I/V Graph Of Thermistor

Expanding and refining the given content, we aim to provide a comprehensive and understandable explanation of thermistors, focusing on the Negative Temperature Coefficient (NTC) type and contrasting it with the Positive Temperature Coefficient (PTC) type and metallic conductors.

Thermistors: An Overview

A thermistor is a type of resistor whose resistance varies significantly with temperature. The term “thermistor” is a blend of “thermal” and “resistor”. Thermistors are widely used in a variety of applications, such as temperature sensing, circuit protection, and temperature compensation.

Negative Temperature Coefficient (NTC) Thermistor

An NTC thermistor is a type of thermistor that exhibits a decrease in resistance as the temperature increases. This characteristic can be observed from the I-V (current-voltage) curve of an NTC thermistor. In such a curve, the ratio of voltage (V) to current (I), which represents resistance, decreases with an increase in current. This implies that the resistance of the NTC thermistor diminishes as the current through it increases.

The Underlying Physics

The behavior of an NTC thermistor can be attributed to its semiconductor material composition. When the potential difference across the thermistor increases, it leads to an increase in current flow. This, in turn, causes a rise in the temperature of the thermistor. The increase in temperature results in a higher vibration of lattice ions within the semiconductor material, which tends to reduce the drift velocity of charged particles (electrons and holes). Despite this reduction in drift velocity, the overall effect of temperature rise is an increase in the number of free electrons and holes due to thermal excitation. This increase in charge carriers outweighs the effect of reduced drift velocity, leading to a decrease in resistance.

Visualization Through the I-V Graph

iv graph of thermistor

An illustrative I-V graph of an NTC thermistor demonstrates this behavior, showing how the resistance changes with varying current and, implicitly, temperature. The graph reveals the intrinsic relationship between current, voltage, and temperature in an NTC thermistor.

Positive Temperature Coefficient (PTC) Thermistor

Contrasting with NTC thermistors, PTC thermistors have a resistance that increases as the temperature rises. This type of thermistor is useful in applications requiring temperature-based resistance increase for protective and regulatory functions.

Comparison with Metals

Unlike thermistors, metals typically exhibit a positive temperature coefficient of resistance. This means that, as the temperature increases, the resistance of metallic conductors also increases. This behavior is opposite to that of NTC thermistors and aligns with the characteristic of PTC thermistors, albeit through different mechanisms.

Understanding Resistance and Its Measurement

An important concept to grasp when discussing thermistors and their I-V curves is the distinction between the gradient of the I-V curve and the actual resistance of the component. Resistance, denoted as $R$, is the ratio of potential difference (voltage, $V$) to current ($I$), expressed mathematically as $R = \frac{V}{I}$. It is crucial to understand that resistance is not the derivative of voltage with respect to current ($\frac{dV}{dI}$), but rather a ratio that defines the proportionality between voltage and current according to Ohm’s law.

In summary, thermistors, particularly NTC and PTC types, offer a versatile means of temperature sensing and control in electrical circuits, with their unique temperature-dependent resistance characteristics. Understanding the principles behind their operation, including the physics of semiconductor materials and the correct interpretation of resistance, is essential for leveraging their capabilities in practical applications.

Worked Examples

Example 1: Practical Application of NTC Thermistor

A team of engineers is designing a temperature sensor for a battery management system (BMS) to prevent overheating. They decide to use an NTC thermistor due to its ability to decrease resistance with increasing temperature. The sensor circuit is designed to trigger a cooling system when the thermistor’s resistance falls below 10kΩ, which corresponds to a critical temperature. If the room temperature is 25°C and the thermistor’s resistance at this temperature is 15kΩ, explain how the NTC thermistor’s behavior ensures the battery’s safety. Assume the thermistor’s resistance drops by 0.5kΩ for every 1°C rise in temperature.

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The NTC thermistor is ideal for this application due to its negative temperature coefficient, meaning its resistance decreases as temperature increases. At the starting point, the thermistor’s resistance is 15kΩ at 25°C. As the temperature of the battery increases, the resistance of the thermistor decreases by 0.5kΩ for every 1°C rise in temperature. This predictable decrease in resistance allows the BMS to monitor the battery temperature accurately.

To trigger the cooling system at the right moment, the BMS watches for the thermistor’s resistance to drop to 10kΩ, signaling a critical temperature. Calculating the temperature at which this occurs involves understanding how much the temperature needs to increase from 25°C to reach the trigger resistance level:

From 15kΩ to 10kΩ, the resistance needs to decrease by 5kΩ. Since the resistance drops by 0.5kΩ for every degree Celsius increase, the temperature needs to rise by 10°C (5kΩ / 0.5kΩ/°C = 10°C). Therefore, the critical temperature at which the cooling system is triggered is 35°C (25°C + 10°C).

This behavior ensures the battery’s safety by actively monitoring temperature changes and activating the cooling system before the battery overheats, preventing potential damage or failure.

Example 2: Contrasting NTC and PTC Thermistors in Circuit Design

An electronic device incorporates both an NTC and a PTC thermistor for different functions. The NTC thermistor is used to regulate the internal temperature, while the PTC thermistor is intended to protect against current surges. Describe a scenario where the dual function of these thermistors is critical for the device’s operation, explaining how each thermistor contributes to the device’s safety and performance.

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In an electronic device, such as a power supply unit, the NTC thermistor can be used at the power entry point to regulate the internal temperature. It ensures that as the device starts up and the internal components begin to heat up, the resistance of the NTC thermistor decreases, allowing more current to flow smoothly without causing a sudden temperature spike, thereby regulating the temperature.

Conversely, the PTC thermistor is placed in series with the output circuit to protect against current surges. In the event of a short circuit or any fault that causes a current surge, the temperature of the PTC thermistor increases, leading to an increase in its resistance. This rise in resistance limits the current flow, protecting sensitive components from damage.

A critical scenario involves a sudden external short circuit causing a current surge while the device is operating at high temperatures. The NTC thermistor, already at a low resistance due to the high temperature, allows for controlled cooling of the device. Simultaneously, the PTC thermistor responds to the current surge by increasing its resistance, thus limiting the current flow and preventing further heat generation. This dual-function approach ensures the device operates safely under various conditions, balancing temperature control and surge protection.

Example 3: Understanding Resistance and Temperature Coefficients

A laboratory experiment involves measuring the resistance of an NTC thermistor at various temperatures to determine its temperature coefficient. At 20°C, the measured resistance is 12kΩ. When the temperature is increased to 30°C, the resistance drops to 9kΩ. Calculate the average temperature coefficient of resistance (TCR) for the thermistor between these two temperatures.

Note: The temperature coefficient of resistance (TCR) is defined as the ratio of the relative change in resistance to the change in temperature. It is given by the formula:

$$TCR = \frac{\Delta R / R_{0}}{\Delta T}$$

where $\Delta R$ is the change in resistance, $R_{0}$ is the initial resistance

, and $\Delta T$ is the change in temperature.

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Given data:

  • Initial resistance ($R_{0}$) at 20°C = 12kΩ
  • Final resistance ($R$) at 30°C = 9kΩ
  • Change in temperature ($\Delta T$) = 30°C – 20°C = 10°C
  • Change in resistance ($\Delta R$) = 9kΩ – 12kΩ = -3kΩ (a decrease in resistance)

Plugging these values into the formula for TCR:

$$\begin{aligned} TCR &= \frac{-3kΩ / 12kΩ}{10°C} \\ &= \frac{-0.25}{10} \\ &= -0.025\, \text{Ω/Ω/°C} \end{aligned}$$

This negative value of TCR confirms the NTC behavior of the thermistor, indicating that its resistance decreases as the temperature increases, at an average rate of -0.025 Ω/Ω/°C between 20°C and 30°C. This measurement is crucial for applications requiring precise temperature monitoring and control.

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