How to Choose the Right Thermistor for Your Temperature Sensor

When faced with thousands of thermistor types, sizing can present considerable difficulties. In this technical article, I’ll walk you through some important parameters to keep in mind when choosing a thermistor, especially when comparing the two commonly used thermistor types for temperature sensing (NTC thermistor resistors or silicon-based linear thermistors). NTC thermistors are widely used due to their low price, but provide less accuracy at extreme temperatures. Silicon-based linear thermistors offer better performance and higher accuracy over a wider temperature range, but are generally more expensive. As we will describe below, other linear thermistors are on the market that offer more cost-effective, high-performance options that help address a wide range of temperature sensing needs without increasing the overall solution cost.

The appropriate thermistor for your application will depend on many parameters such as:

· Bill of Materials (BOM) costs.

· Resistance tolerance.

• Calibration point.

· Sensitivity (change in resistance per degree Celsius).

· Self-heating and sensor drift.

BOM cost

The thermistor itself is not expensive. Since they are discrete, their voltage drop can be changed by using additional circuitry. For example, if you are using a non-linear NTC thermistor and want a linear voltage drop across the device, you may choose to add additional resistors to help achieve this. However, another alternative that can reduce the BOM and overall solution cost is to use a linear thermistor that itself provides the required voltage drop. The good news is, with our new line of linear thermistors, both. This means engineers can simplify designs, reduce system cost and reduce printed circuit board (PCB) layout size by at least 33 percent.

Resistance tolerance

Thermistors are classified by their resistance tolerance at 25°C, but that doesn’t fully explain how they change with temperature. You can use the minimum, typical, and maximum resistance values provided in the Device Resistance vs. Temperature (RT) table in a design tool or datasheet to calculate the associated tolerance over a specific temperature range.

To illustrate how tolerances vary with thermistor technology, let’s compare an NTC to our TMP61 silicon-based thermistor, both of which are rated for ±1% resistance tolerance. Figure 1 illustrates that the resistance tolerance of both devices increases as the temperature deviates from 25°C, but there is a large difference between the two at extreme temperatures. It is important to calculate this difference so that you can choose a device that maintains a lower tolerance over the relevant temperature range.

Figure 1: Resistor Tolerance: NTC vs. TMP61

Calibration point

Not knowing where the thermistor is within its resistance tolerance can degrade system performance because you need a larger error margin. Calibration will tell you the expected resistance value, which can help you greatly reduce the margin of error. However, this is an additional step in the manufacturing process, so the calibration should be kept as low as possible.

The number of calibration points depends on the type of thermistor used and the temperature range of the application. For narrower temperature ranges, one calibration point is suitable for most thermistors. For applications that require a wide temperature range, you have two options: 1) use NTCs calibrated three times (due to their low sensitivity and higher resistance tolerance at extreme temperatures), or 2) use a silicon-based linear thermal The resistance is calibrated once, which is more stable than NTC.

Sensitivity

A large change in resistance (sensitivity) per degree Celsius is just one of the challenges when trying to get good accuracy from a thermistor. But unless you get the correct resistance value in software by calibrating or choosing a low resistance tolerance thermistor, a larger sensitivity won’t help either.

Since the NTC resistance value decreases exponentially, it has extremely high sensitivity at low temperature, but as the temperature increases, the sensitivity also drops sharply. Silicon-based linear thermistors are not as sensitive as NTCs, so they provide stable measurements over the entire temperature range. The sensitivity of silicon-based linear thermistors typically exceeds that of NTCs at about 60°C as temperature increases.

Self-heating and sensor drift

Thermistors dissipate energy in the form of heat, which affects their measurement accuracy. The heat dissipated depends on many parameters, including material composition and the current flowing through the device.

Sensor drift is the amount by which a thermistor drifts over time, usually specified in a datasheet by an accelerated life test given by a percent change in resistance value. If your application requires a long lifetime with consistent sensitivity and accuracy, choose a thermistor with low self-heating and low sensor drift.

So, when should you use a silicon linear thermistor like the TMP61 on an NTC?

Looking at Table 1, you can see that, for the same price, silicon-based linear thermistors can benefit from their linearity and stability in almost any condition within their specified operating temperature range. Silicon-based linear thermistors are also available in commercial and automotive versions and in NTC Common Standard 0402 and 0603 packages for surface mount devices.

Table 1: NTC vs. TI silicon-based linear thermistors

For a complete RT table of TI thermistors and an easy temperature conversion method with example code, download our Thermistor Design Tool.