Question

In thermistors, the resistance:

• A. Remains unaffected with change in temperature
• B. Increases with increase in temperature
• C. Increases linearly with increase in temperature
• D. Decreases with increase in temperature

Hint:

RTD vs. Thermistor memory rule:
• Metals (RTD): Temperature \(\uparrow\) \(\rightarrow\) Resistance \(\uparrow\) (Positive Temperature Coefficient)
• Semiconductors (Thermistor): Temperature \(\uparrow\) \(\rightarrow\) Resistance \(\downarrow\) (Negative Temperature Coefficient)

Answer 1

The Correct Option is D

Concept: Thermistors (Thermally Sensitive Resistors) are specialized semiconductor devices manufactured from oxides of transition metals. Unlike traditional metallic sensors (like platinum RTDs) which exhibit a Positive Temperature Coefficient (PTC), standard commercial thermistors are typically Negative Temperature Coefficient (NTC) elements.

Step 1: Explaining the semiconductor behavior in NTC thermistors.
In a semiconductor material, valence electrons are tightly bound at low temperatures. As the temperature rises, thermal energy frees a significant number of charge carriers (electrons and holes) into the conduction band. The rapid, exponential surge in free charge carriers drastically outweighs any increased atomic lattice scattering. Consequently, the electrical conductivity surges, causing the overall electrical resistance to drop sharply.

Step 2: Mathematical modeling.
The non-linear relationship governing an NTC thermistor is described by the Steinhart-Hart or basic exponential equation: \[ R(T) = R_0 \exp\left[ \beta \left( \frac{1}{T} - \frac{1}{T_0} \right) \right] \] From this formula, it is mathematically evident that as the temperature \( T \) increases, the exponent decreases, causing the resistance \( R(T) \) to decrease dramatically.