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The technician must be knowledgeable in electrical theory to be able to diagnose and repair major appliances properly. Although this section cannot cover all there is to know about electricity, it will provide the basics. In the field of major appliances, the greatest number of potential problems are in the electrical portions of the appliance. ELECTRICAL WIRING The flow of electricity from a power source to the home can be made easier to understand by comparison to a road map. Electricity flows from a power source to a load. This is similar to a major highway, which runs from one location to another. High-voltage transformers are used to increase voltages for transmission over long distances. The power lines that go to different neighborhoods are like the smaller roads that turn off the major highway. The electricity then goes to a transformer that reduces the voltage going into the home. This is the intersection between the small roads and the medium-sized highways. The small road that goes into the neighbor hood, and all the local streets, are like the wiring that goes inside the home. When connecting all the streets, roads, and highways together, the city is accessible. That is similar to having electricity flowing from the power source to all of the outlets in the home. Imagine driving down a road and coming to a drawbridge (in this case the switch) and it opens up. This stops the flow of traffic (electricity). In order for traffic (or electricity) to flow again, the drawbridge must close. WHAT IS A CIRCUIT? A circuit is a complete path through which electricity can flow, and then return to the power source. Figure __1 is an example of a complete circuit. To have a complete path (or closed circuit), the electricity must flow from point A to point B with out interruption. When there is a break in the circuit, the circuit is open. For example, a break in a circuit is when a switch is turned to its “off” position. This will interrupt the flow of electricity, or current, as in Fig. __2. When a broken circuit is suspected, it is necessary to discover the location of the opening. Light bulb __1 The complete circuit Current flows from point A, through the light bulb, and back to point B. __2 With the switch open, the current flow is interrupted. CIRCUIT COMPONENTS In an appliance, an electric circuit has four important components: • Power source: This source might be a battery, or the electricity coining from the wall outlet. Without the applied voltage, current cannot flow. • Conductors: A conductor will usually be a wire, and sometimes the metal chassis (frame). The function of the wire conductor is to connect a voltage source to a load. • Loads: These are the components that do the actual work in the appliance. A load is anything that uses up some of the electricity flowing through the circuit. For example, motors turn the belt, which turns the transmission. That, in turn, turns the agitator in a washing machine. Some other examples are heating elements and solenoids. • Controls: These control the flow of electricity to the loads. A control is a switch that is either manually operated by the user of the appliance, or operated by the appliance itself. THREE KINDS OF CIRCUITS You will come across three kinds of circuits: 1. Series circuits 2. Parallel circuits 3. Series-parallel circuits; a combination of series and parallel circuits. Series circuit In Fig. __3, the series circuit components are joined together in successive order, each with an end joined to the end of the next. There is only one path that electricity can follow. If a break is anywhere in the circuit, the electricity, or current flow, will be interrupted and the circuit will not function (Fig. __4). Figure __5 shows some of the many different shapes of series circuits, all used in wiring diagrams. In each series circuit, there is only one path that electricity can follow. There are no branches in these circuits where current can flow to take another path. Electricity only follows one path in a series circuit. __4 If there is a break in the wiring, all of the light bulbs are off Parallel circuits In Fig. __6, the components are connected across one voltage source and form a parallel circuit. The voltage to each of these branches are the same. The current will also flow through all the branches at the same time. The amount of current that will flow through each branch is determined by the load, or resistance, in that branch. Figures __7 and __8 show examples of parallel circuits. If any branch has a break in it, the current flow will only be interrupted in that branch. The rest of the circuits will continue to function. __3 A series circuit __6 A parallel circuit Series-parallel circuits A series-parallel circuit is a combination of series circuits and parallel circuits. In many circuits, some components are connected in series to have the same current, but others are in parallel for the same voltage (Fig. __9). This type of circuit is used where it is necessary to provide different amounts of current and voltage from the main source of electricity that is supplied to that appliance. __5 When you look at wiring diagrams, you will find series circuits in all sorts of shapes. __8 Notice that as in series circuits, the same parallel circuit can be drawn in many different ways. __7 Another parallel circuit Series-parallel portion __9 Series-parallel circuit Series and parallel rules apply to this type of circuit. For example, if there is a break in the series portion of the circuit (Fig. __9), the current flow will be interrupted for the entire circuit. If the break is in the parallel portion of the circuit, the current will be interrupted for only that branch of the circuit. The rest of the circuits will still function. TYPES OF ELECTRIC CURRENT There are two types of electric current: • Direct current Direct current, or dc, flows continuously in the same direction (Fig. __10). • Alternating current Alternating current, or ac, flows in one direction, and then reverses itself to flow in the opposite direction, along the same wire. This change in direction occurs 60 times per second, which equals 60 Hz (Fig. __11). Direct current (dc) is used in automobile lighting, flashlights, and cordless electric appliances; such as toothbrushes, shavers, drills, and in some major appliances. __10 A simple dc electrical circuit: Current will flow from the negative side of the battery through the switch and load, and back to the positive + side of the battery. A simple dc electrical circuit IL Series portion: Parallel portion Sine wave of single-phase alternating current Alternating current (ac) is used in most homes. This current can be transmitted more economically over long distances than direct current. Alternating current can also be easily transformed to higher or lower voltages. OHM’S LAW Ohm’s law states: The current which flows in a circuit is directly proportional to the applied voltage, and inversely proportional to the resistance. In other words: the greater the voltage, the greater the current; and, the greater the resistance, the less the current. OHMS Resistance is measured in ohms. Resistance opposes the flow of electrons (current). The amount of opposition to the flow is stated in ohms. An instrument that measures resistance is known as an ohmmeter. Figure __12 is a schematic showing an ohmmeter connected to read the resistance of R1. The resistance of any material depends on the type, size, and temperature of its material. Even the best conductor offers some opposition to the flow of electrons. Figure __13 shows another type of meter, the multimeter, used for measuring ohms. The fundamental law to find resistance is stated: the resistance (R) in ohms is equal to the potential difference measured in volts (V) divided by the current in amperes (A). The equation is: R = V-i- A. AMPERES Current is measured in amperes. The term “ampere” refers to the number of electrons passing a given point in one second. When the electrons are moving, there is current. Current can be measured in amperes, which is a measurement of the quantity of electron flow multiplied by time. The ammeter is calibrated in amperes, which we use to check for the amount of current in a circuit. An instrument that will measure amperes is known as an ammeter. Figure __14 is a schematic showing an ammeter connected in a circuit to measure the current in amperes. Figure __15 shows an ammeter that is used in diagnosing appliance electrical problems. Current is the factor that does the work in the circuit (light the light, ring the buzzer). The fundamental law to find current is stated: the current in amperes is equal to the potential difference measured in volts divided by the resistance in ohms. The equation is: A = V R. Peak __11 A waveform of a single-phase alternating current Ammeter VOLTS Electromotive force is measured in volts. This is the amount of potential difference between two points in a circuit. It is this difference of potential that forces current to flow in a circuit. One volt (potential difference) is the electromotive force required to force one ampere of current through one ohm of resistance. __12 An ohmmeter connected to read resistance. __13 A test instrument for measuring ohms. __14 An ammeter connected in a circuit, measuring amperes. __15 In the ammeter, the jaws clamp around a wire to measure the amperage of a circuit. An instrument that will measure voltage is known as a voltmeter. Figure __16 is a schematic showing a voltmeter connected in the circuit to measure the voltage. Voltmeter 1 is connected to read the applied (or source) voltage. Voltmeter 2 is connected to measure the voltage drop, or potential difference, across R2. Figure __17 shows an actual volt-ohm-milli-ammeter (VOM) that is used in measuring voltage. The fundamental law to find voltage is stated: the potential difference measured in volts is equal to the current in amperes multiplied by the resistance in ohms. The equation is: VAxR. WATTS Power is measured in watts and an instrument that will measure watts is known as a wattmeter (Fig. __18). One watt of power equals the work done in one second, by one volt of potential difference, in moving one coulomb of charge. One coulomb per second is equal to one ampere. Therefore, the power in watts (W) equals the product of amperes times volts. The equation is: W = A X V. Voltmeter __16 A voltmeter connected in a circuit to measure voltage. __17 The volt-ohm-milliammeter with test leads. OHM’S LAW EQUATION WHEEL The equation wheel in Fig. __19 shows the equations for calculating any one of the basic factors of electricity. Figure __20 shows the cross reference chart of formulas, as used in this text. If you know any two of the factors (V voltage, A = amperage, R = resistance, W = power), you can calculate a third. To obtain any value in the center of the equation wheel, for direct or alternating current, perform the operation indicated in one segment of the adjacent outer circle. Volt-ohm milliammeter __18. The wattmeter is used to measure power. Digital wattmeter Conversion chart for determining amperes, ohms, volts, or watts Amperes = A, Ohms = Greek letter Omega, Volts = V, Watts = W __19. The Ohms Law equation wheel. __20 The cross-reference chart of formulas. E A 2400-watt heating element is connected to a 240-volt circuit. How many amps does it draw? When finding amperage, the formula will be found in the amperes section of the wheel. Then, solving for amperage: What is the resistance (ohms)? WIRING DIAGRAM SYMBOLS These wiring diagram symbols are commonly used in most wiring diagrams. Study each symbol so that you can identify them by sight (Tables __1, __2, __3, and __4). Table __1 Lamps Incandescent Miscellaneous Adjustable component (arrow drawn through component at approx. 45°) Operating coil (solenoid relay) Solenoid Starter Rectifier (diode) Resistor or heater Capacitor (polarized) Capacitor (non-polarized) Heater Transformer Relay (show device used) Coil-operated Heat-operated Germicidal Ballast (Fluorescent Neon) Double-pole Limit switch N.O. (Normally open) N.C. (Normally closed) Circuit protectors; Circuit breaker 0— Circuit breaker with thermal O.L. Fuse; Thermal fuse Table __2 Temperature-actuated components (Note: Symbols shown to be used for thermostats, bimetal switches, overload protectors, or other similar components, as required. Temp. actuated (close on heat rise) Temp. actuated (open on heat rise) S.P.S.T. (open on heat rise) S.P.D.T. S.P.S.T. (two contacts) S.P.S.T. (adj.) (close on heat rise), (with aux. ‘off’ contacts) S.P.S.T. (with internal heater) (close on heat rise) S.P.S.T. (with internal heater) (open on heat rise) Combination devices Relay-magnetic (arrangement of contacts as necessary to show operation) Relay-thermal (arrangement of contacts as necessary to show operation) Timer (defrost) Manual and mechanical switches Normally closed (SPST) (single-pole, single-throw) Normally open (SPS1) (single-pole, single-throw) Timer switch Automatic switch N.O. (normally open) N.C. (normally closed) Integral switch (timer, clock, etc.) Pushbutton switch (momentary or spring return) Circuit closing _____ N.O. (normally open) Circuit opening N.C. (normally closed) Two circuit SPDT (single-pole, double-throw) Transfer (SPDD (single-pole, double-throw) Multi position Number of terminals Bell Sensor (moisture) Thermocouple Centrifugal switch Pressure switch S.P.D.T. Humidistat Magnetron Thermistor Table __3 Lines and connections Integral conductor External or harness wire Optional or alternate circuit Components Buzzers Adjustable Crossover; Permanent connection -- junction Permanent connection Terminal; Shield Ground (earth) Ground (chassis) Grounded service cord (3-prong plug) 9 Service cord (2-prong) -- Mechanical connection Separable connector; Motors Timer or clock Single-speed Two-speed Three-speed Compressor Relay (show device used) Electronic components__ Thermistor Transistor (PNP) Rectifier (controlled) Silicon-controlled rectifier (SCR) Triac Electronic dryer control Table __4 Lamps Incandescent Germicidal Ballast Fluorescent Neon Miscellaneous Adjustable component (arrow drawn through / component at approx. 45 deg) Operating coil, J (solenoid relay) Solenoid Startup Rectifier (diode) Resistor or heater Capacitor (polarized); Capacitor (non-polarized) Coil-operated Heat-operated Double-pole Limit switch N.O. (Normally open) N.C. (Normally closed) Circuit protectors Circuit breaker Circuit breaker with thermal O.L. Thermal fuse Transformer Wiring diagram symbols 63 TERMINAL CODES Terminal codes are found on all wiring diagrams. To help you identify the color codes they are listed in Table __5. Table __5 Terminal -- Harness wire -- color code - color BK -> Black BK-Y -> Black with yellow etc. TIMER SEQUENCE CHARTS Figure __21 represents a sample timer sequence chart. This detailed chart shows how the timer motor, and timer switch operation, control machine functions. When the timer switch sequence chart information is compared to the wiring diagram, electrical and mechanical diagnoses can be accomplished. WIRING DIAGRAMS In this segment, there are four different wiring diagram examples. Each example will take you through a step-by-step process in how to read wiring diagrams. Example #1 Take a look at a simple wiring diagram for a refrigerator (Fig. __22). Note the black wire on the diagram. It is the wire that goes to the temperature control. The circuit is not energized when the temperature control is in the “off’ position. When the temperature control knob is turned to the on position (switch contacts closed), and the circuit is energized; current will flow through the temperature control, through the red wire, through the overload protector, through the compressor and the relay, and back through the white wire to the line cord. __21 A sample of a timer sequence chart. Switch open __22 A simple wiring schematic of a refrigerator circuit. Switch number; Terminal code __23 The wiring schematic of an automatic washer circuit. Example #2 Trace the active circuits with your finger. In the wiring diagram in Fig. __23, note switch number 1 on the diagram. It is the first switch in line, and it is the main switch that supplies voltage to the timer. No circuits are energized when the timer dial is in the off position (as the diagram indicates). When the user selects the wash cycle with a warm wash, and turns on the timer, switch contacts 1, 2, 3, 6, and 7 are closed. (Use a pencil to close the switches on the diagram.) Voltage is sup plied to the water level switch and the hot and cold water valves. Warm water is now entering into the washing machine tub. When the water level reaches the selected position, the water level switch contacts close from V to number 4, indicating that the water in the tub has reached the selected water level. When the water level switch is in this position the water is turned off. Voltage is now supplied to the timer motor, drive motor, and the agitate solenoid. The washing machine is now agitating and cleaning the clothes. As the timer advances to the spin cycle, timer switch contacts number 2 and 3 open; thus turning off the agitate solenoid and the drive motor. Switch contacts number 8 and 2 now close; thus supplying voltage to the drive motor and the spin solenoid. The timer motor advances to the end of the cycle. Timer switch contacts number 1, 2, and 8 will open. Example #3 The wiring diagram in Fig. __24 is for a refrigerator. Assume that the thermostat is calling for cooling, and the compressor in running. With your finger, trace the active circuits. Refrigerator light; Door switch; Thermostat; Overload; Compressor; Evaporator fan motor; Condenser fan motor __24. The wiring schematic of a refrigerator circuit The thermostat in the wiring diagram for the refrigerator is closed. The evaporator fan motor and the condenser fan motor are also running. Voltage is supplied through the overload to the relay. Current is flowing through the relay coil to the compressor-run winding. You will also notice that the door switch is open, and the refrigerator light is off. When the temperature in the refrigerator satisfies the thermostat, the thermostat switch contacts will open; thus turning off the compressor, the evaporator fan motor, and the condenser fan motor. Example #4 The wiring diagram in Fig. __25 is for a no-frost refrigerator. Note the defrost timer in the lower left part of the diagram. The defrost timer switch con tact is closed to contact number 4, the thermostat is calling for cooling, and the compressor is running. Trace the active circuits with your finger. Voltage is supplied to the defrost timer terminal number 1. Current will flow through the defrost-2 timer motor to the white wire, and back to the line cord. At the same time, current flows through the number 4 contact in the defrost timer to the thermostat. At this point, the current passes through the thermostat to a junction, and splits in two directions. Current will flow through the temperature control, through the overload protector, through the compressor and the relay, and back through the white wire to the line cord. At the same time, the compressor is running, current will flow through the evaporator, and condenser fan motors. When the defrost timer activates the defrost cycle, the defrost timer switch con tact is closed to contact number 2, and the compressor is not running. The evaporator and condenser fan motors will also stop running. The current will flow from terminal number 2, through the defrost bi-metal, through the defrost heater, and back through the white wire to the line cord. With your finger, trace the active circuits. SAMPLE WIRING DIAGRAMS The diagram in Fig. __26, is known as a ladder diagram. It is a simplified diagram using symbols, for parts and control components, attached to wires. The timer is rep resented by a dotted line running vertically through the switch contacts. To read and understand this type of diagram, assume that the complaint is that the dryer is not heating. Look at the section of the diagram marked HEAT. Starting on the left side of the diagram, trace the circuit with your finger. You will notice that Li goes to one side of the timer switch. As your finger moves from left to right, you will pass over the cycling thermostat, hi-limit thermostat, heater, motor centrifugal switch (CS), and on to L2. These are the components that make up the heating circuit. You must also include the motor and the door switch. If the door switch should fail, the motor will not run; thus opening the motor centrifugal switch (CS). If the motor fails, the high limit thermostat will open, shutting off the heater element. If any of these components fail, the dryer will not dry the clothing. __25. The wiring schematic of a no-frost refrigerator circuit. __26 A simple ladder diagram. The diagrams in Figs. __27 and __28 illustrate a pictorial diagram and a schematic diagram. The pictorial diagram shows the actual picture of the components, and the schematic diagram uses symbols for the components. Figure __29 shows a pictorial diagram of a refrigerator. In this type of diagram, you can see where the components are actually located. HOW TO READ THE VOLT-OHM-MILLIAMMETER TEST INSTRUMENT The volt-ohm-milliammeter (VOM) is sometimes called a multimeter, because they can perform more than one function. A typical VOM will allow you to measure volt age, resistance, and current. • Voltage equals electromotive force. • Resistance equals the amount of resistance holding back the flow of current (measured in ohms). • Current equals the amount of electricity flowing through a wire or circuit component (measured in amperes). There are many different types and brands of VOMs (Fig. __30). Most VOMs will have the following: • Test leads. These are the wires coming from the meter to the part being tested. • Meter scales and pointer (or a digital display on a digital meter). These show the amount of whatever value you are measuring. • Function switch. This allows you to select whether you will be measuring ac or dc voltage (volts), current (amps), or resistance (ohms). • Range selector switch. Allows you to select the range of values to be measured. On many meters, you can select functions and ranges with the one switch (as in the meter pictured in Figs. __30A and __30C). All VOMs are used in the same way to measure voltage, current, or resistance. MEASURING VOLTAGE If you don’t have the right voltage in an appliance, the appliance won’t function properly. You can find out whether the appliance is getting the right voltage by measuring the voltage at the wall outlet (receptacle). If an appliance isn’t getting the proper voltage, nothing else you do to fix it will help. For your safety, before using any test instrument, it is your responsibility as a technician to read and understand the manufacturer’s instructions on how the test instrument operates. Making the measurement When measuring voltage, there are several steps you should follow: • Attach the probes (another term for “test leads”) to the meter. Plug the black probe into the meter jack marked negative or common. Plug the red probe into the positive outlet. • Set the function switch to AC VOLTS. • Select a range that will include the voltage you are about to measure. (Higher than 125 Vac if you are measuring 120 volts; higher than 230 Vac if you are measuring 220 volts.) If you don’t know what voltage to expect, use the highest range, and then switch to a lower range if the voltage is within that lower range. • Touch the tips of the probes to the terminals of the part to be measured. • Read the scale. • Decide what the reading means. Making voltage measurements is easy once you know how to select and read the scales on your meter. SELECTING THE SCALES To measure the voltage in appliances, use the ac/dc (Fig. __31). These same scales are used for both ac and dc readings. The numbers on right side of the ac/dc scales tell you what ranges are avail able to you. The meter face in Fig. __32 has three scales for measuring voltage. One is marked 10, another 50, and the third is marked 250. Remember, the scale you read is determined by the position of the range switch. Sample: If the range is set on 250 V, as in Fig. __33, you read the 0- to 250-V scale. Pictorial-Schematic: Refrigerator fan __27. A pictorial diagram and a schematic diagram. Both diagrams show the same components. Schematic; Pictorial __28. A pictorial diagram and schematic diagram. WIRE CODE; COLOR; CODE __29. A pictorial diagram of a refrigerator showing where the components are located. Wall Power cord plug; Door switch- Cabinet light; Motor Fan; Heater; Temperature control __30 A. Volt-ohm-milliammeter. B. VOM and amperage multimeter combination. C. Volt-ohm-milliammeter and ammeter multimeter combination; B. Test lead; C __31. 50A -- An example of scales used on some VOMs. READING THE AC VOLTAGE SCALE When reading the pointer position be sure to read the line marked AC (Fig. __32). The spaces on the voltage scales are always equally divided. When the pointer stops between the marks, just read the value of the nearest mark. In Fig. __34, the pointer is between 115 and 120 volts on the 250 scale. Read it as 120 volts. With an analog meter you’re not gaining anything by trying to read the voltage exactly. MEASURING LINE VOLTAGE Measuring line voltage is the first, and most important, part of checking out an appliance that does not operate. Line voltage is the voltage coming from the wall out let. There should be approximately 120 volts ac at the outlet under “no load” conditions. No load means that no appliance is connected, or that an appliance is connected, but it is turned off. To measure line voltage—no load (120 volts) (Fig. __35): • Set meter to measure ac volts. • Set range selector to the range nearest to, but higher than, 120 volts. • Insert either test lead into one slot of an empty wall receptacle. • Insert the other test lead into the other slot of the same outlet. (Disregard the ground terminal for this test.) Warning!: Do not touch or handle the test leads by the metal portion of the probe. Hold the probe by the plastic grips that are attached to the test leads, to avoid electric shock. __32. The meter face of an analog meter • Read the meter. The reading should be between 115 and 120 volts. • When testing for 240 volts, be sure the range selector is set to the nearest range higher than 240 volts. Note: Most appliances are rated at 120 volts, but will work on voltages ranging from 110 to 125 volts. If the voltage drops more than 10%, the appliance will not operate; and most likely will damage some electrical components, if the appliance keeps running. To measure line voltage—under load (120 volts) (Fig. __36): • Be sure the appliance is plugged into one of the receptacles, and that the appliance is turned on. • Follow steps 1 through 5 for no load conditions, inserting the test leads into the empty receptacle next to the one into which the appliance is plugged. • Under load conditions (appliance is turned on), your reading will be slightly less than under no-load conditions. • When testing for 240 volts, be sure the range selector is set to the nearest range higher than 240 volts. Note: Appliances with drive motors, such as automatic washers, dishwashers, and trash compactors, should also be tested at the moment of start. If the voltage drops more than 10% of the supplied voltage when the motor is started, it means that there is a problem with the electrical supply. __33. The range is set on the 250-V scale. 500V; 50 mA __34, The range is set on the 250-V scale, the pointer indicates __35. Measuring line voltage: no load. When a component is grounded to a chassis, there is no voltage between the component and the chassis. When a chassis is grounded to the earth, there is no voltage between the chassis and the earth. If there is voltage between a chassis and the earth, it’s dangerous. If you stand on the earth (dirt, concrete slab, etc.) and touch the chassis, electricity will flow through your body. You could injure yourself, or even someone else; and death could occur. It is important to be sure that the electrical outlets in the home, from which appliances are powered, must be correctly wired. That will protect the users from electrical shock. If an outlet is wired “backward” (that is, if the black or red “hot wire” is connected to the long slot of the outlet), the appliance connected to that outlet might be unsafe to operate, or blow the fuse, or trip the circuit breaker. Appliances with solid state controls will not function properly if the outlets are wired backward. To test for ground: • First, test for line voltage. (If there is no voltage, you can’t test for ground.) • Notice that the receptacle has a longer and a shorter slot (Fig. __37). If the outlet has been mounted right-side-up, the longer slot will be on the left. • Test for voltage between the short slot and the ground receptacle (the round hole) (Fig. __38). If there is line voltage between these two points, it means that the receptacle is grounded. • If there is no round hole (ground prong receptacle), touch one of your probes to the screw that fastens the cover-plate to the outlet (Fig. __39). To test for polarity: • Test to be sure that there is line voltage between the longer and shorter slots (Fig. __40). • Test to be sure there is line voltage between the short slot and the center screw, or the round hole (ground prong receptacle) (Figs. __41 and __42). • Test to be sure there is no voltage between the longer slot and the center screw, or the round hole (Figs. __43 and __44). Note: If these three tests don’t test out this way, the outlet is incorrectly wired, and should be corrected by a licensed electrician. __36. Measuring line voltage: under load. TESTING FOR GROUND AND POLARITY Ground screw; Neutral (white wire to silver screw) __37. Receptacle identification points. __38 Testing for ground. __39. Testing for ground again. __40. Testing for polarity. (black wire to brass screw) __42 Testing the polarity of a receptacle. __43 Testing the polarity of a receptacle. MEASURING RESISTANCE (OHMS) Electrical appliances need a complete path around which electricity can flow. If there is infinite resistance to the flow of electricity, you have an open circuit, or in finite resistance between the two points being measured. When there is a complete path, you have continuity in the circuit. When you test to find out whether there is a break in the path, you say you are making a continuity check. Continuity checks are made by measuring the amount of resistance there is to the flow of electricity. If there is so much resistance that it is too high to measure (called “infinite”), then you say that the circuit is open (there is no complete path for the electricity to follow). If there is some resistance, it means that there is continuity; but that there is also one (or more) load on the line—a light, a motor, etc. Note: A load is an electrical component that uses electricity to work (e.g., a light bulb, a motor, a heater coil). If there is no resistance between the two points, it means the electricity is flowing directly from one point to the other. If the electricity flowed directly from one point to the other by accident or error, then you say you have a short circuit, or a “short.” SETTING UP THE METER Measuring resistance (ohms) is like measuring voltage, except that the measurements are made with the electricity turned off. Listed below are the steps for measuring ohms: • Attach the leads to the meter. Plug the black lead into the negative outlet, and the red lead into the positive. • If your meter has a function switch, set the function switch to OHMS. • If your meter has a range switch, set the range. The range selector switch will have several ranges of resistance (ohms) measurements (Fig. __45). The ranges are shown like this: R x 1: The actual resistance shown on the meter face, times 1. R x 10: The resistance reading, times ten; add one zero to the reading. R X 100: The resistance reading, times 100; add two zeros to the reading. R x 1K: The resistance reading, times 1000; add three zeros to the reading (K means 1000 ). R X 10K: The resistance reading, times 10,000; add four zeros to the reading. Setting up the meter 81 • Set the range so it is higher than the resistance you expect. If you don’t know what measurement to expect, use the highest setting, and adjust downward to a reading of less than 50. The left side of the scale is too crowded for an accurate reading. • Zero the meter. You should “zero the meter” each time you set the meter. To “zero the meter” means to adjust the pointer so that it reads 0 when the two test leads are touched together. Use the Ohms Adjust knob on the front of the VOM to line up the pointer over the zero on the ohms scale. • Attach the test leads to the component you are measuring. • Take the measurement. In Fig. __45, the range selector switch is on R x 100, and the measurement is 400 ohms. ELECTRICAL SAFETY PRECAUTIONS Know where, and how, to turn off the electricity to your appliance; for example: plugs, fuses, circuit breakers, or cartridge fuses. Know their location in the home. Label them. When replacing parts or reassembling the appliance, you should always install the wires on their proper terminals, according to the wiring diagram. Then, check to be sure the wires are not crossing any sharp areas, or pinched in some way, or between panels or moving parts that might cause an electrical problem. These additional safety tips can also help you and your family: • Always use a separate, grounded electrical circuit for each major appliance. • Never use an extension cord for major appliances. • Be sure that the electricity is off before working on the appliance. • Never remove the ground wire of a three-prong power cord, or any other ground wires, from the appliance. • Never bypass, or alter, any appliance switch, component, or feature. • Replace any damaged, pinched, or frayed wiring that might be discovered when repairing the appliance. __45. With the range set at R x 100, the meter reads 400x. Rx10; Rx1K
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