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Lecture 5

Lecture 5. Current/Voltage Measurement Resistance Measurement. Current/Voltage Measurement. Circuit Model for ideal ammeter/voltmeter. An ideal ammeter has an equivalent resistance of 0 Ohm. An ideal voltmeter has an infinite equivalent resistance. d’Arsonval meter.

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Lecture 5

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  1. Lecture 5 Current/Voltage Measurement Resistance Measurement

  2. Current/Voltage Measurement

  3. Circuit Model for ideal ammeter/voltmeter An ideal ammeter has an equivalent resistance of 0 Ohm. An ideal voltmeter has an infinite equivalent resistance.

  4. d’Arsonval meter When current flows in the coil, it creates a torque on the coil, causing it to rotate and move a pointer across a calibrated scale.

  5. Commercial Rating • Rating: 50 mV and 1mA • Interpretation: When the coil is carry 1 mA, the voltage drop across the coil is 50 mV and the pointer is deflected to its full-scale position.

  6. A DC Ammeter Circuit RA limits the amount of current in the coil.

  7. Example 3.5 (a) • A 50 mV, 1 mA d’Arsoval movement is to be used in an ammeter with a full-scale reading of 10 mA. Determine RA. • How much resistance is added to the circuit when the 10 mA ammeter is inserted to measure current?

  8. A DC Voltmeter Circuit RV limits the voltage drop across the meter’s coil.

  9. Example 3.6 • A 50 mV, 1 mA d’Arsoval movement is to be used in a voltmeter in which the full-scale reading is 150 V. Determine RV. • How much resistance does the 150 V meter insert into the circuit?

  10. Resistance Measurement R1,R2, and R3 are known resistors. Rx is the unknown resistor.

  11. Determine Rx • Adjust the variable resistor R3 until there is no current in the galvanometer. • Calculate the unknown resistor from the simple expression: • Rx=(R2/R1)R3

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