Wheatstone Bridge In Action
To test my Wheatstone bridge, I had one of my colleagues pull a random
resistor from my spare parts box, and paint it over, so I couldn’t read the
stripes.
I built a Wheatstone bridge using a 9V battery, and 100K resistors for R
and R . The unknown resistor was inserted as R , and a decade box was
inserted as R . A digital voltmeter was inserted between points A and B, the
centers of the dividers.
The value of the decade box was adjusted until the voltage displayed on
the meter was as close to 0V as could be reached. In this case, the meter
was flickering between 0.000 and -0.001. At that point, the value of the
decade box matched the value of the unknown resistor. As you can see, we
measured 9,960
Ω
. The mystery resistor was probably a 10K
Ω
, +/- 5%
tolerance resistor.
In practical terms, it would have been easier to just use the digital
multimeter to read the resistor directly, but Wheatstone bridges have uses
in certain types of instrumentation - they can be configured to observe very
small changes in resistance, such as the output of a strain gauge. They’re
also known to torment engineering students in circuit class laboratories!
Experimental Circuit
Since portable digital instruments have mostly superseded traditional
techniques like the Wheatstone bridge, lets explore a more practical
application on the designer’s workbench.
Variable Gain Amplifier
1
3
x
2
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