- AC Circuit Definition: An AC circuit is defined as an electrical circuit that uses alternating current (AC), where the current changes direction periodically.
- Components: AC circuits typically include resistors, capacitors, inductors, and transformers, each playing a specific role.
- Impedance: Impedance in an AC circuit combines resistance and reactance, affecting the total opposition to current flow.
- Series and Parallel Circuits: AC circuits can be configured in series or parallel arrangements, impacting how components interact with the voltage source.
- Phase Difference: Voltage and current in an AC circuit can have a phase difference, leading to variations in when they reach their peak values.
A measurement bridge is an electrical circuit that compares an unknown resistance or impedance with known standards. DC bridges measure resistance. AC bridges balance both magnitude and phase to determine inductance, capacitance or loss. The Wheatstone bridge, Maxwell bridge and Kelvin Bridge all use a null detector, but their connections and error controls differ. A diode bridge is also included below, although it is a rectifier rather than a measurement bridge.
Wheatstone Bridge
A Wheatstone bridge determines an unknown electrical resistance by ratio comparison. Samuel Hunter Christie first described the arrangement, and Charles Wheatstone later popularised it. It can provide high accuracy near its designed range, but precision very-low-resistance work normally uses a four-terminal or Kelvin method rather than a simple bridge or multimeter connection.
Four resistors form two series branches across a source, and a detector connects between their midpoints. In the notation shown, R1 and R3 are known, R2 is adjustable and Rx is unknown. Balance occurs when the midpoint potentials are equal. The ratio of the resistances then satisfies (R1/R2) = (R3/Rx). No current flows through the detector at null, although source current still flows in the two bridge branches. The ratio equation gives the unknown value.
Maxwell’s Bridge
Maxwell’s inductance-capacitance bridge is an AC null bridge. One arm contains the unknown coil resistance and inductance L1. The opposite standard arm contains a known capacitor C4 with a resistor, and the remaining arms set resistance ratios. An AC-sensitive detector replaces the DC galvanometer.
The capacitive standard supplies a phase angle opposite to the inductive unknown. At balance, the detector voltage is zero and the real and imaginary parts of the bridge equation are satisfied separately. In the version shown, capacitor C4 and resistor R4 are parallel. Adjusting the standard and ratio controls determines both coil resistance and inductance. This form is generally suited to medium-Q coils.
Kelvin Bridge
A Kelvin Bridge, also called a Kelvin double bridge, compares a low unknown with a low standard while reducing lead and contact error. For low resistance, the connection drop may be comparable with the quantity being measured. A high voltage is not a general requirement; test current is chosen to produce detectable voltage without excessive self-heating. The second ratio-arm pair R1 and R2, four-terminal connections and correct link arrangement keep lead resistance out of the result when the ratios match. R is the unknown and S is the standard. At balance, the detector between c and d carries no current, while measurement current still passes through R, S and their link. The balance equation is:
Hay’s Bridge Circuit
Hay’s bridge places a resistor in series with the standard capacitor rather than in parallel as in the common Maxwell form. This arrangement is suited to high-Q inductors, where the coil’s impedance angle is close to 90 degrees. Balance gives the coil resistance and inductance from the standard capacitor, series resistance and ratio arms.
Anderson’s Bridge
An Anderson bridge extends the Maxwell’s inductor capacitance bridge with an additional branch and a fixed standard capacitor. It measures coil resistance and self-inductance over a broad range, but its extra arms make balancing and calculation more involved. Some procedures first use a battery and DC detector to set a resistive balance, then apply AC and adjust the remaining control for null. At balance, terminals D and E have equal potential. Branch currents I1, I2 and I3 follow the directions shown in the figure.
Diode Bridge Circuit
A four-diode Diode bridge is a full-wave rectifier, not a null-measurement bridge. During one input half-cycle, one diagonal pair conducts. During the opposite half-cycle, the other pair conducts. Load current therefore keeps the same direction, producing pulsating DC. Practical design must include diode forward drop, reverse-voltage rating, current rating, surge current, thermal loss and any required smoothing or regulation.
