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

http://www.fluke.com/fluke/inen/products/Thermometers.htm

Front Panel Symbols
Symbol Meaning
V V DC

V V AC
mV millivolts (.001V or 1/1,000V)
A Amps
mA milliamps (.001A or 1/1000A)
µA microA (.000001A or 1/1,000,000A)
 Resistance (Ohms)
k   kilo-Ohms, Megohms
)))) Continuity beeper

http://www.fluke.com/fluke/inen/products/Digital-Multimeters.htm

Front Panel Symbols
Symbol Meaning
Capacitance (uF: Microfarads)
(nF: Nanofarads)
Diode test
Hz Hertz (cycles/sec)
REL  Relative or offset reading
Range Manual override of autorange
Hold Touch Hold-last stable reading
MIN MAX Highest, lowest recorded readings
Dangerous voltage levels
Caution: see manual


DMM Specifications
• Display
• Accuracy
• Range &
Resolution

Electronics Electrical

5 Digital Multimeter Basics DMMBASE1.PPT

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


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


Understanding DMM Specs
Range and Resolution

• 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.


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).


• 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.


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.


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


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.


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.

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


What is Crest Factor?
• Crest Factor = Peak / RMS
• For ideal sine wave, CF = 1.414


What is Crest Factor?
• For this current waveform, Crest Factor = 2.9


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


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.


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

Digital Multimeters- Basic Guide

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