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 Symbols Symbol

Meaning

V

V DC

V mV A mA µA  k ))))

V AC millivolts (.001V or 1/1,000V) Amps milliamps (.001A or 1/1000A) microA (.000001A or 1/1,000,000A) Resistance (Ohms) kilo-Ohms, Megohms Continuity beeper

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Front Panel Symbols Symbol

Meaning Capacitance

Hz REL  Range Hold MIN MAX

(uF: Microfarads) (nF: Nanofarads)

Diode test Hertz (cycles/sec) Relative or offset reading Manual override of autorange Touch Hold-last stable reading Highest, lowest recorded readings Dangerous voltage levels Caution: see manual

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DMM Specifications • Display

• Accuracy • Range & Resolution

Electronics

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Electrical

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Understanding DMM Display Specs Display is specified as Digits or as Count

20,000 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

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Understanding DMM Accuracy Specs Accuracy 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

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Understanding DMM Specs Range and Resolution •

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

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As the range increases, the resolution decreases:

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Range:

Resolution:

400.0mV

.1mV

4.000V

.001V (=1mV)

40.00V

.01V

(=10mV)

400.0V

0.1V

(=100mV)

1000V

1V

(=1000mV)

(=1/10 mV)

For maximum resolution, choose the lowest possible range.

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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.

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Measurements using Multimeter • 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).

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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.

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True-rms vs. Average-sensing What does “rms” mean •

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.

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True-rms vs. Average-sensing Average-sensing works for a perfect sinewave • An average-sensing meter assumes a non-distorted sine wave and does the following calculation: • RMS value = 1.11 X Average value

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True-rms vs. Average-sensing What if the waveform is non-sinusoidal? • 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.

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True-rms vs. Average-sensing 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.

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True-rms meter or clamp accurately measures both distorted waveforms and sine waves. Multimeter Type

Average

True RMS

Response to sine wave

Correct

Correct

Response to square wave

10% High

Correct

Response to single phase diode rectifier

40% low

Correct

Response to 3 phase diode rectifier

5-30% low

Correct

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True-rms vs. Average-sensing What is Crest Factor? • Crest Factor = Peak / RMS • For ideal sine wave, CF = 1.414

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True-rms vs. Average-sensing What is Crest Factor? • For this current waveform, Crest Factor = 2.9

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True-rms vs. Average-sensing Crest Factor is an indication of harmonics • 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

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Digital Multimeter Basics

C.F. = 2.39

C.F. = 4.68

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True-rms vs. Average-sensing Summary 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.

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Some Digital Multimeters Provided by FLUKE

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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.

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• 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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