What is a Wheatstone bridge used for?
Measuring an unknown resistance precisely, and reading resistive sensors such as strain gauges, load cells, RTDs and thermistors, where the sensor forms one arm of the bridge.
CIRCUIT DIAGRAM EXAMPLE
Wheatstone Bridge Circuit Diagram and Formula
The Wheatstone bridge measures an unknown resistance by comparing it with known ones. The same circuit sits under strain gauges, load cells and many resistive temperature sensors.
How this circuit works
The bridge is two voltage dividers across the same 5 V battery. R1 and R2 set the voltage at the left midpoint; R3 and Rx set the voltage at the right one. The galvanometer G1 between the midpoints shows any difference.
With these values the left midpoint sits at 5 V × 2.2 kΩ / 3.2 kΩ ≈ 3.44 V. The right midpoint matches it only when the right side divides in the same ratio, which happens when R1 / R2 = R3 / Rx. Then no current flows through G1 and it reads zero.
Rearranged, Rx = R2 × R3 / R1, so this bridge balances at Rx ≈ 1.03 kΩ. In a lab bridge R3 is adjustable: you turn it until the galvanometer reads zero, then work out Rx. The answer depends only on the known resistors, not on the battery voltage or the meter's calibration.
Off balance, the voltage between the midpoints grows with the mismatch; with Rx = 1.1 kΩ it is about 66 mV. Sensor bridges use that output: a strain gauge or RTD changes Rx slightly, and an instrumentation amplifier measures the small difference.
Formulas
- Balance condition
R1 / R2 = R3 / Rx- Unknown resistance
Rx = R2 × R3 / R1 = 2.2 kΩ × 470 Ω / 1 kΩ ≈ 1.03 kΩ- Bridge output
VG = V × (Rx / (R3 + Rx) − R2 / (R1 + R2))Open-circuit voltage between the midpoints. With Rx = 1.1 kΩ: 5 V × (0.701 − 0.688) ≈ 66 mV.
Parts list
| Ref | Part | Value | What it does |
|---|---|---|---|
| B1 | Battery | 5 V | 5 V battery. The balance point does not depend on its voltage. |
| G1 | Meter | — | Galvanometer, or any sensitive meter, between the two midpoints. |
| R1 | Resistor | 1 kΩ | 1 kΩ, top of the left ratio arm. |
| R2 | Resistor | 2.2 kΩ | 2.2 kΩ, bottom of the left ratio arm. |
| R3 | Resistor | 470 Ω | 470 Ω reference arm; adjustable in a lab bridge. |
| R4 | Resistor | Rx | The unknown resistance, Rx. |
Netlist
Recovered from the drawing itself: every pin on each net below is joined by a wire, a junction or a shared supply or ground symbol.
| Net | Connects |
|---|---|
| N1 | B1.-, R4.2, R2.2 |
| N2 | B1.+, R3.1, R1.1 |
| N3 | R3.2, R4.1, G1.2 |
| N4 | R1.2, R2.1, G1.1 |
Common mistakes
- Mixing up the ratio. The balance condition compares the two halves of each side: R1 / R2 on the left with R3 / Rx on the right.
- Connecting the meter between the wrong nodes. It goes between the two midpoints, not across the battery or across one resistor.
- Reading an unbalanced bridge with a low-resistance meter. It draws current and lowers the reading; at balance it makes no difference.
- Ignoring lead resistance when Rx is small. Long leads add to Rx; three-wire and four-wire connections are the usual fix.
Frequently asked questions
What is the balance condition of a Wheatstone bridge?
R1 / R2 = R3 / Rx. At balance both midpoints sit at the same voltage and no current flows through the galvanometer.
Why does the battery voltage not matter at balance?
Both sides divide the same voltage, so changing it moves both midpoints by the same fraction and their difference stays zero.
Draw your own version
Open this circuit in the AI circuit diagram maker and ask for a change in plain English, edit a value, switch to IEC symbols, or export it as SVG, PNG, PDF, a KiCad project or a SPICE netlist. Opening the example and exporting it are free.
Related circuit examples
Inverting op-amp amplifier
A gain of −10 from a 1 kΩ input and a 10 kΩ feedback resistor.
- op-amp
- amplifier
- analog
From circuit diagram to PCB
Open the diagram in ProtoFlow, the free AI PCB design app for Windows, macOS and Linux: pick real parts, lay out and route the board, simulate with SPICE and export Gerbers. Imports and exports KiCad, Altium and EAGLE.
Get for Mac
Free to start. No card required.
