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