How NTC Thermistors & RTD's differ?
Sensors are among the most critical parts of many devices and systems. When it comes to analyzing physical or environmental conditions, temperature is among the most important parameters. As such, when designing an electronic device or system that requires temperature measurement, designers have many options to select from. Two of the most common temperature-sensing technologies, which offer features and functionality for a wide range of uses, are negative temperature coefficient (NTC) thermistors, and resistance temperature detectors (RTD).
What are NTC Thermistors?
NTC thermistors are thermally sensitive resistors that decrease in resistance as temperature increases to exhibit a negative temperature coefficient of resistance. The non-linear relationship between resistance and temperature is beneficial when accurate temperature measurement is needed within a small range of temperatures, such as within 50°C. The change in resistance for every degree of temperature change allows temperature to be measured with high accuracy.
NTCs are made from ceramic metal-oxide semiconductor materials that vary in resistance & temperature characteristics depending on the materials. They are also available in many shapes and form factors for use in a wide scope of applications. NTCs are frequently used within analog measuring circuits where the resistance change can be made linear through the use of a voltage divider and operational amplifier. It is important to note that multiple types of NTCs exist and you should determine which one is best for your intended application. The most common types are point-matched NTCs and interchangeable NTCs. Point-matched NTCs have specific resistance tolerances while interchangeable NTCs are designed to maintain a range of temperatures. Additionally, interchangeable NTCs are designed such that they can be replaced with a thermistor from another manufacturer without the need to re-calibrate the measuring circuit.
What are RTDs?
Like NTCs, RTDs are thermally sensitive resistors. However, they exhibit a positive temperature coefficient, meaning they increase in resistance as temperature increases. In an RTD, the temperature/resistance relationship is linear across a broad range of temperatures, making RTDs useful in applications where measurements are made over many temperatures from -50°C to 500°C. RTDs are made from precious metals either formed into a wire-wound resistor or deposited onto a substrate via a thin-film process. The majority of RTD devices use nickel or platinum as the sensing element, as both these materials have a clearly-defined temperature coefficient of resistance over many temperatures.
RTDs are used in analog measuring circuits to convert a resistance reading into a temperature reading. In many configurations, RTDs are grouped with other resistors in a bridge formation to allow more accurate reading and minimize errors. RTDs come in many classes of accuracy, defined by various standards. For different accuracy classes, the temperature accuracy widens linearly as temperature increases or decreases away from 0°C. The temperature range in which an RTD remains accurate varies by class, so it’s important to know the range of temperatures over which the RTD will function.
While both RTDs and NTCs provide the same basic purpose of temperature measurement via resistance reading, they are different technologies with their own unique benefits. When selecting the technology that is best suited to your needs, key factors such as operating temperature range, the amount of accuracy required in the measurement, the range of temperatures where measurements will be made, and other design factors should all be considered.
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