How does a positive temperature-coefficient device change its output resistance with temperature?

• A. Output resistance decreases with higher temperature.
• B. Output resistance remains constant.
• C. Output resistance increases with higher temperature.
• D. Output resistance first increases then decreases unpredictably.

Answer: (C)

A positive temperature-coefficient device increases its output resistance as temperature rises. In such devices, warming makes the material less conductive, so the current faces more opposition and the resistance goes up. Some PT-C Thermistors show a gradual rise with temperature, while others (Curie-point types) stay relatively stable until a certain temperature, then their resistance increases sharply. The defining behavior is a rising resistance with higher temperature. If resistance decreased with temperature, that would be a negative temperature coefficient; if it stayed constant, there would be no temperature effect; and if it changed unpredictably, that would not match a standard PTC character.