Thermistor Solutions
Microchip provides various single-chip solutions for thermistors. These thermistors are used to send alert signals to the microcontroller or switch a fan or an LED indicator on/off when the temperature exceeds preset limits. For example, a chilled water reservoir may use a thermistor to detect the change in temperature of the reservoir to detect the water level.
Figure 1: Configuration of the TC621 dual trip point temperature switch
Figure 1 shows how to configure the TC621 dual trip point temperature switch. The thermistor and the alert limit setting resistors directly connect to the device and the alert outputs trigger with ±0.3°C accuracy. The advantage of this device is that it provides a closed-loop temperature switch solution for thermistor applications.
Increasing the Sensor Sensitivity
Although thermistors provide a low-cost solution and are easy to implement, they are highly non-linear, particularly if the application temperature range exceeds approximately 50°C. The vendors typically provide look-up tables and correction factors. However, in the non-linear region, the change in resistance per degree Celsius or the sensitivity is reduced exponentially as temperature increases.
Figure 2: Hand-held thermometer
For example, the handheld instrument depicted in Figure 2, equipped with an integrated thermistor sensor to measure ambient temperature, should maintain consistent measurement sensitivity in both the frigid Alaskan winter and the scorching Arizona summer. Microchip provides various programmable gain amplifier solutions to increase the sensor sensitivity within the non-linear region. Figure 3 shows how to configure a digital potentiometer and a low-power amplifier as a programmable gain amplifier.
Figure 3: Configuration of a digital potentiometer and a low-power amplifier
as a programmable gain amplifier