What is a Bridge?Bridge Circuit is a simplest form consists of a network of four resistancearms Forming a closed circuit. A source of current is applied to two oppositejunctions. The current detector is connected to other two junctions.

What is the use of Bridge circuit?Bridge circuits are used for measuring the unknown Resistance, Inductance, Capacitance, Frequency etc.

D.C Bridges A.C Bridges

• Megaohm Bridge • Anderson Bridge

• Hay Bridge

• Maxwell Bridge

• DeSauty Bridge

• Schering Bridge• Wien Bridge

Inductance Capacitance Frequency

• Kelvin Bridge

Resistance

• Introduction

• Maxwell’s Inductance Bridge

• Maxwell’s Inductance Capacitance Bridge

• Advantages Of Maxwell bridge

• Limitations of Maxwell Bridge

Maxwell’s bridge can be used to measure inductance by

comparison either with a variable standard self inductance

or with a standard variable capacitance. These two

measurements can be done by Maxwell’s bridge in two

different forms.

1) Maxwell’s Inductance Bridge

2) Maxwell’s Inductance Capacitance Bridge

MAXWELL’S INDUCTANCE BRIDGE

Using this bridge, we can measure inductance by comparing it with a variable standard self inductance arranged in bridge circuit

Lx = R3/R1*L3

Rx = R2/R1*(R3+r)

Maxwell’s Inductance CapacitanceBridge

Using this bridge, we can measure inductance by comparing with a variable standard capacitor. The circuit diagram is as shown

Rx = R2*R3/R1

Lx = R2*R3*C1

The quality factor of the coil is given by:-

Q = R1*C1

• The balance equation is independent of losses associated with inductance

• The balance equation is independent of frequency of measurement

• The scale of resistance can be calibrated to read the inductance directly

• The scale of R1 can be calibrated to read the Q value directly

Advantages of Maxwell Bridge

Limitation of Maxwell Bridge

1)The variable standard capacitor is very expensive.

(2) The bridge is limited to measurement of low quality coils (1 < Q < 10) and it is also unsuitable for low value of Q.

De Sauty Bridge

• Introduction

• Advantages of Desauty bridge

• Limitation of Desauty Bridge

Introduction• i)A four-arm bridge used to compare two capacitances; two adjacent arms

contain capacitors in series with resistors, while the other two arms contain resistors only. Also known as Wien-DeSauty bridge.

• ii)De Sauty Bridge measures an unknown capacitance in term of a standard capacitance i.e. comparing two capacitance’s Two ratio arm of this bridge consist pure resistor and two consist capacitor where one is of known value and another is standard capacitor.

Let,

C1 = Capacitor whose capacitance is to be measured

C2 = a standard capacitor

R1, R2 = non-inductive resistors

Balance is obtained by varying either R1 or R2. For balance, point B and D are at the same potential.

C1 = C2*R2

R1

The balance of the condition can be achieved by varying either R1 or R2.

2 2

1

Advantage’s and Disadvantage’sOf DeSauty Bridge

• Advantages : the advantage of this bridge is its simplicity to obtain a perfect balance.

• Disadvantages : a perfect balance can be obtained, only if both the capacitors are free from dielectric losses i.e. when air capacitors are used.

Schering Bridge

• Introduction

• Advantages of schering Bridge

IntroductionIt is one of the most widely used a.c bridges for the measurement of unknown capacitors, dielectric loss and power factor.

As shown in the fig C1 is perfect capacitor to be

measured. R1 is series resistance, C2 is standard

capacitor having very stable value. R3 and R4 are

nonconductive resistances while C4 is variable

capacitor.

Equation of balanced Schering bridge:-

Advantage of Schering bridge

• The balance equation is independent of frequency.

• It is used for measuring the insulating properties of electrical cablesand equipment's.

• It can measure small capacitors at low voltages precisely.