Instrumentation & Measurements Lec 7

Electrical Instrumentation & Measurements
AC & DC Bridges
Lecturer: engr. Sumera Jabbar
Class: 3rd semester

Bs Electrical Engineering Technology
Institute of Southern Punjab Multan

Objective
• Ability to explain operation of DC bridges, working principle, operation and applications.

What is bridge?
• A bridge circuit is one kind of electrical circuit where the two branches of the circuit are
linked to a third branch .
• A bridge circuit is two port network with a source of excitation connected to one port and a
detector to other.
• The components to be measured is inserted in one branch of the network and network is
adjusted until the detector indicates no output.
• At this point bridge is said to be balanced, the unknown value may be determined from
known values of the circuit.
• Bridges circuits are commonly used to measure many quantities in electronics such as
resistance, capacitance, inductance, impedance and admittance.
Types of Bridges
1. Dc bridges resistance bridge

• Wheatstone bridges
• Guarded Wheatstone bridge
• Kelvin bridges
2. AC bridges
• Capacitance bridges
• Inductance bridges
Resistance Bridge
• Depending on the magnitude of the quantity to be measured, several resistance bridges
are available as alternatives to the voltmeter- ammeter method and ohmmeter for the
measurement of DC resistance.
• The wheat stone bridge is used for most resistances greater than several ohms.
• The kelvin bridge is used for low values from 1 micro ohm to 1 ohm.
• Modification of the Wheatstone bridge allows for the location of faults in cables.
• These types of bridges are mostly dc excited.
Wheatstone Bridge
• Wheatstone bridge, also known as the resistance bridge, is used to calculate the
unknown resistance by balancing two legs of the bridge circuit, of which one leg
includes the component of unknown resistance. It was invented by Samuel hunter
christie in the year 1833, which was later popularized by sir Charles Wheatstone in
1843.
• The circuit is composed of two known resistors, one unknown resistor and one variable
resistor connected in the form of a bridge. This bridge is very reliable as it gives
accurate measurements.
Construction of Wheatstone Bridge
• A Wheatstone bridge circuit consists of four arms of which two arms consists of known
resistances while the other two arms consist of an unknown resistance and a variable
resistance. The circuit also consists of a galvanometer and an electromotive force
source. The emf source is attached between points a and c while the galvanometer is
connected between the points b and d. The current that flows through the galvanometer
depends on the potential difference across it.
Wheatstone Bridge Principle
The Wheatstone bridge works on the principle of null deflection, i.e. The ratio of their
resistances are equal and no current flows through the circuit. Under normal conditions,
the bridge is in the unbalanced condition where current flows through the galvanometer.
The bridge is said to be in a balanced condition when no current flows through the
galvanometer. This condition can be achieved by adjusting the known resistance and
variable resistance.
Derivation
2nd derivation
Wheatstone Bridge Application
• The Wheatstone bridge is used for the precise measurement of low resistance.
• Wheatstone bridge along with operational amplifier is used to measure physical
parameters such as temperature, light, and strain.
• Quantities such as impedance, inductance, and capacitance can be measured using
variations on the Wheatstone bridge.
Wheatstone Bridge Limitations
• For low resistance measurement, the resistance of the leads and contacts becomes
significant and introduces an error.
• For high resistance measurement, the measurement presented by the bridge is so large
that the galvanometer is insensitive to imbalance.
• The other drawback is the change in the resistance due to the heating effect of the
current through the resistance. Excessive current may even cause a permanent change
in the value of resistance.
Guarded Wheatstone Bridge
• Basic wheat stone bridge is useful in most cases , its performance deteriorates
substantially when measuring high resistances, such as insulation of cables and other
materials.
• Resistance of this type are generally greater than 1MΩ.
• The major problem of high resistance measurements is the leakage current that travels
around and over the binding post of the resistor being measured.
• Because these currents can bypass the resistor being measured and flow through the
detector branch, they affect the measurement.
• To reduce this measurement error, the leakage current is diverted away from the
unknown resistance by using a guard wire and guard ring.
• The guard ring surrounds the positive terminal post of the bridge circuit and minimizes
the leakage current by minimizing the potential difference.
• Only the current flowing through Rx flows through the detector, as a result, a true
reading is obtained.

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Electrical Instrumentation & Measurements

Class: 3rd semester

Institute of Southern Punjab Multan

• Ability to explain operation of DC bridges, working principle, operation and applications.

1. Dc bridges resistance bridge

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