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Transcript
Measuring Voltage and Current • d’Arsenval analog meter movement ECE 201 Circuit Theory 1 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 1 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 1 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 1 4 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 1 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 1 6 Ammeter Example • A 50 mV, 1 mA d’Arsonval movement is to be used in an ammeter with a full-scale reading of 1 A. Determine RA. (Movement resistance is equal to (50 mV/1 mA) = 50Ω) ECE 201 Circuit Theory 1 7 Solution 999 mA 1 mA (999mA)( RA ) 50mV 50 RA 50.05m 999 ECE 201 Circuit Theory 1 8 Ammeter Example continued • How much resistance is added to the circuit when the meter is inserted to measure current? ECE 201 Circuit Theory 1 9 1A + 50 mV 50/999 Ω - Rm 50 Ω 50mV 0.050 1A or 50 (50) 999 0.050 Rm 50 50 999 ECE 201 Circuit Theory 1 10