Wheatstone Bridge Calculator
Calculate unknown resistance in a Wheatstone bridge from three known resistances. Enter values for instant results with step-by-step formulas.
Formula
Rx = (R2 x R3) / R1 at balance (galvanometer reads zero)
At balance, the voltage across the galvanometer is zero, meaning R1/R3 = R2/Rx. Solving for Rx gives Rx = (R2 x R3) / R1. The bridge voltage for an unbalanced bridge is Vg = Vs x [R3/(R1+R3) - R4/(R2+R4)].
Worked Examples
Example 1: Finding Unknown Resistance
Problem: In a Wheatstone bridge, R1 = 100 ohms, R2 = 200 ohms, R3 = 150 ohms. Find the unknown resistance Rx for balance with 10V supply.
Solution: Balance condition: R1/R3 = R2/Rx\nRx = (R2 x R3) / R1\nRx = (200 x 150) / 100\nRx = 30,000 / 100 = 300 ohms\nVerification: R1/R3 = 100/150 = 0.667\nR2/Rx = 200/300 = 0.667 (balanced)\nBridge voltage at balance = 0V
Result: Unknown Rx = 300 ohms | Bridge is balanced | Galvanometer reads 0V
Example 2: Unbalanced Bridge Voltage
Problem: A bridge has R1 = 120 ohms, R2 = 120 ohms, R3 = 100 ohms, R4 = 105 ohms with 5V supply. Find the bridge output voltage and deviation.
Solution: Balanced R4 would be: (120 x 100) / 120 = 100 ohms\nActual R4 = 105 ohms (5% deviation)\nVoltage at node A = 5 x 100 / (120 + 100) = 2.273V\nVoltage at node B = 5 x 105 / (120 + 105) = 2.333V\nBridge voltage = 2.273 - 2.333 = -0.060V = -60mV
Result: Bridge output: -60mV | 5% deviation from balance | R4 should be 100 ohms
Frequently Asked Questions
Why is the Wheatstone bridge so accurate for measuring resistance?
The Wheatstone bridge achieves exceptional accuracy because it is a null measurement method, meaning it detects the absence of current rather than measuring a current magnitude. At balance, the result depends only on the ratios of known resistors and is independent of the supply voltage, galvanometer sensitivity, and lead resistance. This makes it immune to many error sources that affect direct measurement methods. Precision Wheatstone bridges can measure resistance to accuracies of 0.01 percent or better. The accuracy is limited primarily by the precision of the three known resistors and the sensitivity of the null detector to detect small imbalances.
What are common applications of Wheatstone bridge circuits?
Wheatstone bridges are used extensively in sensor and measurement applications. Strain gauges use Wheatstone bridge configurations to measure tiny changes in resistance caused by mechanical deformation, enabling force, pressure, and weight measurements. Resistance temperature detectors (RTDs) use bridge circuits to convert small temperature-dependent resistance changes into measurable voltage signals. Load cells in scales and weighing systems are essentially Wheatstone bridges with four strain gauges. Bridges are also used in gas detection sensors, humidity sensors, and precision laboratory instruments for calibrating resistors and measuring unknown resistances with high accuracy.
What happens when a Wheatstone bridge is unbalanced?
When a Wheatstone bridge is unbalanced, a voltage difference appears across the galvanometer terminals, causing current to flow through the detector. The magnitude and direction of this voltage indicates how far the unknown resistance is from the balanced value and in which direction. In sensor applications, this unbalanced voltage is the useful output signal that varies with the measured quantity. The relationship between bridge output voltage and resistance change is approximately linear for small deviations from balance but becomes nonlinear for large changes. Signal conditioning electronics amplify and linearize this output for practical measurement systems.
What is bridge sensitivity and how is it maximized?
Bridge sensitivity is defined as the change in galvanometer voltage per unit change in the unknown resistance, expressed in millivolts per ohm. Maximum sensitivity occurs when all four arms have equal resistance at balance. Increasing the supply voltage increases sensitivity linearly but is limited by power dissipation in the resistors. For strain gauge bridges with very small resistance changes (typically 0.1 percent), high excitation voltages of 5 to 10 volts and high-gain amplifiers are needed to produce usable output signals. Temperature changes in the resistors can create false signals that mask the real measurement, so temperature compensation techniques are essential for high-sensitivity applications.
What is the difference between a quarter, half, and full Wheatstone bridge?
A quarter bridge uses one active sensing element (like a strain gauge) as one arm of the bridge, with the other three arms being fixed precision resistors. This is the simplest configuration but provides the least sensitivity and requires temperature compensation. A half bridge uses two active elements in adjacent or opposite arms, doubling the sensitivity and providing some temperature compensation. A full bridge uses four active elements, one in each arm, providing maximum sensitivity (four times a quarter bridge) and excellent temperature compensation. Full bridges are standard in load cells and pressure transducers where maximum accuracy and stability are required.
How does lead wire resistance affect Wheatstone bridge measurements?
Lead wire resistance adds to the resistance of the arm it connects to, causing measurement errors. For a remote sensor connected by long cables, the two lead wires might add 1 to 10 ohms to a 100-ohm sensor, creating a 1 to 10 percent error. Three-wire connections solve this problem by routing one lead wire into each side of one bridge arm, so the lead resistance is split equally and cancels at balance. Four-wire (Kelvin) connections eliminate lead resistance effects entirely by using separate pairs of wires for current excitation and voltage sensing. For precision measurements with long cable runs, three-wire or four-wire connections are essential.