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Basic electrical measurements

Basic electrical measurements. Using handheld electronic test tools. Goals of this presentation. Understand safety specifications and how to operate handheld electronic testers in a safe manner Understand how handheld electronic testers and accessories perform basic measurements

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Basic electrical measurements

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  1. Basic electrical measurements Using handheld electronic test tools

  2. Goals of this presentation • Understand safety specifications and how to operate handheld electronic testers in a safe manner • Understand how handheld electronic testers and accessories perform basic measurements • Learn how to set a digital multimeter (DMM) to the correct function and range for a given measurement • Learn how to measure a variety of electrical parameters and test electrical components • Determine the proper measurement tool for safe and accurate measurements • Understand the differences between average responding and true-rms measurement on non-linear loads

  3. Digital multimeter basics Agenda • Chapter 1: A first look at the DMM • Chapter 2: Multimeter safety • Chapter 3: Multimeter specifications • Chapter 4: Multimeter measurements • Ohm’s Law: basic volts, amps, ohms measurement • Special functions: Min/Max, Peak Hold • Voltage: understanding high input impedance • Current: using current clamps • Resistance: DMM source voltage and multiple sources • Testing components: diodes, caps • Measuring temperature • Chapter 5: Non-linear loads • True-rms vs. average-sensing

  4. Chapter 1 A first look at the digital multimeter • Visual inspection • Front panel symbols • Hands-on safety inspection: • Test leads and probes • Amps inputs: fuses • Volts/Ω inputs: overload protection

  5. HOLD Front panel features: • Volts / Ω / inputs How is this input protected? • Amps, mA, mA inputs How is this input protected? • CAT IV - safety rating • Range: select manual ranging • button Second function • button Hold function

  6. Check out the back... Look at the back of the meter: • Safety warning • Fuse ratingsHow are fuses specified? • Certifications • Battery access

  7. Front Panel Symbols Symbol Meaning V V dc V V ac mV Millivolts (.001 V or 1/1,000 V) A Amps mA Milliamps (.001 A or 1/1000 A) µA MicroA (.000001 A or 1/1,000,000 A) Ω Resistance (Ohms) k Ω, M Ω Kilo-ohms, megohms )))) Continuity beeper

  8. Front panel symbols Symbol Meaning Capacitance (uF: microfarads, (nF: nanofarads) Diode test Hz Hertz (cycles/sec) dB Decibels Range Manual measurement ranging Hold TouchHold/AutoHOLD - last stable reading MIN MAX Highest, lowest recorded readings Dangerous voltage levels Caution: see manual

  9. TouchHold Displays last stable reading • Turn dial to Vdc • Press Hold • Take measurement • Remove probes • Press hold a second time and you are in Autohold • Turn dial to ΩPress HoldMeasure resistor Remove probes Measure second resistor Automatic Touch Hold / Shift (second function) Hold updates automatically.

  10. First look at the DMM Summary • What we learned: • Meaning of front panel symbols • Back panel safety warning and other info • TouchHold & Autohold functions -- how they work

  11. Chapter 2 Multimeter safety • Test leads & probes • Fuses • Overload protection • IEC 61010 standard

  12. Safety inspection Test leads and probes Check test lead resistance:Step 1: Insert leads in V/  and COM inputsStep 2: Select  , touch probe tips Good leads are 0.1 - 0.5 How do you check a single test lead? Visually check for: • New category rating(CAT III-1000 V or 600 V CAT IV recommended) • Double insulation • Shrouded connectors, finger guards • Insulation not melted, cut, cracked, etc. • Connectors not damaged: no insulation pulled away from end connectors • Probe tips: not loose or broken off

  13. Safety inspection Amps inputs need fuses • In a power circuit, use current clamp accessory or stand alone clamp meter • In low energy ckt, 10 A or less, open the circuit: • Measure in series (current is the same in a series circuit). The amps circuit resistance must be small to have a minimal effect on the current. This low impedance input requires fuse protection. A, mA/uA inputs Caution!!! Don’t leave the leads in mA or A input jacks and then take voltage measurements. Ammeter circuit inside DMM COM

  14. Safety inspection Checking meter fuses on most meters Step 1: Plug test lead in V/  input. Select  Step 2: Insert probe tip into mA input and read value Step 3: Insert probe tip into A input and read value Is the fuse okay? What would an open fuse read?

  15. Safety inspection High impedance on Volts/ inputs • Volts measurements need high impedance circuit • Voltage measurements are in parallel Voltage is the same across each parallel branch • Parallel circuits divide current: High impedance branch = less currentLow impedance branch = more current What about protection for ohms measurement?

  16. Safety inspection Overload protection on volts inputs With leads in V/  and COM inputs: Step 1: Select V and put probes in a live outlet. Will you damage the meter if you... Step 2: Select mV Step 3: Select  Step 4: Select A. Overload protection is only to the DMM’s rated voltage.

  17. Common DMM / tester hazards • Arc from transients (lightning, load switching) Protection: Independent certification to meet CAT III-1000 V or CAT IV 600 V • Voltage contact while in continuity or resistance Protection: Overload protection in ohms up to the meter’s volt rating • Measuring voltage with test leads in current jacks Protection: High energy fuses rated to the meter’s voltage rating Use meters / testers without current jacks • Shock from accidental contact with live components Protection: Test Leads double insulated, recessed / shrouded, finger guards, CAT III – 1000 V. Replace when damaged • Using meter or tester above rated voltage Protection: Good karma

  18. Multimeter safety Summary • What we learned: • How to check for good test leads • Why amps inputs need fuse protection • Low input impedance circuit • How to check for open fuses in the meter • Function of overload protection on V/inputs

  19. Chapter 3 DMM specifications • Display • Accuracy • Range and resolution Electrical Electronics

  20. Understanding DMM display specs Display is specified as digits or as count • Digits: 3 1/2, 4 1/2, etc. • Example: 3 1/2: starting from the least significant digit, 3 “full” digits from 0-9, 1 “half” digit at less than 9. Ex: 1999 • Can be confusing: how do you specify 3999? • Count: 6000 5000 4000 3200 etc • 4000 count display reads from 0-3999 • 3200 count display reads from 0-3199 • Hands-on: 6000 count display • Select V, measure battery 5000 count

  21. Understanding DMM accuracy specs Accuracy is specified in percentage • Closeness with which an instrument reading approaches the true value being measured; largest allowable error • Percentage of reading (digital multimeters) vs. percentage of scale or range (analog meters): Example: 1 % scale vs. 1 % reading% scale: If scale or range is 1000 V, an accuracy of 1 % is equal to +/- 10 V. 120 V reading could = 110-130 V% reading: 1 % accuracy with 120 V reading = 118.8-121.2 V • Least significant digit unstable:Example: Accuracy spec = +/-(1 % +2)Reading of 200.0 mV= 197.8 - 202.2 mV

  22. 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: Turn Fluke 179 to Vac and hit Range button (Auto disappears): Range: Resolution:600.0 mV .1 mV (=1/10 mV)6.000 V .001 V (=1 mV)60.00 V .01 V (=10 mV)600.0 V 0.1 V (=100 mV)1000 V 1 V (=1000 mV) (To exit Manual Range, hold Range button for 2 secs) • For maximum resolution, choose the lowest possible range

  23. ABCs of DMM specs Summary • What we learned: • Display specifications: Digits or counts • Accuracy specifications: Percent of range or percent of reading • Range and resolution specs: Low range, high resolution (e.g.: 400.0 mV) High range, low resolution (e.g.: 400.0 V)

  24. Chapter 4 DMM measurements Basic measurements: Ohm’s Law Special functions: Min/Max How DMMs measure voltage: Understanding high input impedance How DMMs measure resistance: No other voltage please How DMMs measure current: Using clamp-on accessories Testing components: Capacitors, diodes, LEDs How DMMs measure temperature

  25. Ohm’s Law (V=IR)Can you prove it, Mr.  • Battery voltage: V = • Resistor: R = • Calculate current: I CALCULATED = V / R = • Measure current: create series circuit with resistor and battery and measure current (use mA inputs): I MEASURED =

  26. MINMAX Special functions DMM as recorder: Min/Max/Avg • Capture sags: (>100 ms) • Fluke 179: push MIN MAX button. (Meter beeps with each new MIN or MAX) • Scroll through Max, Min and Average screensby pushing MIN MAX button. • Record voltage sag as motor is turned on. FAST MN MX

  27. How DMMs measure voltage Measuring volt / input impedance Step 1: Meter 1 (179): Select ohms Meter 2: Select Vdc Use meter 1 to measureinput impedance of meter 2. Meter 2 input Z = ______Ω Step 2: Reverse procedure Meter 1 select Vdc, meter 2 select ohms: Meter 1 input Z = ______Ω

  28. How DMMs measure voltage Advantages of High Input Z • Exercise: Gum wrapper battery • Step 1: Construct battery with foil, wet card and penny (don’t overlap penny onto foil) • Step 2: Select mV-dc and measure battery voltage

  29. How DMMs measure voltage Demonstrating “ghost” voltages • Turn meter to Hz. Lay leads parallel to power lines. What does the display read? • Voltage from hot to capacitively coupled ground: • Effect of floating ground:

  30. How DMMs measure resistance • The meter supplies voltage to the circuit • Presence of external voltage in circuit being measured causes meaningless readings and can damage a meter without overload protection • How it works: Measured V1 across a precision R1 is compared to measured V2 across an unknown Rx

  31. How DMMs measure resistance Open circuit voltage • First measure “open circuit voltage” of meter when in ohms mode Meter 1: V (dc) modeMeter 2:  mode V OUT (METER 2) =Reverse the procedure. V OUT (METER 1) = • Now connect both meters in  mode across a known resistor. Both meters are sourcing voltage. What is the  reading?

  32. How DMMs measure current Current clamp accessories • In power circuits, clamps are used to measure amps • Two types of clamps: ac or ac/dc (Scope clamps have BNC connectors: AC or AC/DC both output mV ) AC AC/DC Output signal Current Voltage 1 milliAmp 1 milliVolt Scale factor per Amp per Amp Sensor Hall effect Current transformer Battery No Yes

  33. How DMMs measure current AC current clamp accessories • Current transformer (CT) style preferred for ac: • CT clamps have good noise immunity: recommended for ac variable speed drives and other noisy environments • How to use: use A inputs • They are CTs with 1:1000 turns ratio: 1 A on primary (circuit being measured) =1 mA on secondary (input signal to DMM) • Connect probe to amps jacks of DMM • Select mA function on the Fluke 179 • True-rms measurements require a true-rms meter.

  34. How DMMs measure current AC/DC current clamp accessories • AC/DC clamps: use V inputs of DMM • Use Hall-effect technology: require batteries in clamp • 1 mV per amp • Select Vdc or mVdc to measure dc current • Select Vac to measure ac current • To measure ac+dc, use the following formula: • V total = Vac ² + Vdc² • Example: Vac = 5 V, Vdc = 5 V, but V total = 10 VV total = 5² + 5² = 25 + 25 = 50 = 7.07 V • True-rms measurement (of ac current) requires a true-rms meter

  35. How DMMs measure current Measuring load current and inrush Plug the ac current clamp accessory into the meter: Fluke 179: use mA inputs Remember: 1 mA = 1 A • Select mA function • Select auto range and connect to mA input and common • Measure motor inrush current: • Select MIN MAX

  36. How DMMs measure current Single phase measurements • Measuring load current: measure hot conductor • Checking for shared neutrals: • Measure with load on and off: current in neutral with load off indicates shared N • If neutral current > hot current, indicates shared N • Ground current: • Measure hot and neutral separately. Difference is leakage current. Assumes non-shared neutral.

  37. How DMMs measure current Three phase measurements • Current imbalance on motor loads: % current imbalance = Deviation from average Average Max (of three phases)Example: A = 50 A, B = 30 A, C = 40 A Average = (50 + 30 + 40) / 3 = 40 A Max deviation = 10 A (10 / 40) x 100 = 25 % imbalance Motors should not exceed 15 % - 25 % current imbalance • Neutral currents at panelboard: • Fundamental N current caused by uneven distribution of single-phase loads among the three phases • 3rd harmonic N current from non-linear single-phase loads 3rd harmonic is additive in neutral x 100

  38. How DMMs measure temperature Temperature accessories • Integrated temperature function • Use type K thermocouple probes (requires no adapter) • Non-contact: Infrared probe • Non-contact can measure electrically live or moving parts • 1 mV dc per ºF or ºC • 4:1 distance-to-target ratio: 4” away reads 1” circle • Internal 9 V battery (10 min. auto shut-off saves battery) • Contact: Thermocouple module • Uses mV dc function (requires input Z of 10 M  ) • Adapter for type-K thermocouple probes Comes with a general purpose bead probe • Switch selectable for ºF or ºC • Internal 9 V battery

  39. How DMMs measure temperature Temperature accessories • Type-K thermocoupletemperature probes • Mini-connectors plug into adapter • Different probes are specialized to measure: • Liquids and gels • Air and gases • Food • Surfaces including hot rollers and plates • Pipes (probe designed to clamp onto pipe)

  40. How DMMs measure temperature Some DMMs have integrated temperature measurement functions • Adapter accepts type K thermocouple probes. Remove for voltage measurement. MIN MAX temperature. • Select TEMP (C/F) Select MIN MAX. • Measure hot (Max) and cold (Min). Temp OC/OF Temp function

  41. Testing components Capacitors • Capacitors store electrical charge • Caution ! • Before measuring a cap, disconnect circuit power and make sure it’s discharged. Use Vdc to test if cap is discharged(= 0 V). • The 179 will display “disc” while discharging cap. • How it works: • The meter charges the cap with a known current for a known period of time, measures the resulting voltage (up to 1.2 V) and calculates the farads.

  42. Testing components Capacitors • Fluke 179: • Turn dial to Capacitance • Press yellow button to select • With probes in voltage jacks, measure cap • Measurement note: • 1.0 µF (microfarads) = 1000 nF (nanofarads) • 0.1 µF = 100 nF

  43. Testing components Diodes Diodes turn ac to dc. • A good silicon diode will have a voltage drop of approximately 0.5-0.7 V when it is forward biased (conducting). It will be open when it is reverse biased. • To test a diode, the DMM forces a test current through the diode in the forward bias direction and measures voltage drop across the diode.

  44. Testing components • Forward bias = ____ V Red lead anode Black lead cathode • Reverse bias = ____ V Red lead anode Black lead cathode • Shorted: 0 in both directions • Open: OL in both directions Diodes

  45. Testing components Diodes • Diode forward bias = ____V(Red lead) + ---- P / N ---- - (Black lead) • Diode reverse bias = ____V(Black) - ---- P / N ---- + (Red) • LED forward bias = ____V(Red) + ----- P/N/P/N/P/N ----- - (Black) • Transistor: finding the base lead(Black) - ----- N / P / N ----- - (Black) + (Red)

  46. DMM measurements Summary What we learned • It’s the law: Mr. Ohm was right. • MIN MAX and other recorder functions • Voltage measurements:The ups and downs of high impedance inputs • Resistance: DMM is the only voltage source • Current: Capturing inrush current • Use of temperature accessories • Components: Capacitor and diode checks

  47. Chapter 5 Measurement Issues withnon-linear loads • True-rms vs. average-sensing • Crest factor

  48. True-rms vs. average-sensing How accurate is your meter? • When can you use an average-sensing meter and when do you need a true-rms meter? • Are you measuring a sine wave or something less ideal than a sine wave?

  49. 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 equals Vdc.

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

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