Multi-Range Analog Ammeter 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

Basic Analog Meter Movement ECE 201 Circuit Theory I

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 ECE 201 Circuit Theory I

Resulting CCW Rotation Flux “Bunching” Mechanical forces on the conductor ECE 201 Circuit Theory I

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” ECE 201 Circuit Theory I

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 ! ILoad Imeter Ishunt ECE 201 Circuit Theory I

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! ECE 201 Circuit Theory I

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

Calibration of your Meter 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. ECE 201 Circuit Theory I