EET 110 - Electronics Survey

Chapter 8 - Measuring Voltage, Current, and Resistance

Objectives

Compare the operation of the analog and digital meters.

Correctly read an analog scale and digital meter display.

Connect a multimeter to measure correctly voltage, current and resistance.

Outline the safety precautions to be observed when using multimeters

Explain multimeter specifications and special features

Analog and Digital Meters

Three electrical quantities are routinely measured voltage (potential) - measured with voltmeter current (electron flow) - ammeter resistance - ohmmeter

Might include analog or digital meters

Analog meters

Analog meters consist of a galvanometer movement - an Electro-magnet within a permanent magnet.

The amount of meter deflection depends on the strength of the magnetic field caused by current flowing in the coil.

In reading an analog meter, deflection past half provides the greatest accuracy.

Analog meters.

Selecting the scale to read is important.Excessive deflection (pegging the meter)

might damage the unit.Jewel movement is fragile - susceptible to

damage from shockFunction select involves both switch and lead

connection.Range selection is done with passive

components

Using the Analog meter

Select function and large rangeverify that the meter is ‘zeroed’select AC or DC readingconnect to circuit to be measuredadjust to get mid scale deflectionread from appropriate scale avoiding

parallax error (use mirror)

Analog Resistance

Resistance is measured using internal batteries.

Ohms are read from right to left (rather than left to right)

Full deflection is 0 ohmsScale is logarithmic - not fixed.

Digital Meters

multimeter function is similar to that found in analog meters\

accuracy and circuit loading is normally substantially less with Digital meters

Requires power to functionAuto ranging meters are availableRange is selected for lowest range that still

displays value

Digital Meters

‘pegging’ the meter is not an issue.Display reading times range gives

measurement.Internal circuitry performs Analog to

Digital conversion may require ‘settle time’

Meter leads

leads are typically pointedattachments are available to provide lugs,

clips, etc.Some meters allow leads to be attached Observe polarity

Measurements

Voltage - Current see labs

Resistance may require ‘zeroing’ analog meters

Continuity provide audible reading testing for 0 resistance.

Other measurement devices

Voltage tester - ‘Wiggins’Clip on current measurement

usually for larger current readings.

Continuity testersMegger - tests for HIGH RESISTANCE

in the order of megohms. Usually provides a large voltage input.

Meter Specs

Input impedance. The amount a circuit is loaded by the meter 20k-30k per volt for analog 10M for digital

Accuracy and resolution Percentage of error based on full scale deflection for analog total % error for digital

Meter Specifications

Battery Life Digital meters require power for all

measurements

Protection Fuse or breaker especially for current readings

Digital/Analog DisplayAuto Ranging

Meter Specifications

Auto PolarityHold featureResponse timeDiode testAveraging or true RMS