Digital Multimeter – Basic Guide <a href="http://www.fluke.com/fluke/inen/products/Digital-Multimeters.htm " title="Digital Multimeters">Digital Multimeters</a> and <a href="http://www.fluke.com/Fluke/inen/Digital-Multimeters/Fluke-117.htm?PID=55996 " title=“Electrical Multimeters">Electrical Multimeters</a> can be used to measure current, voltage, resistance and other parameters for both Industrial & Laboratory Purposes.

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 Objective: Make novice users familiar with the symbols on the front panel. The front panel symbols for dc and ac are international (IEC) standards, but can be a source of confusion for users. 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 We will go through all these functions with hands-on exercises. The farad is the standard unit of capacitance. A nanofarad is .000000001 (1 / billionth) of a farad. http://www.fluke.com/fluke/inen/products/Digital-Multimeters.htm

Display Accuracy Range & Resolution Electronics Electrical DMM Specifications Display Accuracy Range & Resolution Objective: Explain some specification concepts as concisely as possible, and with hands-on when possible. Electronics Electrical http://www.fluke.com/fluke/inen/products/Digital-Multimeters.htm

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 Objective: Clarify display specs. Specification of Counts is clearer than Digits. The digit system is the traditional metric in the Test & Measurement industry, but it is confusing for end-users. http://www.fluke.com/fluke/inen/products/Digital-Multimeters.htm

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 Objective: Explain how accuracy is specified in DMMs. Increased Count by itself (without increased Accuracy) does not bring additional benefits. For example: Meter 1 has a 4000 count display with +/- 1% accuracy. Meter 2 has a 40000 count display with +/- 1% accuracy. Both meters are measuring 2500mV: Meter 1 is in the 4000mV range. +/- 1% of 2500mV is +/- 25mV. Meter 1 reads from to 2475mV to 2525mV. Meter 2 in the 4000.0mV range. +/- 1% of 2500.0mV is still +/- 25.0mV. Meter 2 reads from 2475.0mV to 2525.0mV: higher resolution, but no gain in accuracy. http://www.fluke.com/fluke/inen/products/Digital-Multimeters.htm

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. Objective: Demonstrate relationship between Range and Resolution. They can be specified together. Rather than explain this, use 87 to step through the ranges manually. Auto-ranging automatically chooses the range with the best resolution for the signal being measured. It is the start-up mode of the meter. Manual ranging allows faster readings when the range of those readings is known. http://www.fluke.com/fluke/inen/products/Digital-Multimeters.htm

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

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

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

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. Objective: Explain RMS. Nominal voltages in distribution systems are given in RMS: e.g., 120V, 208V, 230/240V, 480V, 575V, etc., are all rms values. Equipment sizing relies on rms values. Power consumed in R1 is same for both AC and DC source if the VacRMS = Vdc. http://www.fluke.com/fluke/inen/products/Digital-Multimeters.htm

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 Objective: Explain what is meant by average-sensing. The average value of a sine wave = .637 x peak. The rms value of a sine wave = .707 x peak. The ratio .707 / .637 = 1.11. The circuitry to turn ac into dc average is relatively simple and inexpensive. Hence the lower cost of average sensing meters. http://www.fluke.com/fluke/inen/products/Digital-Multimeters.htm

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. Objective: Introduce very common non-linear waveform. This current waveform is typical of single-phase electronic loads. It is the signature current waveform of the Switching Mode Power Supply. Each pulse represents a capacitor bank being recharged at the peak of the incoming voltage sine wave. This waveform is considerably more “peaked” than the sine wave, which is why its rms or effective heating value is so much higher than the dc average. http://www.fluke.com/fluke/inen/products/Digital-Multimeters.htm

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. True-rms meter or clamp accurately measures both distorted waveforms and sine waves. Multimeter Type Average True RMS Response to sine wave Correct Correct Objective: Explain measurement issues with non-linear waveforms. Distorted voltage waveforms are typically “flat-topped” because current is drawn by recharging caps at the peak of the wave. The more severely flat-topped voltage waveforms are, the more they resemble a square wave, hence the tendency of average-sensing meters to read a higher value than the true-rms value. With typical current waveforms, which tend to be peaked, the effect is the opposite: the clamp meter tends to read low. From a troubleshooting point-of-view, this is exactly the combination that tends to mislead people: voltage higher than actual, so that the effects of voltage sag are missed; current lower than actual, so that the effect of overload/overheating is missed. 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 http://www.fluke.com/fluke/inen/products/Digital-Multimeters.htm

True-rms vs. Average-sensing What is Crest Factor? Crest Factor = Peak / RMS For ideal sine wave, CF = 1.414 Objective: Explain CF. Crest Factor is an important specification for true-rms current clamps. http://www.fluke.com/fluke/inen/products/Digital-Multimeters.htm

True-rms vs. Average-sensing What is Crest Factor? For this current waveform, Crest Factor = 2.9 Objective: CF for a common current waveform.

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 C.F. = 2.39 C.F. = 4.68 Objective: Some visual correlation of CF and waveform. Generally, waveforms with CF above 3 do not occur in power circuits. Such high CF signals could occur in electronic circuits where it is important to display the waveform, but not to take a current measurement.

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. Objective: Summary slide. Crest Factor is specified for current clamps. However, there is a common misperception of what this spec means. The CF=3 spec means that at the maximum specified range of the clamp, a signal with CF of 3 or less will be read accurately. However, at lower range, the clamp will accurately read a proportionally higher CF. For example, at half range, CF=6, meaning that the CF of the signal can be less than of equal to 6 and it will still be accurately measured. http://www.fluke.com/fluke/inen/products/Digital-Multimeters.htm

Some Digital Multimeters Provided by FLUKE http://www.fluke.com/fluke/inen/products/Digital-Multimeters.htm

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

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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