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Principles of Computer Engineering: Lecture 4: The Wheatstone Bridge Overview Introduction to the Wheatstone Bridge Use of the bridge to determine unknown resistances To show that the bridge behaves in a non-linear fashion The Wheatstone Bridge We use an “Ohmmeter” to measure an unknown resistance The heart of the simplest Ohmmeter is a so-called “Wheatstone Bridge” circuit If R1 was a variable resistor, we can adjust it until Vab = 0 The Balanced Wheatstone Bridge When Vab = 0, a special condition occurs: the bridge is said to be “balanced”, i.e. Va = Vb This implies that ig = 0, hence from KCL, i4 = i3 and i2 = i1 Further, from Ohm’s Law & KVL; i4R4 = i2R2 and i3R3 = i1R1 The Wheatstone Bridge continued Hence i1 R1 i3 R3 i2 R2 i4 R4 R3 R1 R2 R4 The Wheatstone Bridge: Example Calculate R1 in a Wheatstone bridge when it is balanced and when R2 = 300Ω, R3 = 200Ω, R4 = 100Ω . R3 R1 R2 R4 Answer: R3 200 R1 R2 300 600 R4 100 Principles of Computer Engineering: Experiment 4: The Wheatstone Bridge Wheatstone Bridge When the bridge is balanced there will be no voltage across the terminals ‘a’ and ‘b’ R1 R3 R2 R4 If all resistors are the same value but R1 increases by δR then output becomes R1 R3 R R 1 v v R1 R2 R3 R4 2R R 2 Procedure Setup bridge circuit on breadboard with three 1k resistors in bridge with resistance box as R1 Use power supply to provide 10V to the bridge Adjust R1 until balance is reached (i.e. Vab = 0) Now vary R1 from 100Ω to 1200Ω to give 12 different outputs up to balance point Plot the graph of R1 vs. Voltage Measured R2 (1kΩ nominal) R3 (1kΩ nominal) R4 (1kΩ nominal) R3 R1 R2 R4 By measured R2, R3, R4, calculate R1 for a balanced bridge. R1 for a balanced bridge Calculated Measured R1 (Ω) Calculated Vout 100 200 300 400 500 600 700 800 900 1000 1100 1200 Measured Vout Graph of Voltage vs. Resistance Summary Set up a Wheatstone Bridge circuit and verify its behaviour for different balance conditions Show that the bridge behaves non-linearly Consider sources of errors in this experiment Put all your results and notes into your logbook! Any questions?