What is a Wheatstone bridge?

A Wheatstone bridge is a four-resistor circuit that measures the voltage difference between two divider midpoints. It can determine an unknown resistance by balancing the circuit, or turn a small sensor resistance change into an electrical output. It is a widely used arrangement for strain gauges and other resistive sensing elements.

Think of it as two voltage dividers

The left branch has R1 above R2; the right branch has R3 above R4. Both branches share the same excitation supply. A is the left midpoint and B is the right midpoint. With our chosen polarity, Vout = VA − VB. Equal midpoint voltages give zero output; unequal midpoint voltages give a signed differential output.

A traditional bridge uses a galvanometer between A and B to find a zero-current condition. Modern systems commonly use a high-impedance voltmeter, a differential amplifier or a suitable measurement front end. The divider behaviour follows from Ohm’s law.

Watch what happens when R4 changes

In the demonstration, excitation is 5 V and R1 = R2 = R3 = 1,000 Ω. Select Start to cycle through three R4 values. Each step shows the changed resistor, node B voltage and resulting output polarity together. Stop / reset returns the display to the balanced state.

R4 = 1,000 Ω: VA and VB both equal 2.500 V, giving 0 V output. R4 = 1,100 Ω: VB rises to about 2.619 V and the output becomes approximately −119 mV. R4 = 900 Ω: VB falls to about 2.368 V and the output becomes approximately +132 mV. Values are rounded and assume negligible loading by the measurement input.

Balance does not stop the supply current

At balance, no current flows through the galvanometer branch because A and B are at the same potential. Current still flows through both resistor branches. A zero bridge output therefore does not mean the circuit has stopped drawing power.

With four 1 kΩ resistors and 5 V excitation, each branch contains 2 kΩ. The left branch draws 2.5 mA and the right branch draws 2.5 mA, giving 5 mA total supply current. Meanwhile, the bridge output remains 0 V. The null condition applies to the difference between A and B.

Two measurement methods

Finding an unknown resistor: replace R4 with Rx, adjust a known arm and bring the A-to-B reading to zero. For this arrangement, balance requires R1 × Rx = R2 × R3, giving Rx = (R2 × R3) / R1. Equal resistor values are not required. The bridge formula guide includes the derivation and worked examples.

Reading a sensor: establish a baseline, then continuously measure the imbalance created when force, pressure or temperature changes one or more resistances. The circuit does not need to be rebalanced for every reading. Its unloaded output is Vout = Vs × [R2 / (R1 + R2) − R4 / (R3 + R4)]. See the bridge voltage calculation guide for numerical examples.

Where does it appear in industry?

Force and weighing: a resistive load cell uses strain-gauge elements to convert mechanical deformation into resistance changes. The bridge produces a differential output from those changes. Understanding how a load cell works links that electrical measurement to a weight reading, which requires calibration.

Pressure: a pressure sensor with resistive sensing elements can detect diaphragm deformation through bridge imbalance. Temperature: resistance-based sensors such as Pt100 RTDs and NTC thermistors can be measured in a bridge circuit. Not every pressure or temperature sensor uses this arrangement; modern RTD inputs may use other measurement methods.

Quarter, half and full bridges

A quarter bridge has one active element, a half bridge has two and a full bridge has four. An active element responds to the quantity being measured, such as a strain gauge changing resistance under force. The other arms complete the circuit; some configurations add temperature-compensating elements. A quarter bridge is not a complete circuit made from just one resistor. All four arms must still be present.

Why is it useful for small signals?

The measurement electronics can measure and amplify the small difference directly, rather than separately measuring two larger node voltages and subtracting the readings. Accuracy still depends on resistor tolerances, lead resistance, temperature, amplifier errors and noise. When checking resistance with a multimeter, disconnect excitation and account for other circuit paths.

A millivolt analog voltage needs an appropriate differential analog input or bridge measurement front end. Both input nodes may sit near 2.5 V while the useful analog signal between them is only a few millivolts. Check both the differential measurement capability and the common-mode input range.

Technical references: NI — Measuring Strain with Strain Gages · Analog Devices — Resistive Bridge Basics · Texas Instruments — A Basic Guide to Bridge Measurements.