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Measuring Voltage and Current

Measuring Voltage and Current. d’Arsenval analog meter movement. Analog Ammeter. Use a d’Arsenval meter movement with a parallel resistor (shunt). The parallel resistor diverts some of the current away from the meter movement. Analog Voltmeter.

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Measuring Voltage and Current

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  1. Measuring Voltage and Current • d’Arsenval analog meter movement ECE 201 Circuit Theory I

  2. Analog Ammeter • Use a d’Arsenval meter movement with a parallel resistor (shunt). • The parallel resistor diverts some of the current away from the meter movement. ECE 201 Circuit Theory I

  3. Analog Voltmeter • Use a d’Arsenval meter movement in series with a resistor (multiplier). • The resistor limits the voltage drop across the meter movement. ECE 201 Circuit Theory I

  4. Sources of Error • Any instrument (meter) used to make measurements extracts energy from the circuit. (Known as “loading” the circuit). • The amount of error due to loading depends on the effective resistance of the instrument compared with the resistance in the circuit. ECE 201 Circuit Theory I

  5. Rule of the “one-tenth” • The effective resistance of the ammeter should be no more than 1/10th of the value of the smallest resistance in the circuit. • The current being measured should be the same with or without the ammeter. ECE 201 Circuit Theory I

  6. For a Voltmeter • Since voltage is a parallel measurement, the “loading” effect is minimized when the voltmeter resistance is much higher than the circuit resistance across which the measurement is being made. ECE 201 Circuit Theory I

  7. Problem # 3.32 • An ammeter with a resistance of 5Ω is used to measure the current io in the circuit. • What is the % error in the measured value? ECE 201 Circuit Theory I

  8. io ECE 201 Circuit Theory I

  9. Problem # 3.33 • A d’Arsenval voltmeter is shown. Determine the value of Rv for a full-scale reading of 5 Volts. ECE 201 Circuit Theory I

  10. Choosing the Multiplier Resistor • When 5V is applied, the meter will be at full-scale deflection. • The meter resistance is 50Ω. • Total resistance needs to be 5kΩ. • The series resistor Rv (multiplier) needs to be 4950Ω. ECE 201 Circuit Theory I

  11. Problem # 3.34 • The d’Arsenval voltmeter described in Problem # 3.33 is used to measure the voltage across the 24Ω resistor. ECE 201 Circuit Theory I

  12. What will the voltmeter read?What is the % error in the reading? ECE 201 Circuit Theory I

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