ENGINEERING-43
Wheatstone Bridge
Lab-07
Lab Data Sheet – ENGR-43 Lab-07
Lab Logistics
Experimenter:
Recorder:
Date:
Equipment Used (maker, model, and serial no. if available)
Directions
1. Check out a DMM and Power/Probe Leads for the Power-Supply and DMM
2. Go to the side counter, collect resistors, “bread board”, and leads required to construct the
circuit shown in Figure 1.
3. Make the Measurements and Calculations needed to complete Table I and Table II.
 Use Vs as the baseline for the %-Unbalanced calculation
4. Return all lab hardware to the “as-found” condition
The Wheatstone Bridge Circuit
Due to their outstanding sensitivity, Wheatstone Bridge Circuits are very advantageous for the
measurement of resistance, inductance, and capacitance. Wheatstone bridges are widely used
for strain measurements. A Quarter Bridge Wheatstone bridge is shown in Figure 1.
This circuit consists of 4 resistors arranged in a diamond orientation. An input DC voltage, or
excitation voltage, is applied between the top and bottom of the diamond and the output
voltage is measured across the middle. When the output voltage, VAB = VA – VB, is zero, the
bridge is said to be balanced. One or more of the legs of the bridge may be a resistive
Bruce Mayer, PE • Chabot College • 282217658 • Page 1
transducer, such as a strain gage. The other legs of the bridge are simply completion resistors
with resistance equal to that of the unknown resistance (such as strain gages). As the
resistance of one of the legs changes, by a change in strain from a resistive strain gage for
example, the previously balanced bridge is now unbalanced. This unbalance causes a voltage
to appear across the middle of the bridge. This induced voltage may be measured with a
voltmeter or the resistor in the opposite leg may be adjusted to rebalance the bridge. In either
case the change in resistance that caused the induced voltage may be measured and
converted to obtain the engineering units appropriate to the variable-resistance transducer.
For the circuit of Figure 1 the output, VAB, may be calculated by
 R2
R4 
VAB  VA  VB  VS 


R

R
R

R
2
3
4
 1
Then the %-Unbalanced
%  100 VAB VS
EXTRA CREDIT – 5 Lab-Points
Derive the bridge output equation shown above
 Start with Diagram of Figure 1
 Derivation must proceed in a logical, step-by-step fashion
Bruce Mayer, PE • Chabot College • 282217658 • Page 2
Figure 1 • The Wheatstone Bridge. Vs = 12.00 Vdc. R1 = R3 = 1-3 kΩ. R2 = R4 = 2-6 kΩ.
R1 and R3 Must have the SAME nominal value. R2 and R4 Must have the SAME nominal
Value. The R2:R1 resistance ratio must be >1.4:1
Bruce Mayer, PE • Chabot College • 282217658 • Page 3
 Actual Values
Vs =
R1 =
R2 =
R3 =
R4 =
Table I – Potential Measurements and Calculations
Value Determination
VA
VB
VAB
%-Unbalanced
Calculated
Measured
Now Swap the position of the R1-R3 and R2-R4 resistor pairs; i.e., the resistors that were on
the bottom of the diamond are moved to the top, and vice-versa. Again perform the
calculations and measurements
Table II – Potential Measurements and Calculations
Value Determination
VA
VB
VAB
Calculated
Measured
Run Notes/Comments
Bruce Mayer, PE • Chabot College • 282217658 • Page 4
%-Unbalanced