BRIDGE CIRCUITS

Bridge Circuit is a null method, operates on the principle of comparison. That is a

known (standard) value is adjusted until it isequal to the unknown value.

CONTENTS

• Classification of bridges• Wheatstone bridge• Kelvin double bridge• Mega ohm bridge• Schering bridge

CLASSIFICATION OF BRIDGE CIRCUITS

AC Bridge

DC bridge (Resistance) Inductance capacitance frequency1. Wheatstone bridge 1. Maxwell bridge 1.Schering bridge 1.wein 2. Kelvin double bridge 3. Mega ohm bridge 2. Hay bridge bridge

Wheatstone bridgeThe Wheatstone Bridge was originally developed by Charles Wheatstone to measure unknown resistance

values and as a means of calibrating measuring instruments, voltmeters, ammeters, etc, by the use of

a long resistive slide wire. Although today digital multimeters provide the simplest way to measure a

resistance, The Wheatstone Bridge can still be used to measure very low values of resistances down in the

milli-Ohms range.

Suitable for moderate resistance values: 1 Ω to 10 MΩ

Balance condition: No potential difference across the galvanometer (there is no current through the

galvanometer)

By replacing R4 above with a resistance of known or unknown value in the sensing arm of the Wheatstone bridge corresponding to RX and adjusting the opposing resistor, R3 to “balance” the bridge network, will result in a zero voltage output. Then we can see that balance occurs when R1/R3 = R2/Rx = 1 (BALANCED)

The Wheatstone Bridge equation required to give the value of the unknown resistance, RX at balance is given as:

Where resistors, R1 and R2 are known or preset values

APPLICATIONS OF WHEATSTONES BRIDGE

• The Wheatstone bridge is used for measuring the very low resistance values precisely.

• Wheatstone bridge along with operational amplifier is used to measure the physical parameters like temperature, strain, light, etc.

• We can also measure the quantities capacitance, inductance and impedance using the variations on the Wheatstone bridge.

ADVANTAGES-DISADVANTAGES OF WHEATSTONE BRIDGE

1)It shows a null measurement (i.e.-potential difference across the end points of the

bridge is zero)2)since it is an arrangement of 4 resistors ,it is useful to

find one of them in terms of other three. 3) Te only disadvantage is that the wheatstone bridge is

not applicable forhigh resistances.

KELVIN DOUBLE BRIDGESuitable for resistance values: 1 to 0.00001 ΩWhy it is called double bridge?? it is because It

incorporates the second set of ratio arms as shown below:

The Kelvin double Bridge is a modified Wheatstone bridge and provides high accuracy especially in the measurement of low resistance.

In this the ratio arms p and q are used to connect the galvanometer at the correct point between j and k to remove the effect of connecting lead of electrical resistance t. Under balance condition voltage drop between a and b (i.e. E) is equal to F (voltage drop between a and c)

For zero galvanometer deflection, E = F

Again we reaches to the same result i.e. t has no effect. However equation (2) is useful as it gives error when,

• The only Application of kelvin double bridge is to measure unknown resistance.

Advantages-Disadvantages

• Errors owing to contact resistance, resistance of leads can be eliminated by using this Kelvin double bridge

• The disadvantages are it Requires manual balancing

• Sensitive null detector or galvanometer is required to detect balance condition

• Measurement current needs to be reasonably high to achieve sufficient sensitivity.

Mega ohm Bridge Mega ohm bridge is another important method for measurement of high resistances. It has one

three terminal high resistance located in one arm of the bridge.

Megaohm bridge three terminal resistance

Fig shows the very high resistance with terminals A and B, and a guard terminal, which is put on the insulation. So it forms a three terminal resistance.

The arrangement of above figure illustrated the operation of Megaohm Bridge.

Figure shows the circuit of the completely elf-contained Megaohm Bridge which includes power supplies, bridge

members, amplifiers, and indicating instrument

It has range from 0.1MΩ to 10^6MΩ. The accuracy is within 3% for the lower part of the range to

possible 10% above 10000MΩ. Sensitivity of balancing against high resistance is obtained by use of adjustable high voltage

supplies of 500V or 1000V and the use of a sensitive null indicating arrangement such as a high gain amplifier with an

electronic voltmeter or a C.R.O. The dial on Q is calibrated 1-10-100-1000 MΩ, with main decade 1-10 occupying grater part of the dial space. Since unknown resistance R=PS/Q, the arm Q is

made, tapered, so that the dial calibration is approximately logarithmic in the main decade, 1-10. Arm S give five multipliers,

0.1,1,10,100 and 1000.The junction of ratio arms P and Q is brought on the main panel

and is designated as ‘Guard’ terminal.

CAPACITANCE MEASURMENT

•SCHERING BRIDGE

SCHERING BRIDGE

This bridge is used to measure to the capacitance of the capacitor,

dissipation factor and measurement of relative permittivity.

The connections of the Schering bridge under balance conditions are shown in figure below.

Now when the Schering Bridge is balanced, then

By equating real and imaginary part of the equation we get,

Dissipation Factor

•Dissipation factor tells us about the quality of a capacitor, how close thephase angle of the capacitor is to the

ideal value of 900.

The dissipation factor is given by:

Advantages Of Schering Bridge:1) The balance equation is independent of

frequency. 2) It is used for measuring the insulating

properties of electrical cables and equipments.