Electrical Theory Lesson 2: Basic Instruments and Measurements
This document contains the transcript for the entire lesson.
Page 1: Welcome to Lesson 2 of Electrical Theory. This lesson covers the following objectives:
• Explain the correct procedure for using an ammeter, a voltmeter, and an ohmmeter. • Interpret a linear scale. • Compute shunt resistor values. • Compute multiplier resistor values. • Interpret a nonlinear scale. • Discuss the concept of meter sensitivity. • Understand basic electrical diagrams. • State and explain Ohm’s Law
Page 2: Meters come in two formats; analog and digital. Analog meters use a continuous scale readout that must be interpreted by means of a needle, while the digital multimeter uses a liquid crystal display (LCD) readout that needs no interpretation.
Page 3: A common type of meter movement is the D'Arsonval movement. The movement consists of a permanent‐type magnet and a rotating coil in the magnetic field. An indicating needle is attached to the rotating coil. When a current passes through the moving coil, a magnetic field is produced. The field reacts with the stationary field and causes rotation (deflection) of the needle. This deflection force is proportional to the strength of the current flowing through the coil. When the current ceases to flow, the moving coil is returned to its "at rest" position by hair springs. The coil that rotates in the magnetic field is mounted on precision‐type jewel bearings, much like a fine watch. The jewel‐type bearings and mount is known as a D'Arsonval movement.
Page 4: When connecting a meter to an electrical circuit, proper polarity must be maintained. The meter is equipped with polarity marking, usually a red plus sign (+) and a black negative sign (‐). Some meters use the abbreviation COM, which stands for common, for the negative polarity marking. The meter coil rotates inside the permanent magnet field. If proper polarity is not used, the coil will deflect in the direction opposite to that which it was designed.
Page 5: The operation principle of the iron vane meter movement is shown here. Two pieces of the iron are placed in the hollow core of a solenoid. When the current passes through the solenoid, both pieces of metal become magnetized with the same polarity. Because like poles repel each other, the two pieces of iron are repelled from each other. One piece of metal is fixed in its position. The other piece of metal pivots. The pivoting piece can turn away from the fixed metal. An indicating needle is attached to the moving vane. The needle is equipped with hair springs so that the vane must move against the spring tension for accurate readings. An applied voltage causes current to flow in the solenoid and creates the magnetic field. The moving vane is repelled against the spring according to the strength of the magnetic field. The needle may indicate
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Electrical Theory Lesson 2: Basic Instruments and Measurements
This document contains the transcript for the entire lesson.
either voltage or current. When the iron vane movement is used for a voltmeter, the solenoid is commonly wound with many turns of fine wire. Proper multiplier resistance may be used to increase the range of the meter. A selector switch is used to select proper ranges. When used as an ammeter, the solenoid has a few turns of heavy wire. This is because the coil must be connected in series with the circuit and carry the circuit current. The iron vane meter movement always deflects in the same direction regardless of polarity. Either AC or DC may be measured with this instrument. This type of meter is best suited for high power circuit measurements.
Page 6: The meter scale used to interpret ampere and voltage values is the linear type. A linear meter scale has evenly spaced marks used to indicate the amount of current flowing or voltage present, in the meter movement. This figure shows a typical linear scale for an ammeter. To determine the value of each mark between the major divisions, divide the value of the first major division by the number of spaces in that division. The dial to the right of each scale in this figure is the range selector. The range selector must be correlated to the scale to determine full scale deflection. As the range switch is rotated through the different ranges: – 5 amps, 0.5 amps, and 0.05 amps, interpreting the scale also changes with each major divisions – 0, 1, 2, 3, 4, and 5 and minor divisions. For example, with the scale on 5 amps, then each minor division becomes 0.1 amps, and changing to the 0.5 amp scale, each minor division becomes 0.01 amps.
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Page 8: An ammeter measures electrical current in a circuit. An ammeter will usually measure in amperes, milliamperes, or microamperes, depending on the scale or design of the instrument. The coil in the meter movement of an ammeter is wound with many turns of fine wire. If a large current is allowed to flow through this coil, the ammeter will quickly burn out. In order to measure larger currents, a shunt, or alternate path, is provided for current. Most of the current flows through the shunt, leaving only enough current to safely work the movement coil. The shunt is a precision resistor connected in parallel with the meter coil.
Page 9: This figure shows the proper way to connect an ammeter to an electrical circuit. When an ammeter is connected into the circuit, it becomes part of the circuit in order to allow the current to flow through the meter coil. You are connecting the meter in series with the circuit of device you are trying to measure.
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Page 11: The same basic meter movement that is used in an ammeter is also used to measure voltage. This is providing that the impressed voltage across the coil never exceeds 0.1 volt, as computed, for full scale deflection. To arrange the meter to measure higher voltages, multiplier resistors are placed in
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Electrical Theory Lesson 2: Basic Instruments and Measurements
This document contains the transcript for the entire lesson.
series with the meter movement coil, using a switch. Voltmeters are always connected in parallel with the device being measured.
Page 12: A voltmeter is always connected in parallel or across the circuit. When measuring with the meter set on its highest range. Adjust downward to the proper range to avoid damaging the meter. In addition, be sure that the leads are connected with the correct polarity. The black lead is negative and the red lead is positive.
Page 13: The sensitivity of a meter can be used to gauge meter quality. A quality meter has a sensitivity of at least 20,000 ohms/volt. Precision laboratory meters measure as high as 200,000 ohms/volt. Accuracy, of the meter is commonly expressed as a percentage, such as 1 percent. This means that the true value will be within one percent of the scale reading. Another system of rating meters is the accuracy expressed as a percentage of full scale reading. A meter may have a rating of ±0.05 percent or less. In general, the smaller the percentage, the higher the quality of the meter.
Page 14: When a voltmeter is connected across a circuit to measure a potential difference, it is in parallel with the load in the circuit. This situation can introduce errors in voltage measurement. In this figure two 10,000 ohm resistors from a voltage divider circuit across a ten volt source. The voltage drops across both R1 and R2 are 5 volts each. If a meter with a sensitivity of 1000 ohms/volt on the ten volt range is used to measure the voltage across R1, the meter resistance will be in parallel with R1. The addition of this meter cuts the effective resistance of R1 in half. The combined resistance of the meter and R1 is equal to what’s shown here:
With the meter connected, the total circuit resistance becomes what’s shown here:
Using Ohm’s law, the current can be calculated at approximately 0.00067 amps. Using Ohm’s law again,
ER1 = 3.35 V and ER2 = 6.7 V. The meter has caused an error of more than one volt due to its shunting effect.
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Page 16: A meter used to measure the value of an unknown resistance is called an ohmmeter. The same meter movement that was used in the volt and ammeter can be used for the ohmmeter. A voltage source and a variable resistor are added to the ohmmeter's circuit. A series type ohmmeter is shown here. A three volt battery is used as the source for the ohmmeter. The battery is built into the meter case. The meter movement permits only 0.1 volt for the current of 0.001 amps for full scale deflection. Therefore, a multiplier resistor is placed in series with the meter coil to reduce the voltage applied to the meter coil. The 2900 ohm multiplier resistor, plus the meter coil resistance, is equal to 3000 ohms. Part of this resistance is made up of a variable resistor to allow the total resistance to vary. Because temperature changes or weak batteries can affect the total resistance of the circuit, the
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Electrical Theory Lesson 2: Basic Instruments and Measurements
This document contains the transcript for the entire lesson.
ohmmeter must be calibrated before each use. To use the ohmmeter, first short the test leads together. This applies 0 ohms across the meter. Adjust the ohms adjustment knob until the needle points at zero.
Page 17: Before connecting an ohmmeter to any electrical circuit be sure that the circuit is not energized. The electrical energy in a circuit is not needed to operate the meter movement coil as it is when using a voltmeter or an ammeter. The batteries inside the case provide the source of power for the ohmmeter. Connecting the ohmmeter to an energized circuit will apply the circuit voltage directly to the coil and battery, which can result in damage to the meter.
Page 18: A shunt ohmmeter is connected as shown here. In this circuit, the unknown resistance Rx is shunted, connected in parallel, across the meter. When the ohmmeter is connected in the shunt position, the indicating needle deflects from left to right in the manner of the ammeter and voltmeter. Zero resistance is on the left. The scale increases from left to right.
Page 19: The resistance value is indicated on the ohms scale, which is a nonlinear scale. A nonlinear scale has markings that are not evenly spaced. The nonlinear scale factor increases as the needle travels from zero resistance to infinite resistance. This image shows a typical ohmmeter scale. On the right side of the scale is zero. On the left side is infinity (∞). An infinity reading means that the resistance value is so high that it exceeds the capabilities of the ohmmeter to read it. An ohmmeter comes with a selection of ranges that can be changed by rotating the selector switch. Typical range values are R X 10, R X 100, R X 1k, and R X 10k. These markings mean that the reading indicated on the ohm scale should be multiplied by 10, 100, 1000, and 10,000 respectively.
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Page 21: A common and simple multimeter is used in electronic circuits is the volt‐ohm‐milliammeter, or VOM. A VOM is a voltmeter, ammeter, and ohmmeter, all in one. The VOM has the advantages of being inexpensive and portable. It does, however, usually have a low input resistance (in ohms per volt) on the lowest voltage range. This factor can cause accuracy problems.
Page 22: Digital multimeters (DMM) are the most commonly used meters in the electronics field today. They are rapidly replacing the analog meter. The DMM uses modern electronic circuitry to take electrical measurements and display values, usually on a liquid crystal display screen. Digital meters are more rugged and smaller in size than analog meters. They are also very accurate and very portable.
Page 23: The liquid crystal display shows the meter reading in digits rather than on a scale. Some DMMs simultaneously display digits as well as a bar graph that simulates a linear scale reading. Digital meters not only measure volts, ohms, and current, but can also test electronic components such as transistors and diodes. Most digital meters use an international standard of labels to indicate various meter functions such as AC, DC, and combinations of symbols as shown here. Many digital multimeters are
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Electrical Theory Lesson 2: Basic Instruments and Measurements
This document contains the transcript for the entire lesson.
equipped with protective circuitry to prevent accidental damage when the wrong function is used to take readings. Polarity is usually not an issue when using a digital meter. The meter will automatically adjust for an incorrect polarity, or it will flash a message or symbol on the liquid crystal display, warning the user of the wrong polarity. Some digital meters have an auto range feature. Resistance reading using a DMM still requires you to disconnect the circuit from the power source to prevent damaging the meter.
Page 24: Some manufacturers offer interface cards that can be installed in a personal computer and have test leads similar to meter leads. After the interface board is installed in an expansion slot in the computer, software is loaded. Now the computer will display simulated meters on the monitor. The computer can then be used to take voltage, current, and resistance readings. With the computer you can use the memory and hard drive system to store measurements and retrieve them later.
Page 25: When AC is applied to the meter movement, the needle does not deflect. The meter coil current changes direction in pace with the applied AC voltage. The result is that the magnetic field rises and collapses, and then reverses so rapidly that the coil cannot deflect the needle. The coil simply vibrates under the influence of applied AC voltage. To remedy this, a meter changes the AC voltage into DC by means of a rectifier. A rectifier converts the applied AC current to a DC current of equal value. When the applied AC current is rectified to an equal DC value, the value is referred to as the rms value. The abbreviation, rms, stands for root mean square. Many meter scales, and some DMMs, use this abbreviation. Some meters have a special location to plug in the meter lead when reading AC voltage or current.
Page 26: Resolution is a term that describes the degree of change that must take place before the meter will display the value. For example, if a meter has a resolution of 1/1000th, it can measure a voltage down to 1 millivolt. In general, the better the meter resolution, the more expensive the meter. In digital meters, resolution is also determined by how many digits the meter can display. A digital meter with a five‐digit readout has a better resolution than a digital meter with a four‐digit readout.
Page 27: There are a number of important details to remember when using these meters. An analog meter is a delicate instrument. Handle it with care and respect. When measuring voltage, the meter must be connected in parallel to the device being read. Remember to observe correct polarity. When measuring current, an ammeter must be connected in series with the circuit. When measuring resistance, be certain that no power is applied to the circuit. Always adjust and zero the meter on the proper range before measurements are made. A meter has its greatest accuracy at about two thirds deflection on the meter scale. And never work alone when dangerous voltages are present. When using a multimeter or DMM, it is easy to connect the meter to a voltage source immediately after taking a resistance or current reading. This is the most common mistake made when using a multimeter or DMM. This action will result in damage to the meter or personal injury.
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Electrical Theory Lesson 2: Basic Instruments and Measurements
This document contains the transcript for the entire lesson.
Page 28: Almost all electrical systems contain a ground. A ground provides a safe path for an electrical fault and has a 0‐volt potential. It is typically constructed by connecting a conductor between one of the electrical system conductors and the earth. On AC systems, the neutral conductor is grounded. On DC systems, the negative conductor is grounded. Some electrical systems do not connect directly to the earth. Instead the metal enclosures or metal conduits serve as a ground or a return path for current. If a person touches an energized conductor, that person can easily serve as a path to ground, as shown in the image and experience electric shock. Also, if a system experiences an electrical fault, the metal enclosure or conduit may have a higher voltage potential than the earth ground. Thus, a person touching the enclosure or conductor can experience electric shock. Some devices are double insulated, which means the outside enclosure is not grounded. A person using the device will not be in contact with the ground circuit by using the device.
Page 29: A person can experience different levels of electric shock. These are rated by value of current flow and the effect that each current value has on the human body. Click on the different levels for more information.
Page 30: One of the best ways to avoid fatal electric shock is to keep one hand in your pocket while working with high voltage. This way, any current flowing through your body does not flow through one hand and out the other. The amount of resistance through a person’s body depends on many different things. A person’s skin condition is the most influential resistance factor. A person wit dry skin has a high resistance of about 100,000 ohms. When the surface of the skin is covered with sweat, the resistance can be 10,000 ohms or less. The National Electrical Code (NEC) requires all electrical systems of 50 volts or higher to be grounded. A properly grounded electrical device will produce a low‐resistance path for current. When the properly grounded device comes into contact with an energized conductor, the fuse or breaker will automatically trip or open the circuit.
Page 31: A ground fault interrupter (GFI) provides protection from excessive fault currents through the human body. The NEC requires ground fault protection for specific locations where there is a high probability of electric shock, such as damp or wet locations. A ground fault interrupter provides protection by monitoring and comparing the current through the hot and neutral conductors. A complete circuit has the same current in the hot and neutral conductors. If a ground fault occurs, part of the current will flow to ground. When part of the current flows to ground, the comparator circuit detects an unbalanced condition between the hot and neutral currents. If the difference between the hot and neutral conductor exceeds 5 mA, the comparator circuit will energize the trip coil and cause a contact to open in the hot conductor circuit. This stops the flow of current. After the GFI is tripped, the red reset button needs to be pressed to reset the GFI trip mechanism once more. Ground fault protection devices are commonly incorporated in GFI‐style circuit breakers. Non‐GFI power outlets can be used with
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Electrical Theory Lesson 2: Basic Instruments and Measurements
This document contains the transcript for the entire lesson.
single GFI‐type breakers. The non‐GFI power outlet will then provide the same protection that a GFI outlet provides.
Page 32: When working with electrical systems, there is always a real danger of electrocution. The heart may stop beating because of electric shock. Cardiopulmonary resuscitation (CPR) is an emergency technique consisting of mouth‐to‐mouth resuscitation and a series of chest compressions. The basics of CPR will be covered in this section, but you should take a CPR course from a certified instructor to be competent to administer CPR.
In case of an accident:
Call for help. Check to see if live wires are still in contact with the victim, and move any away with a nonconductive item. Check to see if the victim is responsive.
If the victim is responsive, he or she most likely doesn't need CPR. However, if the victim is not respon‐ sive and not breathing, you will need to administer CPR until help arrives. To administer CPR, you will need to:
Ensure the airway is clear. With the victim lying on his or her back, tilt the head back and lift the chin. Place your mouth over the victim's and give two short breaths.
Position your hands in the center of the victim's chest and firmly press down
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Page 34: Electrical diagrams convey specific information to the technician. They illustrate such items as the size, type, component part number, and component location in relationship to the other circuit components. Diagrams can be used for installation, fabrication, troubleshooting, or to explain the circuit's operation or purpose. Symbols are used to represent circuit components. Wires or conductors are usually shown as lines. Their connections can be shown a number of ways as shown here. One primary type of electrical drawing you will encounter is the schematic diagram. Shown here is a typical schematic diagram. It shows w at parts are needed and how they connect to one another. The main purpose of the schematic is to show how the components relate to each other. The diagrams show which components are in series or parallel with each other. Schematics are an extremely valuable troubleshooting tool. The combination of meters, wiring diagrams, schematics, and electronic theory allow a technician to find circuit problems.
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Electrical Theory Lesson 2: Basic Instruments and Measurements
This document contains the transcript for the entire lesson.
Page 35: Here is a comparison of an elementary line diagram and a wiring diagram. The elementary line diagram is similar to a schematic. It is used primarily in industrial processes to illustrate how a system’s electrical controls relate to each other. The wiring diagram would be used to connect the control system. The elementary diagram clearly illustrates how the circuit operates, while the wiring diagram illustrates the relative positions of the connection points and components as they would actually be found in the equipment.
Page 36: Sometimes a block diagram is used to show how an overall system works. Here is a typical plan of the electrical circuits to be installed in one room of a residence. When constructing an electrical system, you may find using a circuit design software program beneficial. Software systems not only can be used to draw out electronic circuitry, they can actually be used to simulate the circuit as though it was constructed with electronic components. Virtual meters can be connected to different points in the circuit for experimentation and testing. A complete list of materials can be generated from the circuit design. The pattern required for a printed circuit board can be printed. This makes the design and testing process quicker and easier than if the circuit was built using actual components. Once the circuit design is tested to satisfaction, the circuit can be built using actual components
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Page 38: In this lesson you covered the following objectives.
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