bridge circuit consists of dc bridges and ac bridges

1. BRIDGE CIRCUITS
Bridge Circuit is a null method,
operates on the principle of
comparison. That is a known
(standard) value is adjusted until it is
equal to the unknown value.

2. CONTENTS
• Classification of bridges
• Wheatstone bridge
• Kelvin double bridge
• Mega ohm bridge
• Schering bridge

3. CLASSIFICATION OF BRIDGE CIRCUITS
AC Bridge
DC bridge (Resistance) Inductance capacitance frequency
1. Wheatstone bridge 1. Maxwell bridge 1.Schering bridge 1.wein
2. Kelvin double bridge
3. Mega ohm bridge 2. Hay bridge bridge

4. Wheatstone bridge
The 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.

5. 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)

6. 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)

7. 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

8. 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.

9. 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 for
high resistances.

10. KELVIN DOUBLE BRIDGE
Suitable for resistance values: 1 to 0.00001 Ω
Why 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.

11. 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

12. 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.

13. 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.

14. 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.

15. 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.

16. 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

17. 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.

18. CAPACITANCE MEASURMENT
•SCHERING BRIDGE

19. SCHERING BRIDGE
This bridge is used to measure to the
capacitance of the capacitor,
dissipation factor and measurement of
relative permittivity.

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

21. Now when the Schering Bridge is balanced, then

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

23. Dissipation Factor
•Dissipation factor tells us about the
quality of a capacitor, how close the
phase angle of the capacitor is to the
ideal value of 900.
The dissipation factor is given by:

24. 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.