Chapter 5 DC Bridges.ppt

Chapter 5 DC Bridges.ppt - 5.0 DC BRIDGES LO 3 Apply the...

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12/07/18 Chapter 5 : DC & AC Brigdes 1 5.0 DC BRIDGES LO 3 Apply the suitable bridge techniques to measure component values such as resistance, inductance and capacitance. Objectives: Explain how a simple dc and ac bridge circuit operates and derive an expression for unknown parameters. Explain the steps involved in balancing an dc and ac bridge and derive the balance equation. Identify each bridge by name

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5.1 DC Bridge – Wheatstone Bridge Used to measure resistance based on the Wheatstone bridge The Wheatstone bridge remains the most sensitive and accurate method for precisely measuring resistance values. 12/07/18 2 Figure 5.1 Wheatstone Bridge Fig 5.2: Portable Digital DC bridge Chapter 5 : DC & AC Brigdes
Fundamental concept is two voltage dividers, both fed by the same input. The circuit output is taken from both voltage divider outputs. A galvanometer (a very sensitive dc current meter) is connected between the output terminals, and is used to monitor the current flowing from one voltage divider to the other. If the two voltage dividers have exactly the same ratio then the bridge is said to be balanced and no current flows in either direction through the galvanometer 12/07/18 3 Cont’d… Chapter 5 : DC & AC Brigdes

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I 1 R 1 =I 2 R 2 (1) I 1 R 2 =I 2 R 4 (2) Dividing equation 1 by equation 2 results in: R 1 /R 2 = R 3 /R 4 If one of the resistors changes even a little bit in value, the bridge will become unbalanced and current will flow through the galvanometer. 12/07/18 4 Cont’d… Chapter 5 : DC & AC Brigdes
Using bridge null-indication principle For resistor measurement, R4 in circuit was replace with unknown resistor Rx R2 will be change with variable resistor Rg is a internal resistance for galvanometer Adjust R2 to make sure that PMMC indicator show a ‘0’.

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• Fall '17
• puan nor azilah binti ahmad
• Voltage divider, Thévenin's theorem, I1R1

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