- 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 bridge circuit is an electrical circuit configuration used to measure unknown values of resistance, impedance, induction, and capacitance. Many types, like the Wheatstone bridge, Maxwell Bridge, and Kelvin Bridge, work on the same principle and measure quantities accurately. Here is a brief description of how some of these bridges function:
Wheatstone Bridge
A Wheatstone bridge is an electrical circuit developed by Charles Wheatstone, used to determine the value of an unknown electrical resistance. It is highly capable of calculating very low resistances, which other instruments like multimeter cannot measure accurately.
The Wheatstone bridge circuit is a diamond-shaped arrangement of four resistors. It has two parallel legs, each with two resistors in series. A third leg connects between the two parallel legs. Among the four resistors, one resistance value is determined by balancing the two legs. The values of resistors R1 and R3 are known, R2 is adjustable, and Rx is to be calculated. The circuit is connected to an electric supply and a galvanometer between terminals D and B. The adjustable resistor is adjusted until the ratio of the resistances in the two branches is equal (R1/ R2) = (R3/Rx) and the galvanometer reads zero, indicating no current flow. The circuit is then balanced, and the unknown resistor value can be easily measured. The reading of R3 indicates the direction of current flow.
Maxwell’s Bridge
The working principle of Maxwell’s inductance bridge is the same as the Wheatstone bridge, with minor modifications. This bridge has four branches, including an unknown inductance (L1), a variable capacitor (C4), four resistors, and a detector instead of a galvanometer. It measures inductance by comparing the unknown value with a standard variable capacitance.
The basic principle of the bridge is to compensate the positive angle phase of the unknown impedance with the negative phase of a capacitance by putting it in opposite branch. By doing so, the potential difference across the detector will become zero and no current will flow through it. The capacitor C4 and resistor R4 are connected in parallel and the value of both are adjusted so that bridge get balanced.
Kelvin Bridge
Kelvin Bridge is another modification of the Wheatstone bridge which is used to measure low resistance in the range of 1mΩ to 1kΩ with great accuracy. For precise measurement of low resistance, high voltage supply and a sensitive galvanometer are required in Kelvin Bridge. While measuring low resistance, the resistance of connecting wires plays an important role. Wheatstone bridge is used which has two additional resistors as shown in the figure. The resistors R1 and R2 are connected to the second set of ratio-arm and constructed four terminal resistors. Here R is unknown and S is the standard resistor. A galvanometer is placed between c and d so that resistance of connecting wire r can be neglected and does not affect the measurement value. Under the balance condition, galvanometer shows zero and no current flows through the circuit. The equation at balance condition is:
Hay’s Bridge Circuit
Hay’s bridge is another variation of Maxwell’s bridge circuit. In Maxwell’s circuit resistance is kept parallel to the capacitor where as, in Hay’s circuit, the resistor is connected in series with the standard capacitor as shown in the figure. It is very useful if the phase angle of inductive impedance is very large, which could be overcome by taking a low resistance in series.
Anderson’s Bridge
Anderson Bridge is modified version of Maxwell’s inductor capacitance bridge. It is mainly used for measuring self-inductance in a coil by using standard capacitor and resistors. The main advantage of this bridge is that it does not require the frequent balancing of the bridge. To balance the bridge by steady current, variable resistance r is adjusted and AC source is replaced by battery and headphone by moving coil galvanometer. Once the bridge is balanced the potential at the terminal D is similar to the potential at E. the flow of current in respective branches are denoted by I1, I2, and I3 as shown in the figure.
Diode Bridge Circuit
It is a bridge circuit having an arrangement of four diodes that gives the same output polarity for every input polarity. Diode bridge circuit which also called bridge rectifier is used where ever there is a need to change alternating current into direct current. It is also used to detect the amplitude of radio signals. When the positive terminal of the input is connected to the upper left and negative to the lower right, the current flows from upper supply terminal to the output flowed by red path and returns back to the lower supply terminal through the blue path as shown in the figure.
