What are active and passive sensors?

The question of whether a sensor is active or passive depends on whether the sensor independently generates a signal and is therefore active, or a signal is generated using auxiliary energy, making the sensor passive.

Thermocouples are one example of active sensors as they generate thermoelectric voltage due to ambient conditions that can be displayed on a measuring device with a purely analog display.

Passive sensors traditionally include all resistance-based, capacitive or inductive sensors, and therefore also resistance thermometers. They require a measuring current that is fed via the measuring device. This auxiliary energy is then modulated due to environmental influences and can be evaluated as a signal. With passive sensors, self-heating needs to be taken into account as it can influence the measurement results. As a result, the measuring current for resistance thermometers is typically limited to ≤ 1 mA.