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 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 Important note to instructors: This presentation is designed to be used in two ways; As a stand-alone overview of test tools As a guide for “hands-on” demonstrations of test tools using digital multimeters and clamp accessories (if available) Feel free to modify this presentation to meet your classroom needs and lab equipment availability. The Fluke 179 True-rms DMM is used for discussion purposes throughout this presentation. Other DMMs can be used as appropriate, like the 87V of the 289. For hands-on lab two students per station is the best ratio. Each station: DMM and clamp accessory One per class if available: temperature probe, clamp meter Loads: Hair dryer, heavy duty drill (larger the better), PC/TV Voltage sources: 3-prong-to-2-prong adapter (no ground); socket-style light dimmer with extension socket that screws into dimmer, line splitters, CFL light bulb Components & miscellaneous: One set per station: batteries (9 V), resistors (1 kilohm, etc.), caps, diodes, LEDs, transistors (NPN), spare fuses for meters, calculator

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 only for 87V or 289 Voltage: understanding high input impedance Current: using current clamps Resistance: DMM source voltage and multiple sources Testing components: diodes, caps, resistors Measuring temperature Chapter 5: Non-linear loads True-rms vs. average-sensing These are very approximate time estimates for each module: Module 1: 30 min. - one hr. Module 2: 45 min. - one hr. Module 3: 15 min min. Module 4: Two hrs. Module 5: 15 min. (demo only) - one hr. (full lecture and hands-on) These modules are organized so that there is a good flow if followed in order. However, presenters may choose to select and re-organize material for their own shorter presentations.

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 Objectives: This presentation material is designed to put the meter in the hands of the student as soon as possible. In addition, the material is designed to introduce “safety thinking” and safety practices as soon as possible.

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 Objective: Introduce the following concepts, all of which will be gone into in some detail later on in the presentation. 1. Amps and volts inputs require different protection. 2. CAT-voltage rating is safety/measurement category rating of meter. 3. Explain most prominent function buttons. Shortly, we will talk about input protection on the volts and amps inputs. For now, the point is that the DMM is a “hybrid” instrument with very different circuitry on its two inputs. Consequently, the protection requirements of the two circuits are different. CAT IV-voltage is a new rating associated with IEC Second Edition, a relatively recent standard for DMM safety design. CAT III is a minimum rating for meters that will be used on power circuits, such as three-phase circuits. The voltage is the maximum voltage rating of the instrument (specifically, ac or dc voltage to ground). This material is covered in detail in Module 2 on DMM safety. DMM starts up in Autorange. HOLD

Check out the back... Look at the back of the meter: Safety warning
Fuse ratings - How are fuses specified? Certifications Battery access Objectives: Continue inspection of meter, with emphasis on safety information. There is a lot of information on the back of the meter that users often ignore. This exercise gets the meter in the students’ hands and gets them asking questions. Fuses are specified for 44/100 A (440 mA) and 11 A inputs. They are rated 1000 V. Questions might arise about the IR (interrupt ratings): Why are they only 10 kA or 17 kA, when branch circuit breakers can have IRs of 22 kA, and main three-phase breakers can go to 100 kA of interrupt current. The quick answer is that the input impedance of the leads and the internal circuitry limits the fault current to a maximum of 10 kA or 17 kA at 1000 V and under, regardless of the available fault current. These custom-designed fuses have been tested on source circuits at the Bussman facility of over 100 kAIC. This will be covered in the safety presentation. Holding the Hold button down during turn-on shows all display symbols. The battery indicator is very important information that is overlooked by many users.

Front Panel Symbols Symbol Meaning V V dc V V ac
mV Millivolts (.001 V or 1/1000 V) A Amps mA Milliamps (.001 A or 1/1000 A) µA MicroA ( A or 1/ A) Ω Resistance (Ohms) kΩ, MΩ Kilohms, megohms )))) Continuity beeper Objective: Make novice users familiar with the symbols commonly found on the front panel of DMMs. The front panel symbols for dc and ac are international (IEC) standards, but can be a source of confusion for users.

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 We will go through all these functions with hands-on exercises. The farad is the standard unit of capacitance. A nanofarad is (one one-billionth) of a farad.

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 Ω Press Hold Measure resistor Remove probes Measure second resistor Automatic Touch Hold / Shift (second function) Objectives: Immediate hands-on measurement exercise to explain TouchHold. We recommend you use a battery for measurements because, at this point in the presentation, we have not talked about safety, and the student has not yet tested 120 Vac. Hold updates automatically.

First look at the DMM Summary What we learned:
Meaning of front panel symbols Back panel safety warning and other information TouchHold & AutoHOLD functions - How they work

Multimeter safety Test leads & probes Fuses Overload protection
Chapter 2 Multimeter safety Test leads & probes Fuses Overload protection IEC standard

Safety inspection Test leads and probes
Check test lead resistance: Step 1: Insert leads in V/ and COM inputs Step 2: Select  , touch probe tips - good leads are  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 Objective: Get the student to think about the safety integrity of test leads and probes, often the weak point in the overall safety of the meter. Module 2 on safety focuses on the meter itself. Test leads are easy to neglect. However, a safety-designed meter with cheap or beat-up test leads, or leads not designed for high energy circuits, is like a new car with bald tires. Double insulation, finger guards, shrouds and recessed input jacks protect against electric shock from accidental contact with live circuits. Shrouds protect against the possibility that the probes could be connected across live voltage and the banana plugs be pulled from the meter. CAT ratings (III-1000 V or IV 600 V) are visible on the meter and probes. Test lead resistance is in the ohm range. A single lead can be tested by looping it between the V and COM inputs.

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 Objective: Introduce the concept of low input impedance on the ammeter circuit and why it requires fuse protection. It may be useful to recall that the DMM was originally designed with electronic circuits in mind, where it is often possible to open circuits and make series measurements. This is obviously not an option with power circuits. Module 4 on DMM measurements has a section on the use of the current clamp accessory. Since the input impedance is so low in the ammeter circuit, it presents a virtual short circuit when making a voltage measurement across a live source. Caution! Don’t leave the leads in mA or A input jacks and then take voltage measurements. Ammeter circuit inside DMM COM

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? Objectives: Measure low input impedance of amps inputs Quick fuse test. (Note: This test does not test for the wrong fuse being installed, as could happen if the original fuse was replaced with a low energy fuse. From a safety point of view it is preferable that a blown fuse not be replaced and the circuit left open if the correct fuse is not available.) Measurements: mA: = 2.0 ohm typ. A: = < 0.5 ohms typ.

What about protection for ohms measurement?
Safety inspection High impedance on V/ 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 current Low impedance branch = more current Objective: Introduce concept of high input impedance (High Z) on volts inputs. A high input impedance allows the voltmeter to draw a minimal amount of current when making a measurement. High impedance meters will generally not affect circuit performance. Safety implications: The voltmeter circuit has inherent overcurrent protection because of its high impedance. However, it requires overvoltage protection, both for steady state overvoltages and for transient overvoltages (see Electrical Measurement Safety module available on the Fluke Education web site at Fuses could be used in the voltmeter circuit. However, in addition to the cost, there is one major drawback: if the fuse were open, the meter reading could be taken to mean that the circuit being measured was dead! Measurement implications: High Z is especially useful for measuring low energy voltage signals where the source voltage itself would otherwise be drawn down by excessive load current (through the meter). A “stiff” source will by definition not be drawn down. Obviously, as far as the effect of voltmeter measurement on the circuit is concerned, electrical power sources are for all practical purposes ideal stiff sources. What about protection for ohms measurement?

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. Objective: Demonstrate the existence of overload protection. The student is now measuring 120 Vac. There is the real possibility of damage. In this mode, and having been sensitized to safety issues, he or she is asked to successively select Vdc, mVdc, ohms and amps. Typically, trainees are at least a little bit hesitant, and rightfully so. It is not intuitively obvious that the meter will not be damaged. Overload recovery is automatic. The key point to make is that the meter is protected against overvoltages (up to the meter’s rated voltage) as long as the leads are in the volts inputs.

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 Safety hazards are broken into two broad categories, “operator error” which is avoidable and “electrical environment” (later slide) which is unavoidable. 1. The mA/amps jacks on a DMM connect to a very low impedance test circuit inside the meter. When the low impedance of the DMM’s mA/amps input jacks accidentally are placed across a power circuit, they in effect form a short-circuit. See graphics slide entitled “Misuse of DMM in ammeter mode” from the Fluke Electrical Measurement Safety education module. You can demonstrate the input impedance of the amps inputs by measuring it. Put the meter into ohms mode and attach leads. Use the red lead (connected to the V/ohm input) and put the probe tip into the 10 A and the mA input (typical reading is 0.1 ohm and 10 ohms respectively). An OL reading indicates an open fuse. 2. Older analog meters would typically self-destruct if they contacted power circuit voltages while in ohms mode.

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

DMM specifications Display Accuracy Range and resolution Electronics
Chapter 3 DMM specifications Display Accuracy Range and resolution Objective: Explain some specification concepts as concisely as possible, and with hands-on when possible. Refer to the “ABCs of Digital Multimeters” application note from Fluke, which is part of the Basic Electrical Measurements module, available from the Fluke Education web site at Electronics Electrical

Understanding DMM display specifications
Display is specified as digits or as count Digits: 3 1/2, 4 1/2, etc. Example: 3 ½ - starting from the least significant digit, 3 “full” digits from 0-9, 1 “half” digit at less than 9. Example: 1999 Can be confusing: How do you specify 3999? Count: 6000, 5000, 4000, 3200, etc. 4000 count display reads from 3200 count display reads from Hands-on: count display Select V, measure battery 5000 count Objective: Clarify display specifications. Specification of counts is clearer than digits. The digit system is the traditional metric in the test and measurement industry, but it is confusing for students. An explanation of counts, accuracy and resolution is available in the “ABCs of Digital Multimeters” application note.

Understanding DMM accuracy specifications
Accuracy is specified in percentage Closeness with which an instrument reading approaches the true value being measured; largest allowable error Percentage of reading (DMMs) 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 = V % reading: 1 % accuracy with 120 V reading = V Least significant digit unstable: Example: Accuracy spec = +/-(1 % +2) Reading of Mv = mV 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 count display with +/- 1 % accuracy. Both meters are measuring 2500 mV: Meter 1 is in the 4000 mV range. +/- 1 % of 2500 mV is +/- 25 mV. Meter 1 reads from to 2475 mV to 2525 mV. Meter 2 in the mV range. +/- 1 % of mV is still +/ mV. Meter 2 reads from mV to mV, higher resolution, but no gain in accuracy.

Understanding DMM specifications
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: mV .1 mV (= 1/10 mV) V .001 V (= 1 mV) V .01 V (= 10 mV) V 0.1 V (= 100 mV) 1000 V 1 V (= 1000 mV) (To exit Manual Range, hold Range button for two seconds) 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 a 179 to step through the ranges manually. You can do this using a 1.5 V or 9 V battery. 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.

ABCs of DMM specifications
Summary What we learned: Display specifications - digits or counts Accuracy specifications - percent of range or percent of reading Range and resolution specifications – Low range, high resolution (e.g., mV) High range, low resolution (e.g., V)

DMM measurements Basic measurements: Ohm’s Law
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

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 = Objective: Introduce all three basic measurement modes: V, A, R. This exercise will require a calculator.

Special functions DMM as recorder: Min/Max/Avg
Capture sags: (>100 ms) Fluke Push MIN MAX button. (Meter beeps with each new MIN or MAX.) Scroll through Max, Min and Avg screens by pushing MIN MAX button. Record voltage sag as motor is turned on. MINMAX Objective: Demonstrate MIN MAX, a useful recording function of the DMM. They are typically used to capture intermittent events. Use the hair dryer as the load. Measure line voltage with the dryer off. Turn on MIN MAX, then turn on the hair dryer. Go back and look at the voltage drop. Applications: Min/Max is useful for capturing voltage sags or other intermittent events. Min/Max in the 179 is specified to record conditions lasting 100 ms or longer, which is about 6 cycles of 60 Hz. It records the rms value. Avg is the average of all readings during the period of recording. Some DMMs, like the 87V or 289, have a peak measurement function which can be used to detect voltage flat-topping. Voltage flat-topping is caused by single-phase converters charging capacitors at the peak of the wave. Since current is drawn at the peak of the voltage, the cumulative effect is that the voltage distortion appears as a flat-topped sine wave. If the voltage waveform becomes too flat-topped, it will no longer be able to charge the caps (i.e., the cap voltage will drop too low) which could cause equipment malfunction. The combination of voltage drop (long branch circuit runs) and flat-topping can be lethal for modern electronic equipment.

How DMMs measure voltage
Measuring volt / input impedance Step 1: Meter 1 (179) - Select ohms. Meter 2 - Select Vdc. Use meter 1 to measure input 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 Objective: (Note, this exercise requires two meters.) Measure high input impedance of meter. The input impedance is typically in the 10 megohms range. Note: The measurement will not work if Vac is selected instead of Vdc because of the coupling cap in the ac circuit, which blocks the dc voltage being sourced from the meter doing the R measurement.

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: Objective: Demonstrate phenomenon of ghost (coupled) voltage. Stray voltage (a.k.a. ghost voltage, coupled voltage) results from capacitive coupling between a live conductor and a dead one. The high impedance of the DMM will detect this coupled voltage in the dead circuit. However, the circuit is in reality de-energized, since the slightest load would draw the voltage down. Meter will display 60 Hz with no hard connection. After measuring voltage from hot to capacitively coupled ground (typ V), measure from neutral to capacitively coupled ground (typ mV). Use a “cheater” plug with ground prong removed. Ground will float roughly halfway between hot and neutral.

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 Objective: Explain how ohms mode is fundamentally different from volts and amps measurement, because the meter is sourcing voltage to make the measurement. The ratio-metric method is used in most, but not all, DMMs to measure resistance. The other method is using a true current source to source current through unknown. The meter measures the resultant voltage drop across the unknown resistance and calculates the value.

How DMMs measure resistance
Open circuit voltage First, measure “open circuit voltage” of meter when in ohms mode. Meter 1: V (dc) mode Meter 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? Objectives: (This demonstration requires two meters.) Measure meter source voltage in R mode. Demonstrate that the circuit must be dead for an accurate R reading. We know that a meter supplies voltage to a circuit in Resistance mode, but what exactly is this voltage? In this exercise, we measure it. Typical value is .65 V. This voltage may be different if using different model DMMs. When two meters are connected, there are two voltage sources in the circuit, and the meters will not give reliable readings. Sometimes the meter which makes the first reading will remain stable, but the situation is basically unpredictable.

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 Scale factor 1 milliAmp 1 milliVolt Objective: Clarify the difference in output signal between the ac clamps and the ac/dc clamps. The use of mA signal ac clamps and mV signal ac/dc clamps is the key to understanding use of clamp accessories. The other concept to understand is how the clamps use 1000:1 scaling. Current transformers (CTs) are passive devices. Hall effect transducers, on the other hand, output a small voltage signal proportional to the current flow. This signal requires amplification by an Op Amp which in turn requires a power source, hence the battery. The BNC style clamps are typically just basic CT clamps which have a built-in resistor network on the secondary that converts the mA to a mV signal. They are designed to be used with scopes and some power quality analyzers, but can be used with DMMs with a BNC-to-banana plug adapter. They would be plugged into the meter’s V inputs, not the A inputs. per Amp per Amp Sensor Current transformer Hall effect Battery No Yes

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. Objective: Explain how the DMM should be ranged when using ac clamps. Noise immunity: The mA signal is inherently more noise immune than the voltage signal. This is why the 4-20 mA signal is so often used in instrumentation, including in analog I/O in process controllers and PLCs (programmable logic controllers). True-rms is very important for accurate readings on non-linear loads. In fact, electricians who work in commercial buildings where these loads are prevalent, as well as in many industrial facilities, should always use true-rms DMMs with clamps, or alternatively, use a true-rms clamp meter. (See section on measurement of non-linear loads.)

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 True-rms measurement (of ac current) requires a true-rms meter. Objective: Explain how Hall effect clamps can be used to make ac only, dc only or ac and dc readings.

Measuring load current and inrush Plug the ac current clamp accessory into the meter: Fluke 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 Objective: Hands-on current measurements. Exercises: (Use line splitter, with 10:1 option as needed.) 1. Measure current drawn by a hair dryer: high and low settings. 2. Measure inrush of a drill motor: When making MIN MAX measurements, it may help to have the hairdryer on low before turning the drill motor on just to make sure everyone is in the right range, using the right line-splitter (x1 or x10), etc. Measurement notes: The amount of inrush current will vary depending on the type of load being measured. As a practical matter, inrush has two main effects: It can cause voltage sags and it can cause nuisance tripping of motor starters and controllers on motor start-up.

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, Ground current: Measure hot and neutral separately. Difference is leakage current. Assumes non-shared neutral. Inline current measurements (meter in series): Measure current through the DMM using a battery and resistor. Objective: Explain some common uses of current measurements. Explain how ground current can be measured indirectly, since the clamp may not have sufficient low-end accuracy for direct ground current measurement (i.e., currents under 1 A or so). Also explain that generally, inline measurements are only done for electronic circuits. Two of the requirements for inline measurements are that the circuit needs to be opened and de-energized to insert the meter. This could be a safety hazard in high energy circuits. The preferred method of measuring current in high energy circuits is to use a clamp-on current accessory with the DMM. To make an inline measurement with the DMM, use a 9 V battery and a 1 kilohm resistor in series. Place the meter in series with the components using the COM and mA input jacks. Set the function switch to mA ac then push the yellow button to switch to dc current.

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 °F or °C 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 °F or °C Internal 9 V battery Objective: Introduce methods of temperature measurement using DMM accessories. There are two ways to measure temperature: Non-contact infrared (uses DMM’s mV range) Thermocouple (contact – uses DMM’s mV range or temperature function in meters like the Fluke 179 with integrated temperature measurement)

Temperature accessories Type-K thermocouple temperature 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) Objective: Explain variety of thermocouple probes available (for more information visit the following web link by pasting it into your browser’s address window: These probes are the basic measuring sensor and can be used with dedicated thermometers or with DMMs equipped with a thermocouple adapter. If the DMM doesn’t have a temperature function, then an 80TK temperature module is required. If the DMM does have a temperature funtcion, an 80AK-A adapter may be required to connect to the K type mini connector of the probe.

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 °C/°F Temp function Objective: Demonstrate a DMM with built-in temperature measurement like the Fluke This is especially useful in industries like HVAC and process, where temperature measurement is common. In process and HVAC industries, MIN MAX is especially useful for temperature measurement and recording temperature changes over time.

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. Objective: Explain how the DMM measures capacitance.

Testing components Capacitors Fluke 179: Measurement note:
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 Objective: Hands-on measurement of caps. Higher range caps can be measured using the extended range of the Polarized caps should be measured with the red lead on the positive pole.

Testing components Diodes Diodes turn ac to dc. A good silicon
diode will have a voltage drop of approximately 0.5 V to 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. Objective: Introduction to diodes. Diodes are in the front-end of power supplies in electronic equipment. Their job is to turn ac to dc.

Testing components Diodes Forward bias = ____ V Reverse bias = ____ V
Red lead anode Black lead cathode Reverse bias = ____ V Shorted: 0 in both directions Open: OL in both directions Objective: Hands-on measurement of diodes. There are two steps involved in testing for a good diode: Forward bias V typically Reverse bias - OL Additional measurements: LED forward bias V typically (three diodes in series) Forward bias from one leg to other two identifies base of NPN.

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) Objective: Hands-on measurement of diodes. There are two steps involved in testing for a good diode: Forward bias V typically Reverse bias - OL Additional measurements: LED forward bias V typically (three diodes in series) Forward bias from one leg to other two identifies base of NPN.

DMM measurements Summary What we learned:
It’s the law: Mr.  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.

Measurement issues with non-linear loads
Chapter 5 Measurement issues with non-linear loads True-rms vs. average-sensing Crest factor

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? Objective: Introduce concept of non-linear or non-sinusoidal waves. Background: An average-sensing or average-responding meter is most accurate with an ideal sine wave. The more distorted or non-linear the sine wave is (voltage or current), the more inaccurate the average-sensing meter’s reading will be. True-rms meters will read either sine wave or non-sinusoidal waves with accuracy up to the bandwidth of the ac converter. The captured current waveform is typical of single-phase electronic loads. The voltage waveform is typical of branch circuits in commercial buildings where there are lots of computer and other non-linear loads. Supporting Document: “Why true-rms” application note, available on the Fluke Education web site at

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 the same for both ac and dc source if the Vacrms equals Vdc. Objective: Explain rms. Nominal voltages in distribution systems are given in rms, e.g., 120 V, 208 V, 230/240 V, 480 V, 575 V, etc., are all rms values. Equipment sizing relies on rms values.

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 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.

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.

What causes non-sinusoidal waveforms? Waveform distortion is caused by non-linear loads, which includes virtually all electronic loads: Switching-mode power supplies (PC, office equipment) Light switch dimmers and electronic ballast Variable speed drives The diode -capacitor input circuit draws short pulses of line current during the peak of the line voltage Objective: Using a typical single-phase non-linear load, explain the concept of non-linear load. The cap needs to recharge itself as the load draws it down. Current is only drawn by the cap at the peak of the voltage supply waveform. This peaking of the current waveform tends to flat-top the voltage waveform. This is a non-linear relationship between current and voltage. If the load were linear, then current would increase proportionally with voltage; in other words, the current waveform would look like a sinewave. The prevalence of harmonics-generating loads (non-linear loads), in modern electrical distribution systems, causes sine waves to become distorted. Virtually all electronic loads, unless they have additional circuitry specifically designed to minimize the problem, are non-linear. Another way of saying the same thing is that they generate harmonics in the process of rectifying ac to dc power. The reality is that a typical commercial or industrial facility has more and more non-linear loads and that electrical measurements in such an environment can only be made accurately with a true-rms meter.

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 meters or clamps accurately measure 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 three- phase diode rectifier 5 % to 30 % low Correct

What if the waveform is non-linear? Current measurement exercise: Measure these loads with true-rms and avg-sense clamp, noting differences: Linear load (hair dryer/drill, incandescent light bulb) Non-linear load (TV, monitor, PC, dimmer, CFL light bulb) Voltage measurement: Measure voltage using true-rms and average sensing meters while someone makes adjustments at the source. When are the readings closest and when do they differ? Objective: Hands-on measurement using true-rms and avg-sense meters. 1. The term non-linear means the same as non-sinusoidal. 2. Current: Linear measurements should be about the same. Non-linear loads - true-rms will read correct. Average-sensing meter will read low. 3. Voltage: Connect the light dimmer at the source, not at the load. Connect the first power strip in the string into the light dimmer’s extension socket. True-rms will read lower than the avg-sense. Note: The difference will vary as the light is dimmed. The readings will be closest when the light is full bright, since at that point the light is seeing a full sine wave and distortion is minimal (there might still be some distortion from the building voltage source itself). As the light is dimmed, the difference will increase up to some maximum point. This is because the phase-control waveform, shown in the graphic, becomes very non-linear.

What is crest factor? Crest factor = Peak / rms For ideal sinewave, CF = 1.414 Objective: Explain CF. Crest factor is an important specification for true-rms responding instruments.

What is crest factor? For this current waveform, crest factor = 2.9. Objective: CF for a common current waveform.

Crest factor is an indication of harmonics For current or voltage measurements, the higher the CF, the greater the waveform distortion. CF spec is important for accurate measurements. It is only specified for true-rms products. It is more critical for current measurements since harmonic distortion typically is higher for current than for voltage. 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.

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 CAT III-600 V minimum rating Distribution level: power distribution equipment. Objective: Summary slide. Crest factor is specified for true-rms measurement devices. 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 or DMM, a signal with CF of 3 or less will be read accurately. However, at lower range, the clamp or DMM 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 or equal to 6 and it will still be accurately measured. Specifications subject to change without notice. 9/ B AO-EN-N

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