Thermistors – Sign of the Temperature Coefficient of Resistance For a typical thermistor device used in sensing applications, what is the sign of its temperature coefficient of resistance (TCR)?

Introduction / Context:
Thermistors are temperature-sensitive resistors widely used for measurement, compensation, and protection. Two broad types exist: NTC (negative temperature coefficient) and PTC (positive temperature coefficient). In common sensing roles, NTC thermistors dominate, exhibiting a decrease in resistance with increasing temperature.

Given Data / Assumptions:

• General term “thermistor” in typical instrumentation context.
• Focus on most widely used sensing elements.
• Operating over moderate temperature ranges where behavior is well characterized.

Concept / Approach:

In NTC thermistors (e.g., metal oxide ceramics), carrier concentration increases significantly with temperature due to semiconductor-like behavior, causing resistance to drop; thus dR/dT < 0. PTC thermistors exist (e.g., doped polycrystalline barium titanate), but are more specialized for switching/overcurrent protection. Therefore, asked generally, the expected TCR is negative.

Step-by-Step Solution:

Verification / Alternative check:

Manufacturers’ datasheets show resistance–temperature curves descending with temperature (e.g., 10 kΩ at 25 °C dropping to a few kΩ at elevated temperatures).

Why Other Options Are Wrong:

• Positive/zero: do not describe NTC behavior.
• “May be positive or negative” is true across all thermistor families, but the typical sensor thermistor is NTC; the question seeks the prevalent characteristic.

Common Pitfalls:

Assuming all thermistors are NTC in every application; PTC devices are common in resettable fuses and motor protection.

Final Answer:

Negative