EET 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 l voltage (potential) - measured with voltmeter l current (electron flow) - ammeter l 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 shock §Function select involves both switch and lead connection. §Range selection is done with passive components

Using the Analog meter §Select function and large range §verify that the meter is ‘zeroed’ §select AC or DC reading §connect to circuit to be measured §adjust to get mid scale deflection §read 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 ohms §Scale 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 function §Auto ranging meters are available §Range 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 l may require ‘settle time’

Meter leads §leads are typically pointed §attachments are available to provide lugs, clips, etc. §Some meters allow leads to be attached §Observe polarity

Measurements §Voltage - Current l see labs §Resistance l may require ‘zeroing’ analog meters §Continuity l provide audible reading l testing for 0 resistance.

Other measurement devices §Voltage tester - ‘Wiggins’ §Clip on current measurement l usually for larger current readings. §Continuity testers §Megger - tests for HIGH RESISTANCE l in the order of megohms. l Usually provides a large voltage input.

Meter Specs §Input impedance. l The amount a circuit is loaded by the meter l 20k-30k per volt for analog l 10M for digital §Accuracy and resolution l Percentage of error l based on full scale deflection for analog l total % error for digital

Meter Specifications §Battery Life l Digital meters require power for all measurements §Protection l Fuse or breaker l especially for current readings §Digital/Analog Display §Auto Ranging

Meter Specifications §Auto Polarity §Hold feature §Response time §Diode test §Averaging or true RMS