Transcript Multi-Range Analog Ammeter

Multi-Range Analog Ammeter
• Multi-Range
– Multiple Full-Scale Values
– Ranges less than and/or greater than the
device’s full-scale value
– Measure 10 mA with a 0-1 mA device
• Analog Meter
– Moving-pointer
– Works by interaction of magnetic fields
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Basic Analog Meter Movement
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Interaction of Magnetic Fields
Basic DC Motor
Permanent Magnet
Magnetic flux from N to S
Current into A and out of B
Magnetic flux due to current
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Resulting CCW Rotation
Flux “Bunching”
Mechanical forces on the conductor
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Extending the Range of an Ammeter
• Meter movements
available in standard
full-scale values
• We have 0 – 1 mA
• To measure currents
greater than 1 mA, we
need to “divert” the
extra current away
from the meter using
a “shunt”
BasicMeter
+
0.000
A
DC 50
Resistance
BasicMeter
+
0.000
A
DC 50
Resistance
Rshunt
1
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Calculation of the Shunt Resistor
ILoad
Imeter BasicMeter
+
Ishunt
0.000
DC 50
Resistance
Rshunt
1
• Measure ILoad
• Ishunt = ILoad - Imeter
A
• Parallel combination of
meter and shunt
• ImeterRmeter = IshuntRshunt
• Rshunt = Rmeter(Imeter/Ishunt)
• Need to know Rmeter !
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Here’s how NOT to measure the resistance
of the meter movement
• Do not connect the
meter movement to
any analog or digital
multimeter
configured as an
ohmmeter!
• The internal battery
will supply too
much current and
destroy the
movement!
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XMM1
BasicMeter
+
0.000
A
DC 50
Resistance
7
Preliminary (Prelab) Work
• Devise a non-destructive scheme to
determine the internal resistance of the
meter movement.
• You may use any equipment or
components (power supply, function
generator, resistors, potentiometers,etc.).
• Have your method approved BEFORE you
begin the experiment!
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Calibration of your Meter
CurrentLim it
A
100 Ohm
0.000
STANDARD
A DC 1e-009Ohm
0.000
MeterUnderTest
A DC 1e-009Ohm
+
VariableVoltage
12 V
+
B
• Measure the same current with the meter you
designed and a “standard” (commercial) meter.
• Plot the reading of your meter as a function of
the readings obtained with the standard meter.
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