What Is a Wheatstone Bridge? How Does It Work?

A Wheatstone bridge is a classic four-resistor circuit used to precisely measure very small changes in resistance. It's formed by placing two voltage dividers in parallel; by reading the voltage difference at the bridge's midpoints you determine a resistor's value or change.

How It Works: The Balance Condition

The bridge has four arms (R1, R2, R3, R4) and an excitation voltage (Vex) is applied. When the bridge is "balanced" — that is, R1/R2 = R3/R4 — the voltage difference between the midpoints is zero. When one arm (e.g. a sensor) changes, the bridge becomes unbalanced and a small difference voltage appears, proportional to the change in resistance. So the Wheatstone bridge turns tiny changes that are hard to measure directly into a signal that can be amplified "around zero".

Where Is It Used?

• Load cell and strain gauge: Strain gauges measuring force/weight read the resistance change with a bridge (scales, weighing).
• RTD/PT100 and thermistor: To precisely measure resistance that varies with temperature.
• Pressure/torque sensors: Usually built with a bridge output.

Quarter, Half and Full Bridge

Depending on how many arms are active sensors, there are quarter (1 active), half (2 active) and full-bridge (4 active) arrangements. More active arms give higher sensitivity and better temperature compensation. Because the bridge output is very small (mV range), it's usually amplified with an instrumentation amplifier or a bridge ADC (e.g. load-cell ADCs).

Things to Watch Out For

• Excitation stability: The output is proportional to the excitation voltage (ratiometric); supply noise couples into the measurement. A stable reference/supply matters.
• Temperature: With a single active arm, temperature drift causes error; a half/full bridge and same-material arms compensate for it.
• Lead resistance: On long cables the lead resistance couples into the bridge; a 3/4-wire connection or nearby amplification is needed.

Frequently Asked Questions

What is a Wheatstone bridge for?
It's for precisely measuring very small resistance changes (like a strain gauge, RTD); it turns the change into a difference voltage around zero.

What does bridge balance mean?
When R1/R2 = R3/R4 the voltage between the midpoints is zero; this is called balance. When the sensor changes, balance is broken and a readable difference appears.

To amplify the bridge output, a load-cell ADC or an instrumentation amplifier is used.