UNIVERSITY OF MASSACHUSETTS DARTMOUTH

DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING

ECE 201

CIRCUIT THEORY I

EXTENDED - RANGE AMMETER

INTRODUCTION

The basic instrument used to measure electrical quantities is some form of analog (moving pointer) ammeter. A typical d’Arsenval analog meter movement is shown below in Figure 1.

Figure 1. A typical d’Arsenval analog meter movement

By adding circuit components that are external to the meter, we can construct instruments such as multi-range ammeters, voltmeters, and ohmmeters. It turns out that every meter movement displays some amount of resistance (the meter resistance, Rmeter ). In order to design additional configurations, we need to know the meter resistance.

PRELAB ACTIVITY

Devise a NON-DESTRUCTIVE scheme to measure the resistance of the 0-1mA meter that is provided in the lab. You may make use of an external power supply (or battery) and other circuit components (such as resistors) to make your measurement. Design your measurement scheme so that no more than 1 mA will flow through the meter! YOU CAN NOT USE THE MULTIMETER AS AN OHMMETER TO MAKE THIS MEASUREMENT!

Have your measurement scheme approved by the instructor or TA BEFORE you proceed with the actual measurement!


EXTENDED-RANGE AMMETER

The basic meter movement shown can measure from 0 to 1 mA full-scale. In order to extend the range of the meter, some external circuitry must be added. If the current to be measured exceeds the full-scale current of the meter movement, a parallel, or “shunt” branch must be provided.

Consider the case shown below where the desired full-scale Load current to be measured is 5 mA. Assume that the meter resistance is 100 Ω.

Figure 2. Circuit for a 0–1 mA meter to measure a 5 mA full-scale current

The Load current is 5 mA (if the meter resistance is equal to 0). When the shunt resistor is properly chosen, current division causes 1 mA to flow through the meter and 4 mA to flow through the shunt resistor.

Since the meter movement and the shunt resistor are in parallel, they have the same voltage drop across them, so, if the resistance of the meter is known, we can determine the value of the shunt resistor from the following:

where

Imeter is the full-scale current of the meter

and

Ishunt = ILoad - Imeter

The equivalent resistance of the meter circuit is now the parallel combination of the meter resistance Rmeter and the shunt resistor Rshunt.

LAB DESIGN PROBLEM

Using MultiSim, model your given meter and design an external shunt resistor that can be used to convert the 0-1mA meter to measure a full-scale current up to 10 mA. Run a simulation of your design, and when you are satisfied that your circuit meets the specifications, show it to the instructor or TA.

TESTING / CALIBRATING YOUR CIRCUIT

Construct the meter circuit that you designed and test it against a “standard” meter (your digital multimeter). Using the power supply/resistor combination shown in Figure 2, connect your “standard” meter and your “meter under test” in series with the A-B terminals.

Make measurements necessary to sketch a "calibration curve" which plots the current measured by your meter circuit against the current measured by the “standard” digital multimeter. Note that is not necessary to record the power supply voltage!

Use EXCEL to create a data table and to plot the calibration curves. Compare your measurements with the expected measurements.

RESULTS / CONCLUSIONS / REPORT (1 per GROUP)

For your extended-range ammeter,

Submit a copy of the EXCEL data table and calibration curve.

Determine the "equivalent" resistance of your Meter Under Test (meter and shunt).

Comment about the desired resistance (or range of resistance) of a practical ammeter and give a reason for your conclusion.

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