Side-by-side comparison
| Parameter | Balanced | Unbalanced Bridge |
|---|---|---|
| Null Condition | R1/R2 = R3/R4 — galvanometer reads zero | R1/R2 ≠ R3/R4 — galvanometer deflects |
| Output Voltage | V_out = 0 V | V_out = V_supply × (R1/(R1+R2) – R3/(R3+R4)) |
| Measurement Principle | Null method — varies known arm until balance | Deflection method — reads output voltage directly |
| Accuracy | Very high — independent of supply voltage and meter sensitivity | Lower — depends on linearity, supply stability, and meter calibration |
| Supply Voltage Effect | No effect on null condition | Output voltage proportional to supply; instability causes error |
| Sensitivity | Maximized when all four arms equal (R1=R2=R3=R4) | Small ΔR gives small V_out; amplification needed (e.g., INA128) |
| Application | Precision resistance measurement, Kelvin bridge for low R | Strain gauge, load cell, pressure transducer, RTD signal conditioning |
| Typical Example | Wheatstone bridge at balance: P/Q = R/S, used in lab resistance boxes | Half-bridge or full-bridge with INA128 instrumentation amp, 0–5 V output |
Key differences
A balanced bridge satisfies P/Q = R/S exactly, giving zero galvanometer current regardless of supply voltage — this supply independence is what makes null methods so accurate. An unbalanced bridge generates V_out = V_s × ΔR/(4R) for a small resistance change ΔR in one arm (quarter-bridge approximation), which for a 120 Ω strain gauge at 10 V supply and 0.12 Ω change gives only 2.5 mV — requiring a high-gain instrumentation amplifier like the INA128 with CMRR > 100 dB. Maximum bridge sensitivity occurs when all arms are equal.
When to use Balanced
Use a balanced bridge when maximum measurement accuracy is required and you can vary a known arm — for example, using a Kelvin double bridge to measure a 0.001 Ω shunt resistor in a battery management system.
When to use Unbalanced Bridge
Use an unbalanced bridge for continuous, real-time transducer signal conditioning — for example, four 350 Ω strain gauges in a full Wheatstone bridge on a beam, amplified by an INA128 to give a 0–5 V output proportional to applied force.
Recommendation
For instrumentation exams, choose balanced bridge configuration for precision null measurements and unbalanced for dynamic transducer applications. In any problem mentioning strain gauges, RTDs, or load cells, assume unbalanced bridge with an instrumentation amplifier in the signal chain.
Exam tip: The examiner will ask for the condition of balance (P/Q = R/S or P·S = Q·R) and will expect you to derive the output voltage expression for an unbalanced bridge — know V_out = V_s × ΔR/(4R + 2ΔR) ≈ V_s × ΔR/(4R) for small ΔR.
Interview tip: Interviewers at instrumentation companies like Honeywell or Yokogawa ask why a full-bridge configuration is preferred over quarter-bridge in a strain gauge — answer: full-bridge doubles the sensitivity and cancels temperature drift because all four arms are active gauges.