Question

Which of the following is an "Active" transducer?

• A. Strain Gauge
• B. Thermistor
• C. Thermocouple
• D. LVDT

Hint:

Active transducers are self-generating (e.g., Piezoelectric, Thermocouple, Photovoltaic).
Passive transducers are variable-parameter devices (e.g., LVDT, Strain Gauge, RTD, Thermistor) and always require external power.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question asks to identify which of the given transducers is classified as an "Active" transducer.
Transducers are devices that convert a physical quantity into an electrical signal, and they are broadly classified as active or passive.

Step 2: Key Formula or Approach:
The classification is based on the source of energy used for transduction:
- Active Transducers: These are self-generating devices that convert energy from one form to another directly, producing an electrical output (like voltage or current) without requiring any external power source.
- Passive Transducers: These require an external auxiliary power source to operate, as they detect physical changes by varying parameters like resistance, inductance, or capacitance.

Step 3: Detailed Explanation:
Let us analyze each of the options:

• Thermocouple (Active):
- It operates based on the Seebeck Effect. When two dissimilar metal junctions are kept at different temperatures, an electromotive force (EMF) is directly generated across the terminals.
- Because it generates its own voltage signal without requiring an external electrical supply, it is a classic active transducer.

• Strain Gauge (Passive):
- It operates on the principle of piezoresistive effect, where mechanical strain changes the electrical resistance of a wire. It requires an external excitation voltage (usually via a Wheatstone bridge) to convert this resistance change into a voltage signal.
- Thermistor (Passive):
- It is a temperature-sensitive resistor whose resistance changes with temperature. It requires an external power supply to measure the resulting current change.
- LVDT (Linear Variable Differential Transformer - Passive):
- It operates on the principle of variable mutual inductance and requires an external AC excitation source to power its primary coil.

Step 4: Final Answer:
The Thermocouple is the active transducer among the options.