Residential Cabling Technologies Copyright © 2005 Heathkit Company, Inc. All Rights Reserved Electrical Measurements

2 Objectives At the end of this presentation, you will be able to:

3 Define the following terms: ammeter, ohmmeter, voltmeter, open, and short. Describe how to properly use an ammeter. Describe the proper way to use a voltmeter. Describe how to properly use an ohmmeter.

4 Describe how shorts and opens affect circuit operation. Explain the advantages of an oscilloscope compared to an AC voltmeter. Describe why proper safety precautions are essential when you take measurements.

5 Measurement Basics Electrical measurements serve two important purposes: Determine if the circuit is operating properly Provide clues as to why the circuit is not operating The three most basic measurements are current, voltage, and resistance.

6 Meters To take accurate measurements in a circuit, you need a meter. There are two major classes of meters; Analog and Digital. Meters can be specialized or combined.

7 Analog Meters Analog meters use a needle that is deflected across a scale by the factor being measured. Analog meters introduce a certain amount of error in the process.

8 Here you see an example of parallax error. Diagram A represents the correct viewing position. Diagram B illustrates the effect of viewing from the right.

9 Digital Meters Digital meters perform the same basic functions, but provide a digital display or readout. The digital display helps to reduce errors in the measurement.

10 Multimeters Multimeters combine several measurement functions into one unit. Typically three meters are combined into one unit. Multimeters are available in analog and digital models.

11 Using a Meter Read the instruction manual before you use the meter for the first time. Observe correct negative and positive polarities when you measure voltages and current. Always observe the manufacturers recommended safety guidelines when you use a meter. When you need to measure an unknown quantity, select the highest range available on the meter.

12 A Meter Overview Analog meters are usually multimeters. Models are made for special applications that perform only one function. Meter Range Select

13 Range Scales Meter Needle

14 DC mA Ranges Zero Adjust Inputs Resistance Ranges DC Volt Ranges AC Volt Ranges

15 Zeroing the Meter Analog ohmmeters should be zeroed before you take any measurements. To do this: 1. Select the ohm function and range. 2. Connect the test leads together. 3. Adjust the control until the needle indicates zero.

16 Digital multimeters come in two major styles: Manual and Auto-ranging. The manual meter requires the user to select the proper function and range. Display Range & Function Select Range & Function Select Inputs

17 Transistor Tester Current Ranges Voltage Select Voltage Ranges Capacitance Ranges Capacitance Ranges Capacitor Reader Ohm Ranges Ohm Ranges

18 This is an example of an auto-ranging meter. Here, the user selects the function with the dial and the meter selects the correct range.

19 How you connect the meter’s test leads is as important as measurement accuracy. Three inputs are typical : common, current, and voltage/resistance. Current Input Voltage/Resistance Input “Common” Input

20 Meter Functions Three functions are common to most DMM models in use today: Ammeter –measures current flow in a circuit. Voltmeter –measures voltage rises and drops in a circuit. Ohmmeter –measures resistance in a circuit.

21 Ammeter Function Ammeters measure current in a circuit. Current measures a quantity of electrons. A coulomb is the basic unit of electrical charge. One ampere of current flows when one coulomb of electrons passes a point every second.

22 To measure current, you must connect the meter in series with the circuit under test. You must observe the proper polarity. You must make the connection without power applied to the circuit. A

23 To measure the current in a circuit: 1. Remove power. 2. Select the correct function and range. 3. Break the circuit. 4. Connect the meter. 5. Re-apply power and take the measurement. A

24 Voltmeter Function The voltmeter function is used to measure AC and DC voltages in a circuit. When you measure a voltage drop, connect the meter leads on either side of the load. When you measure a voltage rise, connect the meter leads across the voltage supply. Always observe correct polarity when you measure voltages.

25 To measure a voltage, you must connect the meter in parallel with the circuit under test. You must observe the proper polarity. You can make the connection with power applied to the circuit. You must take care when you measure live circuits. V

26 To measure a voltage in a circuit: 1. Select the correct function and range. 2. Determine the proper polarity. 3. Connect the test leads in parallel. 4. Take the measurement. V

27 1. Select AC 2. Select Range 3. Connect Leads

28 1. Select AC 2. Select Range 3. Connect Leads

29 Ohmmeter Function The ohmmeter function is used to measure resistances in a circuit. When you measure a resistance, you must isolate the component you want to measure. When you make the measurement, connect the meter leads across the component. NEVER attempt to measure resistance while power is applied to a circuit.

30 To measure a resistance in a circuit: 1. Select the correct function and range. 2. Disconnect power. 3. Isolate the component. 4. Connect the test leads. 5. Take measurement. 6. Switch the meter off. 1 1 

31 Shorts and Opens Ohmmeters are also used to detect two types of defective circuit conditions: shorts and opens. A short condition occurs when a portion of the circuit that should offer a resistance appears as if no resistance were present. An open condition occurs when a portion of the circuit that should conduct current acts like an open switch.

32 A switch can be used to simulate an open or a short condition. An open appears as an infinite resistance. A short appears as zero resistance. 

33 Safety Safety should be your number one concern in electronics. Current always seeks the path of least resistance to ground. The effects of current and voltage can be influenced by the environment.

34 Introduction to Oscilloscopes Voltmeters measure current and voltage. Voltmeters provide no information about the waveform. To properly diagnose AC problems, it helps to view the waveform.

35 The oscilloscope was developed to solve these problems. Oscilloscopes are available in different sizes and styles. Both Analog and Digital models are available.

36 Analog oscilloscopes are based upon the CRT or Cathode Ray Tube. The CRT uses essentially the same technology as a television set. The CRT converts the input signal into a visual display that replicates the input signal.

37 Trace and Blank Circuits Trace and Blank Circuits Input signal Amplifier Circuits Amplifier Circuits CRT

38 Input Y-Amplifier Time base

39 We need a means to measure the signal that appears on the CRT screen. The Graticule is a grid that is positioned in front of the CRT screen. The graticule is divided into squares to provide a reference for measurement of the signal.

40 This is a typical CRT graticule. Usually the grid consists of 1 cm squares. The horizontal axis is referred to as the X-axis and the vertical axis is the Y-axis. Y-axis X-axis

41 controls

42 Using the Oscilloscope An oscilloscope requires a different set of skills. The basic functions are time and amplitude measurements. Advanced features may require specialized training.

43 Oscilloscope basics: Always read the user’s manual before you attempt to operate an oscilloscope. Always be aware of the limitations of the oscilloscope. Always anticipate the magnitude of the signal you want to measure. Observe all electrical safety precautions as well as any additional precautions provided by the scope manufacturer. Input probes should be grounded to a common circuit ground.

44 The input cable connects to the scope with a BNC connector. Positioning controls allow you to make precise alignment. Volts/Div allows you to set the voltage per division. BNC Connectors Positioning

45 The Volts/Div setting allows us to measure amplitude. If Volts/Div =1 Peak-to-Peak amplitude is 6 divisions or 6 volts.

46 The Time/Div control adjusts the timing per division. Positioning controls allow us to make precise alignment. External triggering is an advanced feature. External Trigger Position Time Selection Display Adj.

47 The time/Div setting allows us to measure frequency. If Time/Div = 20  S The wave period is equal to 84 microseconds (4.2 x 20).

48 Alternate Oscilloscopes Unique testing and flexibility requirements have resulted in several new types of oscilloscopes. Some are designed to meet specific performance requirements. Others are designed with flexibility in mind.

49 Hand-held oscilloscopes are smaller and more portable. They offer a more limited number of functions. They have unique safety considerations.

50 Digital PC-based oscilloscopes come in a variety of styles. These are software- based devices with hardware interfaces. The functions and features depend upon the cost and quality of the software.

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