Selecting Thick-Film Thermistors for Fast-Response Temperature Sensing in Harsh Environments

One of the key challenges for designers of systems ranging from automotive to industrial controls and medical devices is monitoring and controlling component temperatures. Each component has a safe operating area and must operate within those temperature limits to avoid failure.

For example, consider the need to monitor battery temperature in an electric vehicle (EV). Temperature sensors must be sensitive, accurate, reliable, and responsive. At the same time, when multiple sensors are required, cost must be minimized. The thermistor is a very suitable choice for this application.

Let’s review the basics of thermistors and present some examples with unique attributes from the Stackpole Electronics TNC series.

What is a thermistor?

A thermistor (thermal resistor) is a semiconductor resistor whose resistance varies with temperature. Typically constructed from metal-oxide-semiconductor materials, it provides large, precise, and predictable changes in resistance with temperature.

Thermistors are available with either a positive temperature coefficient (PTC) of resistance, in which resistance increases with temperature, or a negative temperature coefficient (NTC) of resistance, in which resistance decreases with temperature.

Thermistors are used as temperature sensors or in-rush current limiters. They are available in various physical packages, including rods, discs, beads, washers, and surface-mount devices (SMDs). They are easy to use, rugged, reliable, accurate, responsive, and inexpensive. As a result of these characteristics, thermistors are used wherever single-point temperature measurements are needed, such as in vehicles, home appliances, ovens, and industrial equipment.

The resistance of NTC thermistors varies exponentially with temperature, with a negative slope (Figure 1).

Figure 1: NTC thermistors have a resistance with an exponential response that decreases (10 kΩ to 1 kΩ) with increasing temperature (25°C (room temperature) to 100°C). (Image source: Art Pini)

The response curve of a 10 kilohm (kΩ) thermistor, due to its exponential nature, shows a wide range of resistance values (from 150 Ω to 250 kΩ) over its operating temperature range. This provides high sensitivity to changes in resistance with changes in temperature. The nominal resistance is quoted at laboratory ambient temperature, 25°C.

Manufacturers specify another thermistor characteristic called beta (B). B is a material-dependent constant that describes the slope of the resistance/temperature curve. The B value is the primary descriptor in a simplified model of the thermistor’s characteristics over a temperature range. In this example, B, defined between 25 and 100°C (B25/B100), provides the resistance values for the given thermistor. Given the value B and the resistance (R0) at a known temperature (T0), it is possible to determine the resistance (R1) at a second temperature (T1) using the equation:

R1 = R0 exp (B (1/T1) - (1/T0))

Where:

Temperature (T1, T0) is expressed in kelvin (K)

Resistance (R1, R0) is expressed in Ω

Applying thermistors

Thermistors, like resistors, are passive devices. To operate, they require a power source, which can be supplied by either a current or a voltage source (Figure 2).

Figure 2: Thermistors require power from either a current source or a stable voltage source. (Image source: Art Pini)

The resistor in series with the thermistor scales the thermistor's voltage to ensure it remains within the device's maximum input range. In the voltage-driven configuration, it enables monitoring of the current through the thermistor. In either case, these resistors must be low-tolerance, high-accuracy components.

The selection of drive current or voltage and series resistance must ensure that the thermistor does not self-heat from the applied power. Thermistor self-heating occurs when current through the thermistor raises its temperature above ambient, resulting in a false temperature reading. The TNC series thermistors have a dissipation constant of less than 1.5 milliwatts per degree centigrade (mW/°C) in air. The dissipation constant is used to determine the maximum current through the device that keeps self-heating within the desired temperature tolerance for any application.

A thermistor-based temperature monitoring circuit would include an analog-to-digital converter (ADC) to digitize the thermistor voltage (VTH) (Figure 3).

Figure 3: Shown is a temperature monitor based on a thermistor. (Image source: Art Pini)

In this example, the thermistor is driven by the ADC's band-gap reference voltage. V_TH is the differential input to the ADC. The differential low-pass resistor-capacitor filter minimizes noise and interference.

Remember that the thermistor voltage is a nonlinear function of the measured temperature. The microcontroller decodes the temperature from the measured voltage.

Rugged, fast-response NTC thermistors

While there are many thermistor options, the TNC series (Figure 4) from Stackpole Electronics is a good choice for designs in harsh environments that require fast response times. This line of small-footprint surface-mount NTC thermistors is differentiated from traditional multilayer NTC thermistors by advanced thick-film deposition technology applied to a ceramic base.

Figure 4: The TNC series of NTC thermistors uses thick-film deposition on a ceramic substrate to provide a mechanically rugged structure. (Image source: Stackpole Electronics)

This construction differentiates the TNC series from traditional multilayer NTC thermistors by using a single ceramic substrate rather than monolithic co-fired ceramic layers, thereby eliminating structural layer delamination under strain. It also produces a component with a consistent, uniform thickness across the entire resistance range. These devices are all AEC-Q200 qualified for automotive and vehicular applications.

TNC series thermistors are available in eight nominal resistance values of 3, 4.7, 5, 10, 22, 33, 47, and 100 kΩ, with resistances ranging from 50 Ω to 500 kΩ over temperature. The line offers B values from 2410 K to 4700 K and resistance tolerances of ±0.5%, ±1%, ±2%, ±3%, ±5%, and ±10%. They operate over a temperature range of -40 to 150°C for devices with B values greater than 3300 K, and -40 to 125°C for B values below 3300 K. The thermistors are available in any of three SMD packages: 0402, 0603, and 0805. Selected combinations of resistance, B values, tolerance, and package size are available.

In addition to their robust construction, these small-footprint thermistors feature a fast thermal response, with a thermal time constant of less than 5 seconds (s). This ensures a rapid response to temperature changes.

Example devices include the TNC0402FTD10K0F338, a 10 kΩ ±1% thermistor with B25/50, B25/85, and B25/100 values of 3380 K, 3409 K, and 3418 K, respectively. The B value tolerance is ±1%. It is available in a 0402 SMD package and has a maximum power rating of 110 mW. As one of the smaller devices in the series, it is particularly suitable for locations where space is a premium.

When modeling any of these thermistors using the B values, select the B temperature range that best matches the intended design range.

The slightly larger TNC0603JTC47K0H450 is available in a 0603 SMD package and has a nominal resistance of 47 kΩ with a tolerance of ±5%. It has a maximum power rating of 120 mW. The B values are B25/50, B25/85, and B25/100 for 4438 K, 4500 K, and 4518 K, respectively, making it a good choice for applications requiring greater sensitivity to temperature changes. The B value tolerance is ±3%.

Where higher power dissipation or greater thermal tolerance is required, the 130 mW TNC0805KTC100KH410 in a larger 0805 SMD package can be a good fit. This is a 100 kΩ thermistor with a resistance tolerance of ±10%. The B values are B25/50, B25/85, and B25/100 for 4071 K, 4100 K, and 4109 K, respectively. The B value tolerance is ±3%.

Conclusion

Stackpole Electronics TNC series thermistors, using thick-film technology, offer accuracy, stability, and long-term durability for sensing, monitoring, and controlling temperature in harsh environments, including automotive, industrial, and medical applications.