The bridge uses for measuring the value of unknown resistance, inductance and capacitance, is known as the AC Bridge. The AC bridges are very convenient and give the accurate result of the measurement.The construction of the bridges is very simple. The bridge has four arms, one AC supply source and the balance detector. It works on the principle that the balance ratio of the impedances will give the balance condition to the circuit which is determined by the null detector.
2. AC BRIDGE
• AC bridges are the circuits that are used for the measurement of electrical
quantities such as inductance, capacitance, resistance. Along with these an
ac bridge allows us to measure storage factor, loss factor, dissipation factor
etc. AC bridges operate with only AC signal.
• An AC bridge is used to provide phase shifting and providing a feedback
path to the oscillator.
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3. AC BRIDGE NETWORK CONSTRUCTION
An AC bridge consists of 4 nodes
with 4 arms, a source excitation
and a balanced detector.
Each of the 4 arms of the bridge
consists of impedance.
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4. Source and detector in an AC bridge network are connected in opposite
nodes.
This is so because if source and detector are connected to the same node, all
the voltage or current of the source will be displayed at the detector.
So, in this condition, the bridge will never come into balance conditions.
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5. There are 2 conditions in
order to balance the
bridge-
The detector current
Id should be zero.
The potential difference
between the detector
node should be zero.
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6. It is a condition when certain
specific applied circuit situation
causes the detector current to
become 0.
BALANCE OR NULL CONDITION INAN AC BRIDGE
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7. An AC bridge is a
derivative of Wheatstone
bridge.
This is so because if
battery and
galvanometer of a
Wheatstone bridge
are replaced by an ac
source and detector
respectively.
It will behave as an
AC bridge.
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9. BALANCED CONDITION OF AC BRIDGE
For the bridge to be balanced, considering the above-shown figure
The current through detector must be 0 that requires the potential
differenceVbd to be 0.
In such a condition voltage drop from a to b will get equal to voltage drop
from a to d, both in magnitude and phase.
we can write the above-stated condition as,
E1 = E2
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10. Applying ohms’ law
I1 Z1 = I2 Z2
At balance
And
Substituting the value of I1 and I2
Z1 (Z2 + Z4) = Z2 (Z1 + Z3)
Z1Z2 + Z1Z4 = Z1Z2 + Z2Z3
Hence,
Z1Z4 = Z2Z3
The above equation is the basic equation for a
balanced AC bridge.This equation is suitable
to use while dealing with a bridge consisting of
series elements.
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11. • On contrary, while dealing with a bridge consisting of parallel elements,
admittances are used.The equation is given as
Y1Y4 =Y2Y3
• So, theoretically, that the product of impedances of one pair of opposite arms
must equal to the product of impedances of another pair of opposite arms in
complex notation.
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12. • Let us now consider impedance in
its polar form
• Z = Z∠θ
• Z represents the magnitude and
• θ represents the phase angle of
complex impedance.
• The above equation can be written
as
(Z1∠θ1) Χ (Z4∠θ4) = (Z2∠θ2) Χ
(Z3∠θ3)
• Z1 = (Z1∠θ1)
• Z2 = (Z2∠θ2)
• Z3 = (Z3∠θ3)
• Z4 = (Z4∠θ4)
• So, here impedance parameters
will get multiplied and angles will
be added.
Z1 Z4 ∠ θ1+θ4 = Z2 Z3 ∠ θ2+θ3
• Separately we can write
magnitude and phase equation as-
Z1 Z4 = Z2 Z3
• The condition in above equation is
called magnitude criteria and
∠θ1+∠θ4 = ∠θ2+ ∠θ3
• This condition is known as phase
criteria.
So, in a bridge balance condition,
magnitude and phase criteria should
be satisfied simultaneously.
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13. Applications of AC Bridges
• AC bridges are used to find unknown impedances along with associated
parameters.
• Communication system and complex electronics circuitry majorly make use of AC
bridges.
• AC bridge circuits are used in phase shifting and for the filtration of undesirable
signals.
• It is also used to measure the frequency of audio signals.
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