Cairo University
3rd Year Mechanical
Faculty of Engineering
1st Term 2025/2026
Mech. Power. Dept.
MEP3030
AC&DC Bridges
Report
Name: Mina Ehab Abass
ID: 9230940
Sec: 4
BN: 18
Under supervision of: Dr. Tharwat Wazier Abou Arab
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1. Introduction
Electrical bridges are fundamental circuits used to measure unknown
electrical quantities with high accuracy. Their operation relies on
balancing two sections of the circuit, which allows engineers to
determine values such as resistance, inductance, capacitance, and
overall impedance.
DC bridges are mainly employed to measure pure resistances under
steady (constant) conditions, whereas AC bridges apply the same
balancing principle to alternating-current systems, making it possible
to measure complex impedances.
For engineering students, mastering these bridge circuits is essential
for precise instrumentation, reliable circuit analysis, and effective
calibration in electrical and electronic applications.
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2. DC Bridges
2.1 Wheatstone Bridge
•
Used for precise measurement of unknown resistance.
•
Operates when the bridge reaches a
balanced condition, meaning no current
flows through the galvanometer.
•
Balance equation:
( R1 / R2 = Ra / Rb)
•
Applications: sensor measurements .
2.2 Kelvin Double Bridge
•
Used for very low-resistance
measurements.
•
Employs additional ratio arms to
compensate for stray resistances.
•
Common in precision testing of
transformer windings and conducting
materials.
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3. AC Bridges
3.1 Maxwell Bridge
•
Measures unknown inductance using a
known capacitance.
•
Balancing occurs when real and imaginary
parts of impedances are equal.
•
Suitable for coils with medium-quality
3.2 Schering Bridge
•
Used to measure capacitance and dielectric loss in
insulating materials.
•
Widely used in testing cables, capacitors, and
insulation systems.
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4. Comparison Between AC and DC Bridges
Parameters
DC Bridges
AC Bridges
Excitation
DC Source
AC Source
Measured Quantities
Resistance only
R, L, C, frequency
Balance Condition
Magnitude only
Magnitude and phase
Detector
Galvanometer
Vibration
galvanometer/headphones
5. Applications in Engineering
•
Calibration of sensors (RTDs, strain gauges).
•
Measurement of material properties (dielectric constant, loss
factor).
•
Fault detection in cables and electrical equipment.
•
Filter and oscillator design in communication systems.
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6. Conclusion
AC and DC bridges are considered among the most precise and reliable
techniques for measuring electrical quantities in both laboratory and
industrial applications.
DC bridges offer a simple and highly accurate method for determining
resistance, while AC bridges extend these capabilities to the
measurement of complex impedance—making them essential tools in
modern electronics, power systems, and communication engineering.
Understanding their operating principles, balance conditions, and
practical uses provides engineers with the analytical and measurement
skills needed to work effectively with real-world electrical systems.
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