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How to choose the right thermistor for the temperature sensor?

Views:0     Author:Site Editor     Publish Time: 2020-05-13      Origin:Site

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When is faced with thousands of thermistor types, selection may cause considerable difficulties. In this technical article, I will introduce you to some important parameters to keep in mind when is choosing a thermistor, especially when you want to use two commonly used thermistor types for temperature sensing Resistor 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 of ultrasonic transducer sensor can provide better performance and higher accuracy in a wider temperature range, but usually their price is higher.Following we will introduce that other linear thermistors in 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) costs.
• Resistance tolerance.
• Calibration points.
• Sensitivity (change in resistance per degree Celsius).
• Self-heating and sensor drift.


BOM cost
The thermistor itself is not expensive. Because they are discrete, their voltage drop can be changed by using additional circuits. For example, if you are using a non-linear NTC thermistor and you want a linear voltage drop across the device, you can choose to add additional resistors to help achieve this feature. However, another alternative that can reduce the total cost of the BOM and solution is to use a linear thermistor that provides the required voltage drop. The good news is that with our new linear thermistor series. This means that engineers can simplify the design, reduce system costs and reduce the 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 completely explain how they change with temperature. You can use the minimum, typical, and maximum resistance values is provided in the device resistance and temperature (R-T) table in the design tool or data sheet to calculate the tolerance for the relevant specific temperature range.


To illustrate how the tolerance varies with the thermistor technology, let's compare NTC and our TMP61-based silicon-based thermistor, both of which have a nominal 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 large difference between the two at extreme temperatures. It is important to calculate this difference, so that you can select devices that maintain a low tolerance over the relevant temperature range.

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Calibration point
It is not known that the thermistor's position within its resistance tolerance will reduce system performance because you need a larger error range. Calibration will tell you the expected resistance value, which can help you to greatly reduce the error range. 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 narrow temperature ranges, 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 high resistance tolerance), or 2) use silicon-based linear thermal The resistance is calibrated once, which is more stable than NTC.
Sensitivity
When 200KHz ultrasonic transducer is trying to obtain good accuracy from a thermistor, a 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 a low resistance tolerance, a larger sensitivity will not help.
Because the NTC resistance value decreases exponentially, it has extremely high sensitivity at low temperatures, but as the temperature increases, the sensitivity will drop sharply. The sensitivity of a silicon-based linear thermistor is not as high as that of NTC, so it can perform stable measurements over the entire temperature range. As the temperature increases, the sensitivity of a silicon-based linear thermistor usually exceeds the sensitivity of NTC at about 60 ° C.
Self-heating and sensor drift
The thermistor of ultrasonic wind sensor transducer dissipates energy in the form of heat, which affects its measurement accuracy. The amount of heat dissipated depends on many parameters, including the material composition and the current flowing through the device. Sensor drift is the amount of thermistor drift over time, and usually the accelerated life test given by the percentage change in resistance value is specified in the data sheet. If your application requires a long service life and consistent sensitivity and accuracy, choose a thermistor with low self-heating and small 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 situation 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 available in surface mount device NTC general standard 0402 and 0603 packages.

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