Lab 6
Wheatstone Bridge
Objective Gain practical experience in using a Wheatstone Bridge, a handle on the
precision of our variable resistors, and knowledge of how to improvise using discreet
components in order to improve this precision.
Discussion of Concepts The concept of a Wheatstone Bridge has been discussed in
class, and you should refer to section 2.8 of Hambley. You are going to be given an
unknown resistance to measure, and you are to do it without using an ohmmeter (kind
of). There will be couple of twists in lab, compared to the rosy description in your
textbook. First, you will be limited to using one (1) variable resistor having a nominal
range of 0-100k ohms. Your textbook circuit shows two variable resistances. This is
where the design part of this experiment comes into play. Second, you will find that you
may not be able to adjust the provided variable resistor as finely as you would like. (e.g.,
you might not be able to exactly get your bridge to balance). You will solve both of
these problems by using discrete components (namely: resistors) to augment the given
materials. For example, if you find that you cannot get your initial bridge to balance due
to the size of the other two “known” resistors, you might have to change one of the other
(other than the unknown and the single variable) resistances. If the variable resistance is
too “twitchy” (e.g., you can get -1.0 V and +1.0V but not zero by fiddling with it), you
may need to combine this resistance with series and parallel components to address this
problem. See the preliminary lab section below for some hints on how to do this.
Preliminary Lab Exercise
1. Design and draw a (separate) Wheatstone Bridge circuit that balances with the
following resistances in place of Rx in Figure 2.62 on page 97 of your text.
Rx = 120 Ohm
Rx = 4.7k Ohm
Rx = 10M Ohm
2. Verify your results for part 1 using PSpice
3. Assume that you have a variable resistor having a nominal resistance ranging
from 0 to 100k Ohms. Further assume that you can only adjust this to +/- 5k Ohms. That
is, if you try to set it at 50k Ohms, you may be anywhere between 45k Ohms and 55k
Ohms. Investigate the behavior of this resistance when placed in parallel with a 50k
Ohm resistance. That is, calculate the nominal value (50k in parallel with 50 k) as well
as the two extreme values (45 k in parallel with 50k and 55 k in parallel with 50k).
4 Verify your results for part 3 using PSpice
Procedure
1. You will be given an unknown resistance in lab. Do not measure this resistance
directly at this time.
2. Construct your Wheatston Bridge circuit, using a Voltmeter in place of the
d’Arsonval meter shown in the text. You will be provided with a range of values (aka
“hints”) for the two non-variable resistances in your circuit.
3. Balance your bridge. Keep in mind that you may need to replace discreet components
and/or replace your variable resistance with a series, parallel, or combination circuit
containing the variable resistance. Disconnect the power before making ANY
changes to the circuit. The obvious exception is that you can fiddle with the knob on
the variable resistor while the power is on.
4. Once your bridge is balanced, disconnect the power and replace the voltmeter with
an ammeter. Verify that the bridge is still balanced (with the power back on), making
minute adjustments as necessary.
5. Disconnect the power, and use an ohmmeter to measure the actual value of your
discreet components (except your unknown Rx), including both the variable resistance
and the total resistance of the combination (if any) containing the variable resistance.
Recall that each resistance should be isolated from the circuit for this measurement to
mean anything. (a little bit of thought when building/designing your circuit might make
this step easier, especially regarding any combination involving the variable resistance)
6. Calculate the value of your unknown resistance, Rx.
7. Finally, measure Rx with your ohmmeter. Have the TA initial your lab results.
Analysis
Write up your results, including the design process that went into selecting your
values. Assume that all values obtained by your ohmmeter are correct, and calculate
percent error between your calculated value for Rx and the value measured directly.
(Note: In a real world application we would have precision decade boxes (expensive
variable resistors) and known resistances, and would rely on their nominal values being
correct. It obviously makes no sense to trust the ohmmeter to measure 3 resistances and
assume that a calculation involving these three numbers is more accurate than measuring
Rx directly. Keep this in mind when discussing the practicality aspects of this lab.)