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MODULE 1 : SAFETY; USE OF POWER SOURCES AND METERS
At the end of this experiment, the student should be able to: TLO 1: Identify safety hazards common to the general shop and electronics laboratory. TLO 2: Measure the electrical resistance of your body using the ohmmeter function of the Electronic VOM. TLO 3: Familiarize yourself with the safety precautions to be used when working with electronic circuits. TLO 4: State the proper operating procedure when using a power supply. TLO 5: Demonstrate the correct method of connecting voltmeters and ammeters for circuit measurement. INTRODUCTION: Safety is everyone’s responsibility. Everyone must cooperate to create the safest possible working conditions. Where your personal life and good health are concerned, safety becomes your responsibility. Whether you step in front of a speeding truck, or expose yourself to a lethal electric shock, are matters over which you, as an individual, have more control than anyone else. The safety rules and discussion in this Laboratory Exercises apply to individual students as well as the class as a whole. Power supplies are used to provide power for experimental purposes, for testing, and for repair. Although there are hundreds of different kinds of power supplies in general use, they all have much in common. Meters are essential for measuring product performance and in troubleshooting defective circuits. Like power supplies, all meters of a type are about the same and differ only in range, sensitivity, and circuit protection. All instruments used for industrial and laboratory testing have some method of indicating the results of the tests they are designed to perform. The most common indicating device is a scale. The scale can be graduated to indicate volts, ohms, amperes, watts, decibels, gallons, miles or any desired quantity. Multifunction test instruments may have several scales and measure several properties. One scale could be for volts, another for amperes, another for ohms and still another for decibels. The instrument usually has switches for changing functions and ranges. The general term for these instruments is multimeter; however, when a multimeter is designed to read volts, ohms, and milliamperes, it is often called a VOM. An ammeter (the name is a contraction of ‘ampere-meter’) is a device for measuring the electric current through a wire or a circuit element. The voltage drop across the ammeter itself disturbs the circuit into which it is plugged in, and such disturbance may change the very current the ammeter is used to measure. To minimize the disturbance, the voltage across the ammeter should be as small as possible, which requires that the electric resistance of the ohmmeter should be as small as possible.
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A voltmeter is a device for measuring potential difference (the voltage) between two wires, usually across a circuit element or a group of elements. The current flowing through the voltmeter itself disturbs the circuit into which it is plugged in, and such disturbance may change the very voltage the voltmeter is used to measure. To minimize the disturbance, the current through the voltmeter should be as small as possible, which requires that the electric resistance of the ohmmeter should be as large as possible. An ohmmeter is a device for measuring electric resistance of an isolated circuit element or of a simple circuit without any active power sources. A simple ohmmeter consists of a battery, an ammeter, and a safety resistance Rs, all connected in series with the resistance being measured: ENGAGE
- Why is electrical safety very important at home and at the school or workplace?
- In what ways do you practice electrical safety and best practices. EXPLORE
- Refer to this module and read in advance to have a better understanding of the activities to be performed.
EXPLAIN & ELABORATE
EQUIPMENT/ MATERIALS NEEDED:
Power Source - 0 – 6 Vdc, 10 mA Ammeter - 0 – 10 mAdc Voltmeter - 0 – 10 Vdc Electronic VOM Practical Electronics Trainer R1 - 1 kΩ, 1W S1 - SPST, Component Board M Universal Experiment Board K
PROCEDURES:
Note: In the absence of an actual laboratory set-up, you may answer the module based on the concepts learned from the lecture subjects. As a verification, perform a simulation with your simulator of choice. For every circuit, attach a screenshot of the set-up in your report. Examples of simulators: LTspice (Free), Electronic Workbench, Circuit Construction Kit: DC at phet.colorado.edu
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DEATH LABORED BREATHING PAINFUL SENSATION THRESHOLD EXTREME BREATHING DIFFICULTIES SEVERE SHOCK CANNOT LET GO MILD SENSATION MUSCULAR PARALYSIS
Amperes Fig. 1 – 1
- Current is forced through the resistance of a circuit by voltage, which is electrical pressure or force. A lower resistance in the circuit allows more current to pass through the circuit for a given amount of voltage.
- If you think of a human body as a circuit then the amount of current that can flow between any two points on the body must depend on the resistance between the two points at that time and the amount of voltage or electrical pressure applied.
- Your instructor will show you how to use the ohmmeter function of the Electronic VOM. When you understand how to use it, measure the resistance between the following points on your body: Right hand to left hand ………..... R = _________ ohms Right (left) hand to right (left) ankle ... R = _________ ohms
- Wet the contact areas and measure the resistance again.
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_Right hand to left hand ………….. R = _________ ohms Right (left) hand to right (left) ankle .. R = _________ ohms Is the resistance lower when the areas are wet? __________ If the entire body surface was wet would the resistance be even lower?
Your body resistance is quite low if you are perspiring and your pulse rate is high. Under certain conditions the body’s resistance can be as low as few hundred ohms._
- From the chart of Fig 1 – 1 you know that 0.1 ampere of current can be fatal. Let’s use a form of Ohm’s Law to determine how much voltage is necessary to force 0. ampere of current through the resistance of your body. We’ll use the equation : Voltage = Current x Resistance Since current is 0.1 ampere the formula becomes voltage = 0.1 x resistance.
- Substitute the resistances measured in step (5) into the formula to calculate the voltage necessary to force 0.1 ampere of current between the selected points of your body. Right hand to left hand voltage = 0.1 x _______ = ________ volts Right (left) hand to right (left) ankle voltage = 0.1 x _______ = ________ volts
- Now substitute the resistances measured in step (6). Right hand to left hand voltage = 0.1 x _______ = ________ volts Right (left) hand to right (left) ankle voltage = 0.1 x _______ = ________ volts Obviously you should not test your results. But bear in mind that voltages much lower than those calculated above can cause death under certain circumstances. TLO 3: Familiarize yourself with the safety precautions to be used when working with electronic circuits.
- Probably the best advice you can follow when working with electricity is THINK - - - - - - - PLAN AHEAD
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- Your power sources, when properly handled, will provide years of reliable service and will present no danger to you. The following is a general description of power sources. a) Most power supplies have on – off switches. A panel lamp is generally provided to show when they are on. b) All variable power supplies have some type of control for varying the output voltage. Most of them use variable control which permits setting any desired voltage within the range of the supply. There are some, however, that have switches for selecting a number of set voltage levels. c) The power circuits are usually protected against overload by fuses, circuit breakers, current limiter or similar devices. d) Power is generally available from two or more terminals. They may be binding posts; banana jacks, or power receptacles. e) Power sources used in industrial shops and laboratories should have their enclosures and panels connected to electrical earth ground by means of a three – wire input system or a separate ground lead. The power supplies recommended for use with this laboratory manual use a three-wire input system.
- Proper operation of power sources. a) Before turning the power supply on, carefully inspect your test set - up. Make sure the power supply and meter leads are connected with the correct polarity. b) Make certain all variable voltage controls are set for minimum voltage before applying power. c) After the supply is turned on, slowly advance the variable voltage control to the desired voltage. d) If the output voltage doesn’t change as the voltage control is advanced, check the circuit breaker, it might be open. The on – off lamp shows only that power is applied to the input of the power supply, it does not indicate the presence of voltage at the output terminals. e) Before resetting the circuit breaker, return the voltage control to the minimum setting, and correct any cause of overload. Then press and release the reset button.
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f) If, instead of a circuit breaker, the output terminals are protected by a fuse, check with your instructor before replacing the fuse. TLO 5: Demonstrate the correct method of connecting voltmeters and ammeters for circuit measurement.
- Meters are used to measure quantities or values. The quantities usually measured the current, resistance and voltage. Each measurement requires that the meter be connected into the circuit in a certain way. Be sure to know what you want to measure and how you’re going to do it before connecting the meter and turning on the power.
- Check the polarity of the test leads before you apply power to the circuit. This simple precaution can prevent serious damage to a meter.
- Check the range setting of the meter before applying power to the circuit. The range must be higher than the value being measured, otherwise the meter pointer will be “pegged” at the upper edge and may be damaged.
- Voltmeters are used to measure voltage or electromotive force (emf). This is the electrical force that moves current through the resistance of a circuit. The unit of electronic force is the volt.
- Voltmeters are connected across (in parallel with) the component or circuit as shown in Fig. 1 – 2. + - 6Vdc R1 1K + - VOLTS S VOLTMETER CONNECTS IN PARALLEL ACROSS LOAD RESISTOR R Fig. 1 – 2
- It is important to connect the meter leads to match the polarity of the circuit being measured. In Fig. 1 – 2 the voltage across the resistor marked R1 is being measured. Note that the voltmeter leads are connected with the positive (+) lead to the positive (+) power supply side of the resistor and the negative (-) lead on the negative (-) power supply side. This insures that the meter indicator (or pointer) will move in the proper direction. If the connections are reversed the pointer will try to move below zero.
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- When voltmeters and multimeters such as the Electronic VOM are used to measure circuit voltage, the ground or common lead must be connected to the circuit ground point when it is shown on the schematic diagram.
- Don’t expect measured circuit values to agree exactly with calculated values or values measured by other students. They’ll be close, but a number of factors such as component tolerances, meter accuracy, lead and contact resistance, and even the way you read the ammeter can cause measurements to vary.
EVALUATE
To be given and scheduled via Google Classroom.
REFERENCES:
- Labvolt Manual
- Hayt, Jr., W. H., Kemmerly, J. E., & Durbin, S. M. (2010). Engineering Circuit Analysis. New York: McGraw-Hill.
- Siskind, C. S. (1956). Electrical Circuits. McGraw-Hill.
