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How to choose the right thermistor for a temperature sensor

When faced with thousands of thermistor types, the selection may cause considerable difficulties. In this technical article, I will introduce you to some important parameters to keep in mind when choosing a thermistor, especially when the two commonly used thermistor types for temperature sensing (negative temperature coefficient NTC thermistor) Resistance or silicon-based linear thermistor).

When faced with thousands of thermistor types, the selection may cause considerable difficulties. In this technical article, I will introduce you to some important parameters to keep in mind when choosing a thermistor, especially when the two commonly used thermistor types for temperature sensing (negative temperature coefficient NTC thermistor) Resistance or silicon-based linear thermistor). NTC thermistors are widely used because of their low price, but they provide low accuracy at extreme temperatures. Silicon-based linear thermistors can provide better performance and higher accuracy in a wider temperature range, but they are generally more expensive. In the following we will introduce that other linear thermistors that are on the market can provide more cost-effective high-performance options to help solve a wide range of temperature sensing needs without increasing the overall cost of the solution.

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

• Bill of materials (BOM) cost.
• Resistance tolerance.
• Calibration points.
• Sensitivity (change in resistance per degree Celsius).
• Self-heating and sensor drift.

BOM cost

The thermistor itself is not expensive. Since they are discrete, the voltage drop can be changed by using additional circuits. For example, if you are using a non-linear NTC thermistor and want a linear voltage drop across the device, you can choose to add an additional resistor to help achieve this feature. However, another alternative that can reduce the BOM and the total cost of the solution is to use a linear thermistor that provides the required voltage drop by itself. The good news is that with our new linear thermistor series, these two. This means that engineers can simplify designs, reduce system costs, and reduce printed circuit board (PCB) layout size by at least 33%.

Resistance tolerance

Thermistors are classified according to their resistance tolerance at 25°C, but this does not fully explain how they change with temperature. You can use the minimum, typical, and maximum resistance values ​​provided in the device resistance and temperature (RT) table in the design tool or data sheet to calculate the tolerance within the relevant specific temperature range.

To illustrate how the tolerance changes with the thermistor technology, let us compare NTC and our TMP61-based silicon-based thermistor, both of which have a rated resistance tolerance of ±1%. Figure 1 illustrates that when the temperature deviates from 25°C, the resistance tolerance of both devices will increase, but there will be a big 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 within the relevant temperature range.

How to choose the right thermistor for a temperature sensor
Figure 1: Resistance tolerance: NTC and TMP61

Calibration point

Not knowing where the thermistor is within its resistance tolerance will degrade system performance because you need a larger margin of error. The calibration will tell you the desired resistance value, which can help you greatly reduce the margin of error. However, this is an additional step in the manufacturing process, so 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 a narrow temperature range, one calibration point is suitable for most thermistors. For applications that require a wide temperature range, you have two options: 1) use NTC calibration three times (this is due to their low sensitivity at extreme temperatures and higher resistance tolerance), or 2) use silicon-based linear thermistors The resistance is calibrated once, which is more stable than NTC.

Sensitivity

When trying to get good accuracy from a thermistor, the large change in resistance (sensitivity) per degree Celsius is just one of the problems. However, unless you obtain the correct resistance value in the software by calibrating or selecting a thermistor with low resistance tolerance, greater sensitivity will not help.

Because the NTC resistance value decreases exponentially, it has extremely high sensitivity at low temperatures, but as the temperature rises, the sensitivity also drops sharply. The sensitivity of silicon-based linear thermistors is not as high as NTC, so it can perform stable measurements over the entire temperature range. As the temperature increases, the sensitivity of silicon-based linear thermistors usually exceeds that of NTC at about 60°C.

Self-heating and sensor drift

The thermistor dissipates energy in the form of heat, which will affect its measurement accuracy. The heat dissipated depends on many parameters, including the material composition and the current flowing through the device.

Sensor drift is the amount of drift of the thermistor over time, which is usually specified in the data sheet by an accelerated life test given by a percentage change in the resistance value. If your application requires a long service life and 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 TMP61 on NTC?

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

parameter

NTC thermistor

Silicon-based linear thermistor

BOM cost

・ Low to medium:

Low cost of thermistor

May need additional linearization circuit

· Low:

Low cost of thermistor

No need for additional linearization circuit

Resistance tolerance

· Big:

Huge difference between tolerance and extreme temperature at 25°C

· small:

Within the entire temperature range, less than ±1.5% of maximum tolerance

Sensitivity

・ Inconsistent:

Very big at low temperature

Decrease sharply as the temperature rises

・ Consistent:

Maintain stable sensitivity over the entire temperature range

Higher than NTC which usually exceeds 60°C

Calibration point

· More:

Wide application requires multiple points

· one point:

Only one point is needed for a wide range of applications

Self-heating and sensor drift

· high:

Increase power consumption with temperature

Large sensor drift

・Minimum:

Reduce power consumption with temperature

Sensor drift is small

Table 1: NTC and TI silicon-based linear thermistors

For a complete RT table of TI thermistors and easy temperature conversion methods with sample codes, please download our thermistor design tool.

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