Electrical Resistance

Electrical resistance is a fundamental electronic property that limits the flow of current through a circuit. Traditional fixed resistors perform this function with a constant value throughout the circuit, while certain special types of resistors exhibit variable behavior depending on environmental factors. This variable structure enables electronic systems not only to limit current but also to measure environmental conditions and generate responses.

Thermistors, one of these special resistor classes, transcend their role as passive components by assuming the function of sensitive sensors due to their direct relationship with temperature. Thermistors do not merely measure temperature changes; they also adjust the system’s response to these changes, becoming part of an active control mechanism. Thanks to these features, they have acquired an important place among the measurement, control, and safety components of modern electronic systems.

This approach, which does not limit the concept of electrical resistance to fixed-value components, guides us toward understanding how temperature is integrated into electronics through thermistors.

What Is a Thermistor?

A thermistor is a passive electronic component made of semiconductor material whose resistance changes in direct or inverse proportion to temperature. Its primary function is to detect changes in ambient temperature and respond by altering its resistance value. This characteristic transforms it from a simple resistor into a sensitive sensor that responds to temperature.

Structurally, thermistors are produced by sintering metal oxides at specific temperatures. This semiconductor material alters its electrical properties in response to temperature changes. While the sensitivity of thermistors varies depending on the material used and the manufacturing method, most can provide high-accuracy temperature monitoring in a wide range of applications.

Despite their small physical size, these components can operate effectively over a broad temperature range. For this reason, they are preferred in temperature measurement, circuits requiring temperature-based compensation, and heat-dependent automatic control systems. Although passive in nature, their behavior is highly active; through their interaction with the environment, they endow electronic systems with the ability to “sense” the external world.

Structure and Operating Principle of NTC and PTC Thermistors

Thermistors are divided into two main categories based on their resistance relationship with temperature: NTC (Negative Temperature Coefficient) and PTC (Positive Temperature Coefficient) thermistors. The fundamental difference between these two types lies in their resistance response to temperature.

NTC Thermistors

In NTC thermistors, resistance decreases as temperature increases. This behavior is explained by the increase in carrier concentration with rising temperature, which enhances electrical conductivity. They are commonly used for temperature measurement. In semiconductor structures, higher temperatures generate more free carriers, increasing conductivity and causing resistance to drop.

These thermistors are suitable for precise temperature monitoring in the range of -55 °C to 150 °C. They are used for temperature sensing in battery packs, medical devices, and microcontroller-based systems.

PTC Thermistors

In PTC thermistors, resistance increases as temperature rises. This inverse effect is particularly favored in circuit protection applications. PTC devices conduct current with low resistance at low temperatures but abruptly increase their resistance beyond a specific temperature threshold, thereby limiting current flow. This behavior enables them to function as thermal fuses or current limiters.

Some PTC types have a bimetallic structure and operate like “self-resetting fuses.” They interrupt current flow when temperature rises and resume conduction upon cooling. These properties provide long-lasting circuit protection.

Structural Differences

Both types of thermistors are typically manufactured in disc, bead, or surface-mount chip forms. NTCs are designed for wider temperature ranges and high-precision measurements, while PTCs respond rapidly to sudden temperature increases and are used in safety-critical circuits. Consequently, the ceramic material, sintering duration, and additive compounds used in their construction differ according to their type.

Mathematical Model: Resistance Variation with Temperature

The resistance variation of thermistors with temperature can be clearly explained not only through empirical observations but also through mathematical models. In particular, NTC thermistors exhibit an exponential decrease in resistance as temperature increases. This relationship is represented by mathematical expressions based on specific constants and temperature values.

In an NTC thermistor, conductivity increases with rising temperature, leading to a reduction in resistance. The most common mathematical model used to describe this behavior is known in the literature as the B-parameter model. According to this model:

• The resistance of a thermistor at a given temperature is related exponentially to its resistance at a reference temperature and the temperature difference.
• This relationship is determined by the material characteristic known as the thermistor’s B coefficient.

In applications requiring higher accuracy, the Steinhart–Hart equation, which uses three constant coefficients, is employed. This equation calculates temperature using the logarithmic resistance values of the thermistor and produces more precise results over wide temperature ranges. However, in practical applications, the B-parameter model is usually sufficient.

The numerical precision provided by these models ensures high reliability in microcontroller-based temperature measurement systems. In sensor-centric systems, mathematical modeling enables both accurate temperature detection and timely system intervention.

Steinhart–Hart Equation

Characteristic Curves and Graphical Analysis

The behavior of thermistors can be clearly observed not only descriptively or mathematically but also graphically. In particular, the relationship between temperature and resistance in NTC thermistors manifests as a distinct curve.

The following graph illustrates the resistance change of a typical NTC thermistor across temperatures from 0 °C to 100 °C:

As shown in the graph:

• Resistance decreases rapidly as temperature increases.
• The curve exhibits an approximately exponential decline, consistent with the behavior predicted by the B-parameter model.
• The rate of resistance decrease is steepest at the beginning (between 0–25 °C) and becomes slower at higher temperatures.

This graphical characteristic simplifies the prediction of how a thermistor will respond in electronic systems. In applications requiring rapid response to temperature changes, accurate analysis of this curve is of great importance.

Calibration and Accuracy Criteria

The measurement accuracy of thermistors is not only dependent on manufacturing quality but also directly related to proper calibration and use under appropriate environmental conditions. Therefore, in high-precision applications, selecting the thermistor is only the first step—its calibration is a critical stage.

What Is Calibration?

Calibration involves adjusting the output values of a thermistor by comparing them with known temperature references. During this process, measurements are typically taken at fixed temperatures, and the system is rescaled accordingly. For example, reference resistance values are determined at 0 °C, 25 °C, and 100 °C, and the thermistor’s curve is stored in the system’s memory.

Methods that can be used during calibration include:

• Constant-temperature baths
• Precise digital temperature references
• Calibration ovens

Accuracy Criteria and Tolerances

Thermistors are manufactured with specified tolerances. Common tolerance values range from ±1 °C to ±0.5 °C. This value defines the percentage deviation of the resistance at a given temperature. Additionally:

• B-coefficient tolerance affects the error margin in temperature calculations. This deviation can range from 1% to 5%.
• If Steinhart–Hart coefficients are provided by the manufacturer, lower error rates (e.g., ±0.2 °C) can typically be achieved.
• Resistance reading resolution (ADC sensitivity) also impacts overall system accuracy.

A thermistor that is uncalibrated or used outside standard conditions can cause serious safety issues in fields such as medical, automotive, or industrial automation. Therefore, it is essential not only to mount thermistors but also to test and calibrate them using proper reference standards to ensure system reliability.

Circuit Application: A Simple Thermistor-Based Sensor Circuit

The resistance response of thermistors to temperature changes makes them highly suitable for use as analog sensors in microcontroller-based systems. Below is the schematic structure and operating principle of a basic temperature sensor circuit using an NTC thermistor.

Circuit Structure and Components

• 1 NTC thermistor (10 kΩ @ 25 °C)
• 1 fixed resistor (10 kΩ)
• 1 analog input (e.g., Arduino, ESP32, STM32, etc.)
• 1 5 V or 3.3 V power supply

The circuit operates on a voltage divider principle. The thermistor and fixed resistor are connected in series, and the voltage at their junction is applied to the analog input.

Operating Principle

• As ambient temperature changes, the resistance of the thermistor changes.
• This resistance variation affects the output voltage of the voltage divider.
• The analog input reads this voltage and generates data correlated to temperature.

On the microcontroller side, this analog value is converted into a temperature reading by comparing it with a pre-defined temperature-resistance lookup table or a mathematical model (e.g., the B-parameter equation).

Application Areas

• Simple ambient temperature indicators
• Fan control systems
• Battery protection circuits
• Temperature threshold control applications

Such circuits offer low-cost, easy-to-implement, and highly educational foundational sensor architectures for both application developers and engineering students. Moreover, factors such as ADC resolution and reference voltage accuracy directly influence measurement precision. Therefore, selecting appropriate components according to the application is essential.

Thermistors for Future Technologies: IoT and Wearable Systems

Thermistors are becoming essential components not only of traditional electronic systems but also of the future’s connected and intelligent technologies. Two key areas drive this transformation: IoT (Internet of Things) and wearable technologies.

IoT Applications

IoT systems consist of smart devices that collect data from the physical world and transmit it to digital environments for analysis. Temperature sensors are among the most critical components of these systems. NTC thermistors are widely used in this field due to their low cost and energy efficiency.

In smart home systems, heaters that adjust based on room temperature; in industrial automation, production control systems based on temperature; and in agriculture, irrigation systems sensitive to soil temperature—all rely on thermistors as primary data providers. This enables advantages such as energy savings, environmental control, and process efficiency.

Wearable Technologies

In wearable devices, temperature measurement serves not only to enhance user comfort but also to monitor health. Smartwatches can detect sudden changes in body temperature to track various health parameters. Thermistors also play a role in indirectly measuring heart rate, stress levels, or fever-like conditions in these systems.

Thermistors used in these applications are extremely small and operate with ultra-low power consumption. Additionally, due to their compatibility with flexible circuit boards, they have begun to be integrated into wearable textile products.

Integration and Future Perspective

Advances in microcontroller and wireless communication technologies have enabled thermistors to be directly integrated into systems using protocols such as BLE, Wi-Fi, or Zigbee. This has not only made real-time temperature monitoring possible but also widespread.

In conclusion, although thermistors are analog components, they assume a strategic role as data sources in the digital world of the future. Both IoT devices and personal technology products are beginning to better understand their environments and users through these simple yet effective components.

From Passive Element to Intelligent Sensitivity

For many years, electrical resistance was viewed solely as a fixed circuit element that limited current. However, with the introduction of thermistors, this class of passive components evolved into a sensitive structure capable of responding to environmental changes. Thanks to their high sensitivity to temperature, thermistors can now perform not only measurement but also control, protection, and data generation functions.

Today, a thermistor is not merely a resistor; it can also function as a sensor, an actuator, or even a decision-maker. These components, with their extensive range of applications—from industrial systems to home electronics, from healthcare technologies to wearable devices—are among electronics’ simplest yet most effective solutions.

As future technologies evolve into interconnected, autonomous, and precise systems, thermistors will continue to occupy a vital place among their essential components. A response initiated by a simple temperature change has now become part of data-driven decisions that influence complex systems. In this sense, thermistors represent far more than just the “face of electrical resistance dancing with temperature.”