An instructor is demonstrating a Wheatstone bridge circuit t…
An instructor is demonstrating a Wheatstone bridge circuit to a biomedical instrumentation class. The bridge has four arms (with resistors R1 to R4) and is powered by a DC supply. When all four resistors are equal, the voltmeter reads zero. The instructor then replaces R1 with a strain gage bonded to a test beam and presses on the beam, causing the voltmeter to give a reading. How does replacing one fixed resistor with a strain gage in a Wheatstone bridge allow the circuit to detect small resistance changes? The bridge amplifies the supply voltage in proportion to the gauge resistance, giving a larger absolute output. The bridge stores charge until enough resistance change accumulates, then releases it as a pulse proportional to total displacement. Starting from a perfectly balanced (zero-output) condition, any tiny resistance change in the gage arm immediately creates an imbalance voltage Vab that is directly related to ΔR, allowing small displacements to be detected. The other three fixed resistors act as reference standards that calibrate the gauge reading automatically on each measurement.