Transcript Measuring Voltage and Current

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