digital multimeters - basic guide

Digital Multimeter – Basic Guide

What is a Digital Multimeter?

• A Digital Multi-meter (DMM) is simply an electronic instrument that

measure electrical parameters.

• A DMM measures

• AC / DC volts,

• AC / DC current

• Resistance

• DMM may have a variety of special features that are

designed for a wide number of applications.

• Frequency

• Temperature

• Capacitance

• Continuity in circuit

• Diode check

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Front Panel SymbolsSymbolMeaningV V DC

V V ACmV millivolts (.001V or 1/1,000V)A AmpsmA milliamps (.001A or 1/1000A)µA microA (.000001A or 1/1,000,000A) Resistance (Ohms)k kilo-Ohms, Megohms )))) Continuity beeper

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Front Panel Symbols

SymbolMeaningCapacitance (uF: Microfarads)

(nF: Nanofarads)Diode test

Hz Hertz (cycles/sec)REL Relative or offset readingRange Manual override of autorangeHold Touch Hold-last stable readingMIN MAX Highest, lowest recorded readings

Dangerous voltage levelsCaution: see manual


5 Digital Multimeter Basics DMMBASE1.PPT

DMM Specifications

• Display

• Accuracy

• Range & Resolution

ElectricalElectronics



Understanding DMM Display Specs

Display is specified as Digits or as Count

• Digits: 3 1/2, 4 1/2, etc.

– Example: 3 1/2: (read as three and half digit DMM)

– starting from the least significant digit (right most), 3 “full” digits

from 0-9

– Left most digit - 1 “half” digit (can read less than 9). Ex: 1999

• Count: 3200, 4000, etc...

– 4000 count display reads from 0-3999

– 3200 count display reads from 0-3199

20,000 count



Understanding DMM Accuracy SpecsAccuracy is specified in percentage %

• Closeness with which an instrument reading approaches the true value being measured;

or largest allowable error.

• Percentage of reading (digital multi-meters) vs. percentage of scale or range (analog

meters):

Example: 1%scale vs. 1% reading

% scale: If scale or range is 1000V, an accuracy of 1% is equal to +/- 10V. 120V reading

could = 110-130V.

% reading: 1% accuracy with 120V reading= 118.8-121.2V.

• Least significant digit unstable:

Example: Accuracy spec = +/-(1%+2)

Reading of 200.0mV= 197.8 - 202.2mV



Understanding DMM Specs

• Resolution is the smallest change in measured value to which the instrument will respond.

• As the range increases, the resolution decreases:

Range: Resolution:

400.0mV .1mV (=1/10 mV)

4.000V .001V (=1mV)

40.00V .01V (=10mV)

400.0V 0.1V (=100mV)

1000V 1V (=1000mV)

• For maximum resolution, choose the lowest possible range.

Range and Resolution


Measurements using Multimeter• Measuring voltage –

Testing for proper supply voltage is usually the first step when troubleshooting a circuit.

How to make voltage measurements :

• Select V~ (ac) or V (dc), as desired.

• Plug the black test probe into the COM input jack. Plug the red test

probe into the V input jack.

• If the DMM has manual ranging only, select the highest range so as

not to overload the input.

• Touch the probe tips to the circuit across a load or power source (in

parallel to the circuit).

• View the reading, being sure to note the unit of measurement.


• Resistance Measurement – Most DMMs measure down to 0.1Ω, and some measure as high as 300 MΩ (300,000,000 ohms). Infinite resistance (open circuit) is read as“OL” on the meter display.

How to make resistance measurements:

• Turn off power to the circuit.• Select resistance (Ω).• Plug the black test probe into the COM input jack. Plug the red test probe

into the Ω input jack.• Connect the probe tips across the component or portion of the circuit for

which you want to determine resistance.• View the reading, being sure to note the unit of measurement– ohms (Ω),

kilohms(kΩ), or megohms (MΩ).

Measurements using Multimeter


• DC and AC current – Current measurements taken with the DMM alone require placing the meter in series with the circuit being measured. This means opening the circuit and using the DMM test leads to complete the circuit.

How to make current measurements

• Turn off power to the circuit.• Cut or unsolder the circuit, creating a place where the meter probes can be inserted.• Select A~ (ac) or A (dc) as desired.• Plug the black test probe into the COM input jack. Plug the red test probe into the

amp or milliamp input jack, depending on the expected value of the reading.• Connect the probe tips to the circuit across the break so that all current will flow

through the DMM (a series connection).• Turn the circuit power back on.• View the reading, being sure to note the unit of measurement.

Measurements using Multimeter



True-rms vs. Average-sensing

• RMS is the Root Mean Square or effective heating value of any ac voltage or current waveform.

• RMS is the equivalent DC heating value of an AC waveform.

Power consumed in R1 is same for both AC and DC source if the VacRMS = Vdc.

What does “rms” mean




• An average-sensing meter assumes a non-distorted sine wave

and does the following calculation:

• RMS value = 1.11 X Average value

Average-sensing works for a perfect sinewave




• For this current waveform, the effective or True-rms value = 1.85 x

Average value.

• An average-sensing meter’s reading (1.11 x Average) would be 40

% too low.

What if the waveform is non-sinusoidal?




• Average-sensing meters typically measure RMS high for voltage and low for

current where there is waveform distortion.

• True-rms meter or clamp accurately measures both distorted waveforms and sine

waves.

True RMS

Correct

Correct

Correct

Correct

Average

Correct

10% High

40% low

5-30% low

Multimeter Type

Response to sine wave

Response to square wave

Response to single phase diode rectifier

Response to 3 phase diode rectifier

What if the waveform is non-sinusoidal?




• Crest Factor = Peak / RMS

• For ideal sine wave, CF = 1.414

What is Crest Factor?




• For this current waveform, Crest Factor = 2.9

What is Crest Factor?



• For current measurement, the higher the CF, the greater the waveform distortion.

• CF spec important for current clamp, since current distortion is typically higher than voltage distortion.

C.F. = 1.43 C.F. = 2.39 C.F. = 4.68

Crest Factor is an indication of harmonics



Minimum specifications for measurements on electrical power systems:

• True RMS– Accurate for both linear and non-linear loads

• Crest Factor 3– Accurate for current waveforms with CF not exceeding 3– CF=3 at max range; CF=6 at half-range

• IEC 1010-1 CAT III-600 V– Distribution level: power distribution equipment.

Summary


Some Digital Multimeters Provided by FLUKE


Why Choose Fluke ?

• Fluke designs its DMMs to the latest, most demanding safety standards.

• Fluke offers many DMMs with different combinations of features like Touch Hold, analog bar graphs, and enhanced resolution.

• Accessories for high current and temperature measurements are available to extend the capabilities of DMMs.


• Company Name : Fluke Corporation - India

• Website: http://www.fluke.com/fluke/inen/home/default.htm

• Address: Division of DHR Holding India Pvt. Ltd. 10th Floor, Sigma Hiranandani Business Park, Powai

Mumbai, Maharashtra 400076

• Contact Number: 1 800 209 9110

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Thank You

digital multimeters - basic guide

Documents

black test

red test probe

digital multimeter

probe tips

digital multi

input jack

crest factor

waveform distortion