A thermistor is a type of resistor whose resistance varies significantly with temperature. As an electronic components supplier, I often encounter customers who are interested in the properties of thermistors, as these properties determine their applications in various electronic circuits. In this blog, I will delve into the key properties of thermistors and discuss their implications in practical use.
1. Temperature - Resistance Relationship
The most fundamental property of a thermistor is its temperature - resistance relationship. There are two main types of thermistors based on this relationship: Negative Temperature Coefficient (NTC) thermistors and Positive Temperature Coefficient (PTC) thermistors.
NTC Thermistors
NTC thermistors are the most commonly used type. Their resistance decreases as the temperature increases. This is because, in an NTC thermistor, as the temperature rises, more charge carriers (electrons or holes) are released into the conduction band due to thermal excitation. The mathematical relationship between resistance ($R_T$) and temperature ($T$) for an NTC thermistor can be approximated by the Steinhart - Hart equation:
$\frac{1}{T}=A + B\ln(R_T)+C(\ln(R_T))^3$
where $A$, $B$, and $C$ are the Steinhart - Hart coefficients, which are specific to each thermistor and are determined through calibration.
NTC thermistors are widely used in temperature sensing applications. For example, in a temperature - controlled fan system, the NTC thermistor senses the ambient temperature. As the temperature increases, the resistance of the NTC thermistor decreases, which changes the voltage across a voltage - divider circuit. This voltage change is then used to control the speed of the fan.
PTC Thermistors
PTC thermistors, on the other hand, have a resistance that increases with an increase in temperature. There are two subtypes of PTC thermistors: silicon PTC thermistors and ceramic PTC thermistors.
Silicon PTC thermistors have a linear increase in resistance with temperature over a certain range. They are often used in precision temperature measurement and compensation applications.
Ceramic PTC thermistors, also known as switching PTC thermistors, have a sharp increase in resistance at a certain temperature called the Curie temperature. Below the Curie temperature, the resistance of the ceramic PTC thermistor is relatively low. Once the temperature exceeds the Curie temperature, the resistance increases by several orders of magnitude. This property makes ceramic PTC thermistors suitable for over - current protection. For instance, in a battery charger, a ceramic PTC thermistor can be connected in series with the battery. If the current exceeds a safe level, the temperature of the PTC thermistor rises, and its resistance increases significantly, limiting the current flow and protecting the battery from over - charging.
2. Sensitivity
The sensitivity of a thermistor refers to the change in resistance per unit change in temperature. It is an important property as it determines how accurately the thermistor can measure temperature changes.
For NTC thermistors, the sensitivity is typically higher at lower temperatures. The sensitivity ($\alpha$) is defined as the fractional change in resistance per degree Celsius change in temperature:
$\alpha=\frac{1}{R}\frac{dR}{dT}$
A higher sensitivity means that a small change in temperature will result in a large change in resistance, making it easier to detect temperature variations. However, high - sensitivity thermistors may also be more prone to noise and instability.
3. Accuracy
Accuracy is another crucial property of thermistors. It refers to how closely the measured resistance corresponds to the actual temperature. The accuracy of a thermistor is affected by several factors, including its manufacturing process, calibration, and environmental conditions.
During the manufacturing process, variations in the material composition and structure can lead to differences in the temperature - resistance characteristics of thermistors. Therefore, proper calibration is necessary to ensure accurate temperature measurement. Calibration involves measuring the resistance of the thermistor at several known temperatures and then using these data points to determine the Steinhart - Hart coefficients.
Environmental factors such as humidity, pressure, and mechanical stress can also affect the accuracy of thermistors. For example, high humidity can cause moisture absorption in the thermistor material, which may change its electrical properties.
4. Response Time
The response time of a thermistor is the time it takes for the thermistor to reach a specified percentage (usually 63.2%) of its final resistance value when subjected to a step change in temperature. It is an important property in applications where rapid temperature changes need to be detected, such as in thermal protection circuits.
The response time of a thermistor depends on its physical size, shape, and the thermal conductivity of its surrounding medium. Generally, smaller thermistors have shorter response times because they have less thermal mass. For example, surface - mount thermistors typically have faster response times compared to larger through - hole thermistors.
5. Self - Heating
When a current flows through a thermistor, it dissipates power in the form of heat according to the formula $P = I^2R$, where $I$ is the current and $R$ is the resistance of the thermistor. This self - heating can cause the temperature of the thermistor to rise above the ambient temperature, leading to an error in temperature measurement.
The self - heating effect is characterized by the dissipation constant ($\delta$), which is defined as the power required to raise the temperature of the thermistor by 1°C above the ambient temperature. To minimize the self - heating effect, the current flowing through the thermistor should be kept as low as possible. In some applications, a constant - current source is used to ensure a stable and low current through the thermistor.
Applications and Our Product Offerings
Thermistors are used in a wide range of applications, including temperature measurement and control, over - current protection, and inrush current limiting. As an electronic components supplier, we offer high - quality thermistors with excellent properties. In addition to thermistors, we also provide a variety of other electronic components such as CBB65 AC Motor Capacitor, CD60 Starter Capacitor, and CBB61 AC Motor Starting Capacitor.
If you are interested in our thermistors or other electronic components, we welcome you to contact us for procurement and further discussions. We have a professional team that can provide you with detailed product information and technical support to help you select the most suitable components for your applications.
References
- "Thermistors: Theory, Design, and Applications" by John W. Horton
- "Electronic Devices and Circuit Theory" by Robert L. Boylestad and Louis Nashelsky
- Manufacturer datasheets of various thermistors