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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
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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
44
33
22
11
RiRi
RiRi
24
31 RRRR
The Wheatstone Bridge: Example Calculate R1 in a Wheatstone bridge when it is balanced and
when R2 = 300Ω, R3 = 200Ω, R4 = 100Ω .
Answer:
600300100200
24
31 RRRR
24
31 RRRR
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’
If all resistors are the same value but R1 increases by δR then output becomes
4
3
2
1
RR
RR
2
1243
3
21
1
RRRRv
RRR
RRRv
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
MeasuredR2 (1kΩ nominal)R3 (1kΩ nominal)R4 (1kΩ nominal)
By measured R2, R3, R4, calculate R1 for a balanced bridge.
24
31 RRRR
R1 for a balanced bridge
Calculated Measured
R1 (Ω) Caculated Vout Measured Vout
100200300400500600700800900
100011001200
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?