ECE 201 Circuit Theory I 1

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

ECE 201 Circuit Theory I 2

Basic Analog Meter Movement

ECE 201 Circuit Theory I 3

Interaction of Magnetic FieldsBasic DC Motor

Permanent Magnet Magnetic flux from N to S

Current into A and out of B Magnetic flux due to current

ECE 201 Circuit Theory I 4

Resulting CCW Rotation

Flux “Bunching”

Mechanical forces on the conductor

ECE 201 Circuit Theory I 5

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

DC 50Resistance

0.000 A+ -

BasicMeter

DC 50Resistance

0.000 A+ -

Rshunt

1

ECE 201 Circuit Theory I 6

Calculation of the Shunt Resistor

• Measure ILoad

• Ishunt = ILoad - Imeter

• Parallel combination of meter and shunt

• ImeterRmeter = IshuntRshunt

• Rshunt = Rmeter(Imeter/Ishunt)

• Need to know Rmeter !

BasicMeter

DC 50Resistance

0.000 A+ -

Rshunt

1

ILoad

Ishunt

Imeter

ECE 201 Circuit Theory I 7

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!

BasicMeter

DC 50Resistance

0.000 A+ -

XMM1

ECE 201 Circuit Theory I 8

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!

ECE 201 Circuit Theory I 9

Calibration of your Meter

VariableVoltage12 V

CurrentLimit

100 Ohm

A

B

STANDARDDC 1e-009Ohm0.000 A

+

-

MeterUnderTestDC 1e-009Ohm0.000 A

+

-

• 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.