Measuring Voltage and Current dArsenval analog meter movement

Analog Ammeter • Use a d’Arsenval meter movement with a parallel resistor (shunt). •

Analog Voltmeter • Use a d’Arsenval meter movement in series with a resistor (multiplier).

Sources of Error • Any instrument (meter) used to make measurements extracts energy from

Rule of the “one-tenth” • The effective resistance of the ammeter should be no

For a Voltmeter • Since voltage is a parallel measurement, the “loading” effect is

Problem # 3. 32 • An ammeter with a resistance of 0. 5Ω is

Problem # 3. 33 • A d’Arsenval voltmeter is shown. Determine the value of

Choosing the Multiplier Resistor • When 5 V is applied, the meter will be

Problem # 3. 34 • The d’Arsenval voltmeter described in Problem # 3. 33

What will the voltmeter read? What is the % error in the reading? ECE

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

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 2

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 3

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 4

Rule of the “one-tenth” • The effective resistance of the ammeter should be no more than 1/10 th 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 5

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 6

Problem # 3. 32 • An ammeter with a resistance of 0. 5Ω is used to measure the current io in the circuit. • What is the % error in the measured value? ECE 201 Circuit Theory I 7

io ECE 201 Circuit Theory I 8

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 9

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

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 11

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