Transducers

Transducers convert energy from one form to another. Learn active vs passive types, strain gauges, thermocouples, LVDTs, piezoelectric devices, loading effects, and the industrial 4–20 mA current loop.

A transducer converts one form of energy into another. Sensors (input transducers) turn physical quantities into electrical signals; actuators (output transducers) such as speakers and motors do the reverse. In measurement, the term usually refers to the sensing element plus its immediate conversion stage.

Understanding how each transducer works tells you its signal level, excitation needs, error sources and the conditioning circuit it requires.

An analogy

A transducer is like a currency exchange: it converts mechanical, thermal or optical 'currency' into electrical currency, charging a small fee (losses and errors) in the process.

Active vs passive

Active (self-generating) transducers produce their own signal: thermocouples, piezoelectric crystals, photovoltaic cells, tachogenerators. Passive transducers change a property (R, L, C) and need external excitation: strain gauges, RTDs, LVDTs, capacitive sensors.

Strain gauges and thermocouples

A metal-foil strain gauge's resistance changes with strain (GF ≈ 2); the change is tiny (0.1 %), so gauges are used in Wheatstone bridges. A thermocouple junction of two metals produces a voltage proportional to the temperature difference between hot and cold (reference) junctions, which is why cold-junction compensation is essential.

LVDT and piezoelectric

An LVDT (linear variable differential transformer) moves an iron core between two secondary coils; the difference voltage is proportional to displacement, with no contact and infinite resolution. Piezoelectric crystals generate charge under force — excellent for vibration and dynamic pressure, but they can't measure static force because the charge leaks away.

Loading

Connecting a measuring circuit draws energy from the transducer and changes the measurement: a high-output-impedance sensor into a low-input-impedance amplifier reads low. Buffers, charge amplifiers (for piezo) and current signalling avoid it.

The 4–20 mA loop and HART

Industrial transmitters send a current of 4–20 mA over two wires. Current is immune to wire resistance and most noise, a 'live zero' of 4 mA distinguishes 0 % from a broken wire (0 mA), and the loop powers the transmitter itself. HART superimposes digital data (FSK at 1.2 kbit/s) on the same pair for configuration and diagnostics.

Formulas

• Strain gauge: ΔR / R = GF · ε
• Thermocouple: V ≈ S · (T_hot − T_cold) (K-type ≈ 41 µV/°C)
• 4–20 mA scaling: value = min + (I − 4)/16 × (max − min)
• Max loop resistance: R_max = (V_supply − V_transmitter,min) / 20 mA

Worked example: Strain gauge resistance change

Given: GF = 2.0; R = 350 Ω; ε = 500 µε

Find: ΔR.

• ΔR/R = 2.0 × 500e-6 = 0.001
• ΔR = 0.001 × 350 = 0.35 Ω

Answer: 0.35 Ω (0.1 %) — needs a bridge to measure

Worked example: Thermocouple output

Given: K-type ≈ 41 µV/°C; Hot junction 225 °C, cold junction 25 °C

Find: Output voltage.

• ΔT = 200 °C
• V ≈ 41e-6 × 200 = 8.2 mV
• The instrument must add the cold-junction temperature back (CJC) to report 225 °C

Answer: ≈ 8.2 mV

Common mistakes

• Wrong: Using copper extension wire for a thermocouple. — Right: Use matching extension/compensating cable; new metal junctions create errors.
• Wrong: Using a piezo sensor to measure a static weight. — Right: Piezo outputs decay under static load; use a strain-gauge load cell.

Applications

• Process plants: Thousands of 4–20 mA transmitters report pressure, flow, level and temperature to PLCs and DCSs.
• Weighing: Load cells (strain-gauge bridges) in scales, silos and truck weighbridges.
• Vibration monitoring: Piezoelectric (IEPE) accelerometers detect bearing faults in rotating machinery.

Key takeaways

• Transducers convert energy forms; active ones self-generate, passive need excitation.
• Strain gauges: ΔR/R = GF·ε (tiny); thermocouples need cold-junction compensation.
• Watch loading effects between transducer and instrument.
• 4–20 mA: robust, self-powered, live-zero industrial signalling.

Going further

• Charge amplifiers: Piezo sensors are read with charge amplifiers whose output depends on sensor charge and feedback capacitance, not on cable capacitance — so long cables don't change sensitivity.
• IEPE sensors: Integrated-electronics piezoelectric sensors include a built-in amplifier powered by a constant current (2–20 mA) over the same coax that carries the signal.
• Digital fieldbuses: Foundation Fieldbus, PROFIBUS PA and IO-Link carry multiple digital measurements and diagnostics per device, progressively replacing analogue 4–20 mA.