermistor is changed, relative to the degree of cooling provided by the flow of liquid or gas. A useful thermistor graph for this phenomena is shown in Figure 1. The small size of the thermistor allows for this type of application to be implemented with minimal interference to the system. Applications such as vacuum manometers, anemometers, liquid level control, fluid velocity and gas detection are used with the thermistors in voltage-versus- current mode. FIGURE 1: When a thermistor is overheated by its own power, the device operates in the voltage-versuscurrent mode. In this mode, the thermistor is best suited to sense changes in the ambient conditions, such as changes in the velocity of air flow across the sensor. Current-Over-Time Mode The current-over-time characteristics of a thermistor also depends on the dissipation constant of the thermistor package as well as element’s heat capacity. As current is applied to a thermistor, the package will begin to self-heat. If the current is continuous, the resistance of the thermistor will start to lessen. The thermistor current- time characteristics can be used to slow down the affects of a high voltage spike, which could be for a short duration. In this manner, a time delay from the thermistor is used to prevent false triggering of relays.
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