Wheatstone Bridge
What is a Wheatstone Bridge?
The Wheatstone Bridge is a classic circuit used to measure an unknown resistance by comparing voltage levels in two voltage divider networks.
It works by balancing two branches of resistors.
When the bridge is balanced, the voltage difference between the center nodes is zero.
That means:
VA = VB
When this happens, the ratio of the resistors on one side equals the ratio on the other side.
How the Bridge Works
Each side of the bridge acts like a voltage divider.
The node voltages are determined by:
VA = Vs × (R2 / (R1 + R2))
VB = Vs × (R4 / (R3 + R4))
When VA equals VB, the bridge is balanced and:
R1 / R2 = R3 / R4
This lets us solve for an unknown resistor R4 using ratios instead of direct measurement.
Watch the Video
Why This Circuit Is Useful
The Wheatstone Bridge is still used today in:
• Strain gauges
• Temperature sensors
• Pressure sensors
• Instrumentation circuits
It’s extremely sensitive and can detect very small resistance changes.
It’s extremely sensitive and can detect very small resistance changes.
Lab Demonstration
In the video above, we built a Wheatstone Bridge and measured the node voltages to determine when the bridge was balanced.
When the voltage difference between the center nodes reached zero volts, we knew the resistor ratios matched.
This confirmed the theory and showed how the bridge can be used to determine an unknown resistance.
Lab Calculations – Wheatstone Bridge
In this lab, we built a Wheatstone Bridge powered by a 5-volt source.
Resistor values used:
R1 = 4.7kΩ
R2 = 10kΩ
R3 = 4.7kΩ
R4 = 10kΩ
Step 1 — Total resistance of each branch
Left branch resistance:
R_left = R1 + R2
R_left = 4.7k + 10k
R_left = 14.7kΩ
Right branch resistance:
R_right = R3 + R4
R_right = 4.7k + 10k
R_right = 14.7kΩ
Both sides are equal, so we expect the bridge to balance.
Step 2 — Current in each branch
Using Ohm’s Law:
I = V / R
I_left = 5V / 14.7kΩ
I_left ≈ 0.00034 A
I_left ≈ 0.34 mA
I_right = 5V / 14.7kΩ
I_right ≈ 0.34 mA
Both currents match, which is what we expect in a balanced bridge.
Step 3 — Node voltage at point A
Point A is between R1 and R2.
This acts like a voltage divider.
VA = Vs × (R2 / (R1 + R2))
VA = 5 × (10 / 14.7)
VA ≈ 3.40 V
Step 4 — Node voltage at point B
Point B is between R3 and R4.
VB = Vs × (R4 / (R3 + R4))
VB = 5 × (10 / 14.7)
VB ≈ 3.40 V
Step 5 — Meter reading
Since VA ≈ VB:
V_meter = VA − VB
V_meter ≈ 0 V
This confirms the bridge is balanced.
Key takeaway
When the ratios of the resistors on each side match, the midpoint voltages match, and the bridge reads zero volts.
This is how the Wheatstone Bridge can be used to measure an unknown resistance with high accuracy.
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