1. Principles of Computer Engineering: Lecture 4: The Wheatstone Bridge

2. 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

3. 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

4. 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

5. The Wheatstone Bridge continued
Hence

6. The Wheatstone Bridge: Example
Calculate R1 in a Wheatstone bridge when it is balanced and when R2 = 300Ω, R3 = 200Ω, R4 = 100Ω .
Answer:

7. Principles of Computer Engineering: Experiment 4: The Wheatstone Bridge

8. Wheatstone Bridge
When the bridge is balanced there will be no voltage across the terminals ‘a’ and ‘b’
If all resistors are the same value but R1 increases by δR then output becomes

9. 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

10. Measured R2 (1kΩ nominal) R3 (1kΩ nominal) R4 (1kΩ nominal)
By measured R2, R3, R4, calculate R1 for a balanced bridge.
R1 for a balanced bridge
Calculated
Measured

11. R1 (Ω)
Caculated Vout
Measured Vout
100
200
300
400
500
600
700
800
900
1000
1100
1200

12. Graph of Voltage vs. Resistance

13. 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?