Wheatstone Bridge Calculator

Bridge sensitivity depends on the sensor element and the detection electronics. A full Wheatstone bridge with a 350 Ω strain gauge bridge excited at 10 V produces approximately 10-30 mV at full scale, representing resistance changes of about 0.1-0.3%. With a 24-bit ADC and a precision instrumentation amplifier, resistance changes of less than 0.001% can be resolved, corresponding to microstrains or millikelvin temperature changes. In laboratory Wheatstone bridge instruments with galvanometer detection, the null point can be set with milliohm precision when using decade resistance boxes as the adjustable arm.

DC excitation is simpler and standard for most resistive sensor applications including strain gauges, RTDs, and thermistors. AC excitation is required for bridges that include capacitors or inductors, such as impedance bridges for component measurement. AC excitation at 50-100 Hz also avoids galvanic effects at electrode-electrolyte interfaces in electrochemical sensing applications, where DC current would cause electrolysis and change the electrode properties. Some precision resistance bridges use AC excitation even for purely resistive elements to take advantage of lock-in amplifier detection techniques that reject noise outside a narrow bandwidth around the excitation frequency, greatly improving sensitivity.

At balance the detector reads zero, and the balance condition R1/R2 = R3/Rx involves only ratios of resistances, not the supply voltage. Doubling the supply doubles the current in both halves of the bridge equally, so the two arm voltages stay matched and the null is unchanged. This makes the Wheatstone bridge a pure ratio measurement that is immune to supply-voltage drift, which is one reason it dominated precision resistance measurement for over 150 years. The supply voltage does matter for sensitivity, however: a higher excitation voltage produces a larger output signal for a given small imbalance, improving resolution up to the point where self-heating of the resistors becomes a problem.