Understanding Transducer Types: Active vs. Passive

Transducers are devices that convert one form of energy into another. In measurement systems, they typically convert a physical quantity (like temperature, pressure, or displacement) into an electrical signal.

Transducers can be broadly classified into two main types based on their power requirements:

• Active Transducers: These transducers do not require an external power source for their operation. They generate their own electrical output signal using the energy from the input physical quantity. Examples include thermocouples and piezoelectric sensors.
• Passive Transducers: These transducers require an external power source (excitation) to operate. They modify some characteristic of the applied power source (like resistance, capacitance, or inductance) in response to the input physical quantity. Examples include strain gauges, thermistors, and LVDTs.

Analyzing the Options

Let's examine each option to determine if it fits the definition of an active transducer:

• Strain Gauge: A strain gauge works by changing its electrical resistance when it is stretched or compressed. To measure this change in resistance, an external voltage or current source (excitation) must be applied, typically as part of a Wheatstone bridge circuit. Therefore, a strain gauge is a passive transducer.

• Thermistor: A thermistor is a type of resistor whose resistance is strongly dependent on temperature. While it senses temperature, it needs an external circuit to supply current or voltage to measure the resistance change. Thus, a thermistor is a passive transducer.

• LVDT (Linear Variable Differential Transformer): An LVDT is a linear displacement sensor. It operates on the principle of electromagnetic induction. It has a primary coil and two secondary coils, and its output voltage depends on the position of a movable ferromagnetic core. An AC voltage must be applied to the primary coil for it to function. Hence, an LVDT is a passive transducer.

• Thermocouple: A thermocouple is a temperature-measuring device consisting of two dissimilar conductors that connect at two points. When the two junctions are at different temperatures, a voltage difference is produced between them, known as the Seebeck effect. This generated voltage is proportional to the temperature difference and does not require any external power source for operation. Therefore, a thermocouple is an active transducer.

Identifying the Active Transducer

Based on the analysis, the thermocouple is the only device among the options that generates its own electrical output signal without needing an external power source. This characteristic defines it as an active transducer.