In this presentation ,all types of Ac Bridges discussed very clearly with neat circuit diagram and phasor diagram..

1. AC
BRIDGE
S
Prepare d by -
Amrendra kumar
Regd.no-14781A0203

2. Synopsis
• Comparison between AC and DC Bridge
• Maxwell’s Inductance bridge
• Maxwell’s capacitance bridge
• Anderson bridge
• De Sauty’s bridge
• Schering bridge

3. Comparison between AC & DC Bridge
For DC Bridge
(a) R1 * R3 = R2 * R4
For AC Bridge
(b) Z1 *Z3 = Z2 * Z4

4. Maxwell’s inductance bridge
• The bridge circuit is used for medium inductances and can be
arranged to yield results of considerable precision.
• As shown in Fig., in the two arms, there are two pure resistances
so that for balance relations, the phase balance depends on the
remaining two arms.

5. Conti.
• L1 = unknown inductance of resistance R1
• L4 = variable inductance of fixed resistance R4
• R4 = variable resistance connected in series
with inductor L4.
R2 and R3 are fixed known resistances
• At balance, (R1 + jωL1)R3 = (R4 + jωL4 )R2
• Finally, L1 = L4(R2/R3)
R1 = R4(R2/R3)

6. Phasor Diagram

7. Maxwell’s inductance capacitance bridge
• In this bridge, an inductance is measured by comparison with a
standard variable capacitance.
• The connection is shown in figure.
• One of the ratio arms has a resistance and capacitance in parallel.

8. Conti.
• L3 = unknown inductance
• C = variable standard capacitor
• R1, R2, R4 = known pure resistances.
• R3 =effective resistance of inductor L3
• At balance, R1(R3 + jωL3) = R2R4
(1 + jωCR1)
• Finally, L3 = CR2R4
R3 = R4R2/R1
Q = ωL3/R3

9. Advantages
• The balance equation is
independent of frequency.
• It is useful for measurement of
wide range of inductance at power
and audio frequency.
Disadvantages
• It cannot be used for measurement
of high Q values (Q≥10).
• It cannot be used for measurement
of very low Q values, because of
balance converge problem.

10. Anderson’s bridge
• This bridge, in fact, is a modification of the Maxwell’s inductance-
capacitance bridge.
• In this method, the self-inductance is measured in terms of a standard
capacitor.
• Figure shows the connections and the phasor diagram of the bridge for
balanced conditions.

11. Conti.
• L1 = self inductance to be measured.
• C = fixed standard capacitor
• R2, R3, R4 , R5 = known pure resistances.
• R1 = resistance connected in series with L1.
• At balance,
(R1 + jωL1) (R3/jωC) = R2 R4+ R3R5
(R3 + R5 + 1/jωC) (R3 + R5 + 1/jωC)
• Finally, R1 = R2R4/R3
L1 = CR2 + R4 + R5 + (R5R4/R3 )

12. Advantages
• Anderson’s bridge balance is easily
obtained for low Q coils.
• The bridge can be used for accurate
determination of capacitance in
terms of inductance.
Disadvantages
• It is complex.
• The bridge balance equations are
not simple. They are rather more
tadious.

13. De Sauty’s bridge
• This bridge is the simplest method of comparing two capacitances.
• The connection diagram of this bridge is shown in figure.

14. Conti.
• C2 = capacitor whose capacitance is to be measured
• C3 = a standard capacitor.
• R3, R4 = pure resistances.
• At balance, R1 -j = R4 -j
ωC3 ωC2
• Finally, C2 = C3R4
R1

15. Advantages
• The bridge is simple.
• It is economical.
Disadvantages
• If both the capacitors are not free
from dielectric loss , then it is not
possible to achieve bridge balance.
This method is only suitable for the
measurement of lossless capacitors.

16. Schering bridge
• It is used extensively fo the measurement of capacitors.
• It is also useful for measuring insulating properties i.e. phase angles very
nearly 90o.
• One of the ratios are consists of a resistance in parallel with a capacitor and
standard arm consists only a capacitor.
• The standard capacitor is a high quality mica capacitor or an air capacitor for
insulation measurement.

17. Conti.
• C2 = capacitor of unknown capacitance.
• r = a series resistance representing the loss in
the capacitor C1.
• C1 = a standard capacitor.
• R3 = a pure resistance.
• C4 = a variable capacitor.
• R4 = a variable pure resistance.
• At balance , r + 1 R4 = 1 R3
jωC2 (1 + jωC4R4) jωC1
• Finally, r = R3C4 & C2 = C1 R4 & D = ωC4R4
C1 R3

18. Advantages
• The bridge is widely used for
testing small capacitors at low
voltages with high precision.
• Since C4 is a variable decade
capacitance box, its setting in μF
directly gives the value of the
dissipation factor.
Disadvantages
• The calibration of C4 is only for
particular frequency, as ω term
present in the equation.
• Commercial Schering bridge
measures capacitors from 100 pF -
1μF with ±2% accuracy.