Science: Sensor Physics and Transduction Mechanisms
GATE IN tests the physical principles behind sensors: thermoelectric, thermoresistive, piezoresistive, piezoelectric, capacitive, inductive, Hall, photoelectric, electrochemical and optical transduction. Expect a physical effect linked to a circuit model and a sensitivity calculation.
Core rules
- Thermocouple: Seebeck emf depends on the temperature difference between junctions; cold-junction compensation is mandatory. Type K sensitivity is about 41 µV/°C near room temperature, but the response is nonlinear, so tables or polynomials are used.
- RTD: Pt100 follows R(T) = R0(1 + αT) over a limited range with α ≈ 0.00385/°C. Three- and four-wire connections cancel lead resistance; excitation current causes self-heating.
- Thermistor: NTC devices have a large negative temperature coefficient and strong nonlinearity; Steinhart–Hart is the usual model.
- Strain gauge: gauge factor GF = (ΔR/R)/ε; a Wheatstone bridge converts the small resistance change to voltage.
- Piezoelectric sensors produce charge, need a charge amplifier, and cannot measure truly static strain.
- Capacitive: C = εA/d, used for displacement, pressure, level and humidity.
- Hall: V_H = K_H I B; used for current sensing and magnetic field measurement.
- Optical: Beer–Lambert A = εcl; photodiode responsivity converts optical power to current.
Worked example
A Pt100 measures 100°C. Using R = 100(1 + 0.00385 × 100) gives 138.5 Ω. With 1 mA excitation, the signal is 138.5 mV and self-heating is I²R = (0.001)² × 138.5 = 0.139 mW, not 19 µW-check the square. If lead resistance is 0.5 Ω per lead, a two-wire connection adds 1 Ω, equivalent to about 2.6°C error; a three-wire connection compensates it.
Traps that cost marks
- Assuming a thermocouple output is linear in temperature.
- Ignoring lead resistance in a two-wire RTD.
- Using a piezoelectric sensor for a static load.
- Confusing gauge factor with Poisson's ratio.