The DC Bridge is used for measuring the unknown electrical resistance. This can be done by balancing the two legs of the bridge circuit. The value of one of the arm is known while the other of them is unknown

1. DC BRIDGE
KAROLINEKERSIN .E
Asst.prof / BMIE

2. DC BRIDGES
 DC bridges can be operated with only DC voltage signal.
 DC bridges are useful for measuring the value of unknown resistance,
which is present in the bridge.
 Wheatstone’s Bridge is an example of DC bridge.
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3. WHEATSTONE’S BRIDGE
 Wheatstone’s bridge is a simple DC bridge, which is mainly having four arms.
These four arms form a rhombus or square shape and each arm consists of one
resistor.
 To find the value of unknown resistance, we need the galvanometer and DC
voltage source.
 Hence, one of these two are placed in one diagonal of Wheatstone’s bridge and
the other one is placed in another diagonal of Wheatstone’s bridge.
 Wheatstone’s bridge is used to measure the value of medium resistance.
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4. CIRCUIT DIAGRAM OF WHEATSTONE
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5.  In above circuit, the arms AB, BC, CD and DA together form a rhombus or square shape.
 They consist of resistors R2, R4, R3 and R1 respectively.
 The current flowing through these resistor arms is I2, I4, I3 and I1 respectively.
 The diagonal arms DB and AC consists of galvanometer and DC voltage source of V volts
respectively. Here, the resistor, R3 is a standard variable resistor and the resistor, R4 is an
unknown resistor. Balance the bridge, by varying the resistance value of resistor, R3.
 The above bridge circuit is balanced when no current flows through the diagonal arm, DB.
That means, there is no deflection in the galvanometer, when the bridge is balanced.
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6. The bridge will be balanced, when the following two conditions are satisfied.
•The voltage across arm AD is equal to the voltage across arm AB. i.e.,
VAD=VAB
⇒I1R1=I2R2
•The voltage across arm DC is equal to the voltage across arm BC. i.e.,
VDC=VBC
⇒I3R3=I4R4
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7. From above two balancing conditions, we will get the following two conclusions.
•The current flowing through the arm AD will be equal to that of arm DC. i.e.,
I1=I3I1=I3
•The current flowing through the arm AB will be equal to that of arm BC. i.e.,
I2=I4
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8. Take the ratio of Equation 1 and Equation 2.
I1R1I3R3=I2R2I4R4
Substitute, I1=I3 and I2=I4 in Equation 3.
I3R1I3R3=I4R2I4R4
⇒R1R3=R2R4⇒
⇒R4=R2R3R1
By substituting the known values of resistors R1, R2 and R3 in above equation, we will
get the value of resistor,R4.
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9. KELVIN BRIDGE
 A kelvin bridge or kelvin double bridge is a modified version of the Wheatstone bridge,
which can measure resistance values in the range between 1 to 0.00001 ohms with high
accuracy.
 It is named because it uses another set of ratio arms and a galvanometer to measure
the unknown resistance value.
 The basic operation of the Kelvin double bridge can be understood from the basic
construction and operation of the kelvin bridge.
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10. PRINCIPLE OF KELVIN BRIDGE
 A Wheatstone bridge is used to measure resistance equal to or greater than 1 – ohm, but
to measure the resistance below 1 – ohm, it becomes difficult because the leads which
are connected to the galvanometer adds up the resistance of the device along with the
resistance of leads leading to variation in the measurement of the actual value of
resistance.
 Hence in order to overcome this problem, we can use a modified bridge called kelvin
bridge.
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11. Derivation for Finding Unknown Resistance Value
 The Kelvin bridge is of resistance “r” which connects “R” ( unknown resistor ) to
standard resistor “S”.
 The resistance value can be viewed in the galvanometer (from “m to n”). If the pointer in
the galvanometer shows at “m”.
 It means, the resistance value is less and if the pointer shows at “n” means the
resistance value is high.
 Hence rather by connecting galvanometer to “ m and n “ we choose another
intermediate point “d” in kelvin bridge as shown in figure
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12. CIRCUIT DIAGRAM OF KELVIN
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13.  r1 / r2 = P/Q …………(1)
 R + r1 = (P/Q) * (S+r2)
 r 1 / ( r1+ r2) = P / (P+Q)
 r1 = [P / (P+Q) ].r
 we know that r1+r2 =r
 r2 = [Q / (P+Q)] .r
 R +[ P/( P + Q)] * r = P/Q [ S+ (Q/(P+Q)*r)]
 R = (P/Q)*S ………….(2)
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14.  From the above equation, by connecting the galvanometer at point “d” there will be
no effect in the measurement of the actual resistance value, but the only
disadvantage of this process is that it is difficult to implement, hence Kelvin double
bridge is used for getting accurate low resistance value.
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15. ADVANTAGES
• It can measure the resistance value in the range of 0.1 µA to 1.0 A.
• Power consumption is less
• Simple in construction
• Sensitivity is high.
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16. DISADVANTAGES
• For knowing whether the bridge is balanced or no, the sensitive galvanometer is
used.
• To obtain good sensitivity of the device, a high current is required.
• Manual adjustments are to made periodically when required.
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17. APPLICATIONS
 It is used to measure the unknown resistance of a wire.
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18. THANK YOU
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