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By EdwardKolbe DC voltmeter CommercialFisheries Engineer, OSU Marine ScienceCenter, Newport Most multirnetersare designedfor measuringboth DC and AC voltages. AssistantProfessor of Agricultural Engineering This bulletin concentrates on how to Oregon State University measureDC voltages;procedures for making AC measurementsare similar. First, plug one of the leadsinto the Multimeters measure electrical voltage, negativeterminal, properly called a resistance,and current, For wiring and jack, marked ! or "COM" for troubleshootingon boats,they can be common! . Standard electrical practice extremelyuseful. Applications include: usesthe black wire for the ground, checkingthe continuity of wiring which is usuallythe negativeterminal. locating breaksin opencircuits; testing The other lead usually red, to signify fusesand diodes;measuring battery the "hot" sideof the circuit! goesinto voltageand voltagedrop over wires and the positivejack, marked + !. After eIectricalloads; identifying "hot" and selectingthe proper DC voltagerange groundedwires; locatingshort circuitsor with the selector switch, measure DC smallcurrent leaks;and checking voltageby touchingthe -! lead to the alternator and generator output, negativeterminal or wire and the + ! A multimeter, also called a volt-ohm- lead to the positiveterminaI or wire. milliammeter VOM!, is actually three Selectingthe proper voltagerange is toob in one.It is a voltmetercapable of important. It is achieved on most meters measuringdirect-current DC! or by positionmgthe seIectorsmitch, and on alternating-current AC! voltages, Its others,by pluggingthe + ! leadinto selector smitch seIects function and different jacks.Ahvays begin on the sensitivity.By repositioningthe selector highestvoltage range that you reasonably switch or on somemeters, by plugging expectto find in whateveryou are the probesor wire leadsinto a different testing.If the rangeyou start on givesa set of connectors!, the meter becomes an very low readingon the scale,switch to ohmmeter, capable of measuring the next losverrange. The most accurate electricaIresistance. Again, by Ripping readingoccurs when the needIeswings the switch or moving the leads,it over most of the scale. becomes an ammeter that measures small DC currents. Ohrn's Law To understand the measurements madeby a multimeter,it is necessaryto understand Ohm's Law, stated Zxampfe;Engineering handbooks showthat a 50-footlength of No. 16 mathematicallyas strandedcopper wire hasan electrical V=IR, resistance of around 0.2 ohm. Ohm's Law Oregon State University tellsus that if a currentof 5 ampsis where V = DC voltage,in volts, ExtensionMarine Advisory Program Rowingthrough the wire to, say,a VHF A Land Grant / Sea Grant Cooperative I = DC current, in amps radio, the voltagedrop overthat wire is: amps is short for SG 42 June 1977 arnperes!, and V=IR R = electrical resistance, in 5 amps X 0.2 ohm ohms. 1,0 volt, The formula for AC voltage is So,on a 12-voltpower supply,the similar,except that the resistanceR a resistance of this wire would cause a DC phenomenon is replacedby a more 1-volt drop, leavingonly 11 volts to complicatedterm calledimpedance,! suppIythe radio. In this example,a A resistance of I ohrn written IQ! larger wire sizewould be neededto allowsa currentof 1 amp A! to Row, provide the full voltagethat the radio if a voltageof onevolt I V! is applied, requires. A simpleexperiment helps to explain the voltage-measuringprocedure, Take an ordinary 1,5-voltflashlight battery; put the DC voltmeteron a high 3 .C; 15 range,say, 60 volts full scale.This would iO 6O ~O meaii that if the battery were putting out 8p 60 volts, the needlewoulrl swing all the way over to full scale figure 1!, No cs ss two brandsof meter are exactlyalike; figure I showsa typicalDC voltage i"fi, ' ' scale.! When you set the selectorswitch Q at 60, you read the voItagefrom the top row of numbers for example,4 means P 40 volts, and so on! . When you try to read the voltageof a 1.5-voltflashlight battery on sucha high range,the needleswings only a short distance figure 2!, Note that if by mistakeyou shouldswitch the leads,the needle deflects to the left that is, in a Figure 1. The mnltimeter icorkiiig asa coltmeter,measraring a ti0scoltliat tery negativedirection and the meter may be damaged.The distinctionbetsveen positiveand negativeleads is not importantwhen measuringAC voltage on the AC voltmeter.! Now switch to a lower range, say 12 volts, For this range,you will read the voltageon the lower scale figure 1!, so 120 means 12 volts, 60 ineans 6 volts, etc, Measurethe flashlightbattery voltageonce more, This time, the needle comesup higher to about 15 on the scale,corresponding to 1.5 volts. Finally, try the lowestrange that can still measure 1.5 volts on the meter. This might be 3 voltsfull scale,The scalein figure I correspondingto this rangeis the middle one, where 30 means 3 volts and 15 mea.ns 1.5 volts. Remember;If you don't know approximatelywhat the voltageis before you make the measurement, start with a very high rangeand work tfotcn to more sensitivesettings. Stop svith the range I'igure2. This is the sariie DC coltraeter rarige as iii figure 1, bat note the meter is where the needle movement covers most measuriiig a 1.5-colt flashlight battery, of the meter scale.If you shouldtry to measurea 12-voltbattery on the 3-volt range,the needlewould swing up against The ohmmeter The stepsfor measuringresistance are: a pinto the right,and vou might very You canchange the multimeter into 1. Use the meter, is a volttneter to make well ruin the meter, an ohmmeterby just turning the selector surethere is no voltagebetween the switch or bv plugging the lead~into a two pointsof interest,Even a weak Two final notes about voltmeters: different set of jacks.The ohmmeter voltage,such as between two 1. All multimcters h,ive a -< ro- measures how much electrical resistance through-hullfittings, will lead to a atfjustmentknob that appliesonly to in ohms! exists beiween any two points, faulty resistancereading. This the ohmmeter function for measuring The commonestapplication is to check frequently occurswhen checkingout resistances.It has no application in for zeroresistance signifying a complete the bondingof a boat.Two unbonded voltagemeasurement. There is alsoa circuit, or continuity! or infinite metal objectswill often be at zero-adjustingscreco for setting the resistance no connection or an open different voltagelevels because of a iieedle to rest nt 0 volts wlien no circuit!, static charge,a currentleak, or voltageis applied. Youwill only need The accuracy of very-low-resistance dissimilarmetal properties.This will to adjust this on rare occasions readingsis not usuallyvery good.For disrupt the ohmmeterreading and perhapswhen you buy the meter or example,few rnultimeters can directly may evenmake it lessthan zero,! after it hasbeen bumped around a lot, measure the 0.2-ohm wire resistance of Warrsirtg: Using an ohmmeter on a 2. He carefulto readDC voltageon the the earlier Ohm's Law! example. If an hot svire can ruin the meter. DC-voltagescale. The AC-voltage ammeter in the line gives you the current 2. put the selector sroitch on the lowest, scaleis slightly different. reading,you c'm measurethe voltage most sensitive ohmmeter scale. This drop with the multimeter,then, using may be marked" R X I", meaning Ohm's Law, divide the voltage by the that you can read the resistance,8, current to calculate line resistance,! directly off the meter without multiplying by anythingbut 1. DC ammeter UnIike the voltmeter, which measures voltagefrom outsidethe electricalcircuit, an ammeter is used in the circuit. To measure current in a wire, you must break the wire and attach the leads so that the currentfiows throughthe xneter figure 4 which usesthe conventional definition that current fiowsfrom positive to negative!. The ammetershould be used in the circuit so that its + ! lead, or probe,connects ultimately to the + ! battery terminal, and the ! lead to the negativebattery terminal. In this configuration,a positivecurrent will be measuredon the axnmeter.If you installed the axnmeterin the opposite way, the needlewould swing toward the left, off the scale of the xneter. Figure 3. The multirneter is note sicitched into its ohmmeter function. The ammeter function of most multimeters is too sensitive, by itself, to be useful for common electrical circuits on boats. This is because the curreiit drawn by a depth meteror radio, for exaxnple,is generally on the order of severalamps, while current measuredby most multnneters is on the order of only milliamps. Mt'ill- meansthousandth, 1 milliampis I/1000 of an amp.! But metershaving a xnaximumrange of severalhundred miliiampr are goodfor measuringleakage currents, provided you are careful to avoid excessivelyhigh amperage,which will burn out the meter. As with the other functions of the multimeter, the ammeter has several ranges.The highest,which may be 300 mA 00 xnilliamps= 300/1000 amp = 0.300amp!, shouldbe tried first. Be careful: a largercurrent will probably burn out the meter.! As before, a scale reading of 30 means 300 mA, 20 means 200 mA, and soon figure 5! . DC Current Direction The mostaccurate reading occurs when the needleswings over mostof the Figure 4, Het'e the multimeter is toorking as an ammeter. scale.If the needledoes not defiectvery inuch indicating a currentmuch smaller than 300 xnA try a lower range setting, 3. Touch the probestogether. Siiice they is coatingthe xnetalobject touched by Note than a rangelabelerl, for example, make an exceIIent electrical the ohmmetexprobe, Note that on the "60 pA" means60 microamps,or 60 one- connection, the resistance should read ohmmeter scale, "K" means thousand; millionths ,000060! of an amp. zero figure3!, If it doesnot, adjust a readingof 1 K meansone thousand the =ero arljnst knolx until it does, ohms.When a resistancereading is When you separatethe two leads,you very large close to os on the scale- break the electrical connection, and you shouldswitch to the next higher the ohinmeterreading should be oc range setting to get a more accurate a symbolmeaning infinity!, When reading.With the selectorswitch on there is no electrical connection the "R X 100" setting,for exaxnple,a between the ohmmeter leads, the readingof 5 means5 X 100,or 500. resistanceis effectivelyinfinite. It is important to rememberthat each 4. Oncethe ohmmeteris zeroed,you tixneyou changethe resistancerange, canbegin measuringresistances, Since you must zerothe ohmxneter,as in resistancesshould be measuredonly step 3, above, on wires in which no current is fiowing, it doesnot matter which lead on the ohmmetergoes on which end. w corroded or damaged con~ector can producea resistancein a circuitwhere youwould normally find 0 Q.This canbe true alsowhen rust or grease