How it works
Active transducers generate their own EMF without an external power supply — thermocouples (Type K: Chromel-Alumel, ~41 µV/°C), piezoelectric crystals (quartz under stress), and photovoltaic cells fall here. Passive transducers need excitation: RTDs, strain gauges (GF ≈ 2 for metallic foil, 50–150 for semiconductor), LVDTs, and capacitive sensors. Dynamic characteristics matter for fast signals: rise time, time constant τ (for a first-order sensor like RTD in water, τ ≈ 5–10 s), and frequency response. Sensitivity = output change / input change, e.g., 41 µV/°C for Type K thermocouple.
Key points to remember
Key performance specs examiners test: sensitivity, resolution (smallest detectable change), accuracy, precision, hysteresis, repeatability, and linearity. A bonded metallic strain gauge at 120 Ω with gauge factor GF = 2 gives ΔR/R = GF × ε; for a strain ε = 500 µε, ΔR = 120 × 2 × 500×10⁻⁶ = 0.12 Ω. Thermocouples need cold junction compensation because the reference junction must be at a known temperature. Piezoelectric transducers cannot measure static (DC) pressures because charge leaks away — they are suitable only for dynamic measurements above a few Hz.
Exam tip
The examiner always asks you to compare active vs passive transducers with two examples each — thermocouples and piezoelectric for active, RTD and strain gauge for passive is the safest answer.