A Broken Circuit - Tangentsoft

Most of the time in electronics, we strive mightily not to build broken circuits. Sometimes, though, a broken circuit is exactly what we need. ABrokenCircuit Mostofthetimeinelectronics,westrivemightilynottobuild brokencircuits.Sometimes,though,abrokencircuitisexactlywhat weneed. Ifyou’veeverhadtotakethemeasureofcurrentina circuit,you’venodoubtbeenirritatedbytheneedtofinda waytobreakthepowerlineenteringthatcircuitsoyoucaninsert anammeterinserieswiththepowerflow.Therearevariouswaysto avoidtheneedtobreakthecircuit,butit’softenthebest waytomakethemeasurement.Whenitcomestomeasuringthecurrent drawofanentirewall-poweredgadgetofsomesort,breakingthe powerinletcircuitisoftenmostinconvenient.Shouldonegetoutthe screwdriverstoopenthedeviceup,soyoucanbreakintothecircuit atthebackendofthepowerinlet?Noneed!Withabrokencircuit inyourtoolbox,youcanjustinsertitinlinewiththepowerplug justlikeanynormalextensioncable,thenplugyourmeterintoit tohealthebreak,supplyingpowertothedevicetobetestedand gettingacurrentmeasurementatthesametime. BeforeIgofurther,there’sanoff-the-shelf devicethatdoessomethingsimilar,anddoesitveryelegantly:the Kill-a-Watt. Ihaveone,andloveit,butsometimesit’snotexactlytheright thing.ThebrokencircuithassomeadvantagesoveraKill-a-Watt,the bestofwhichisthatitletsyouuseanarbitrarilygoodDMMorVOM togetamoreaccuratemeasurementthanthecheaplittleKill-a-Watt canprovide.(Formoreonselectingagoodmeter,seeanotherof myarticles,HowtoBuyaMultimeter.)Abrokencircuitisabit bulkierandlesselegantthanaKill-a-Watt,butitcanbebuiltfor abouthalfthecost,ifyoualreadyhavethemeter. Otheralternativestothebrokencircuitareclampmetersand shunts.IcoverthoseinTangentTutorial #6:HowtoUseaMultimeter. Parts Thebasicpartsareapowerextensioncable,aprojectbox,anda pairofbananajacks.Youmightalsowantsomededicatedtestleads togobetweenthebrokencircuititselfandyourammeter,though thetestleadsyoualreadyhavemaysuffice.Therearesomeother bitsanpiecesyoumightwanttoaddtothiscollectionofparts, coveredinlinebelow. ExtensionCable Ipreferashortextensioncable,perhaps5feetlong,sothat it’sjustlongenoughtoextendfromapoweroutletnearfloor leveluptoanearbyworkingsurface.Anythingmuchlongerisgoingto beabitbulkytokeepwithyourothertools,nottomentionbeing awasteofgoodextensioncordforwhichyounodoubthavemore frequentneed. Youmightbetemptedtogotheotherway,anduseaveryshortcord, butthatremovesoneoftheadvantagesofthebrokencircuitoverthe standardmodelKill-a-Watt,thatbeingthatyoucanmakethecordlong enoughtogetallthepartsyouwanttofiddlewithupontoanormal workingsurface,insteadofhunchingdownaroundawalloutlet. I’vebuiltacoupleofthesebrokencircuits,andI don’tactuallygooutshoppingforashortextensioncable, shortnessbeingagainstthenatureofextensioncables.Instead, IeithertaketheshortestextensioncableIhaveonhandandhack achunkoutofthemiddletoshortenit—savingthechunk forsomelaterproject—orItakeasectionofftheendofa longcable. Ifyou’regettingthecableforthisprojectbyshortening alongextensioncable,youwillalsoneedareplacementpowerplug, andaninlinepowerreceptacle.Thesearereadilyavailableinthe electricalsectionofhardwarestores.Besuretogetonesofthesame qualitylevelorbetterthantheonethecablecamewith,soasnot todiminishtheoverallqualityoftherepairedcable.Ifyou’ve neverreplacedapowerplugorreceptacle,it’sprettyeasy. Thewiresinyourcableshouldbecolorcoded,asshouldthe contactsinsidetheplugandreceptacle.Thereissomevariancein thesecolorcodingstandards,buttheoneshowninthesepictures isthemostcommon,hereintheUSatleast:whiteforACneutral, blackforACline(“hot”),andgreenforearthground.The plugandreceptaclewilltypicallyhaveasteelscrewfortheneutral connection(possiblyplatedwithzincorsomeothersilverymetal), abrassscrewforthehotconnection,andagreen-paintedscrewfor ground.Thatis,silvertowhite,brasstoblack,andgreentoground; themnemonicvalueofthisconventionshouldbefairlyclear,toa nativeEnglishspeaker,anyway. Ifyou’reusingacheapextensioncordwithnocolorcoding, you’llhavetoworkoutthecorrespondencebetweentheplug connectionsandthewiresyourself. It’seasiesttousearoundcable,asthatmakescutting theholesintheprojectboxeasierthanifyouuseflatcable. Youmightalsowantsomeproperly-sizedstrainreliefbushingsfor yourcable.InthebrokencircuitsI’vemade,Ijustusehot glueontheinnersurfaceoftheboxaroundthecabletopreventthe cablepullingthroughandstrainingtheconnectionsinsidethebox. ProjectBox IusedaRadioShack270-1801projectbox,3 in× 2 in&times1 in. Somethingalittlesmallercouldwork,butbewarethatwe’re workingwithAClinepowerhere.Youwantenoughroominsidethecase tobeabletoseparateeverythingdecently.Inthepictures,youcan seesomeexposedsolderjoints,andsomecoveredbyheatshrink.The heatshrinkisamerenodtowardsafety,andmoreamatterofneatness, really,notsomethingtopreventshorts.I’mdependingmuch moreonairgapshereforinsulation.Heatshrinkisnowherenearas thickasAClineinsulation,butairisanexcellentinsulator. Speakingofinsulation,Icanonlyrecommendusingaplastic projectbox.Youcoulduseametalboxifyoutookcaretokeepthe airgapsniceandbig,butwhyruntherisk?TheRadioShackcaseI mentionedabovecomeswithbothanaluminumcoverandaplasticone; Iusedtheplasticone. BananaJacks Bananajacksarewidelyavailable.I’veseenthematRadio Shack,BestBuy,hardwarestores,etc.They’recommonlyused forspeakerconnections,butalsofortestequipment,whichourbroken circuitis. Ipreferthesortwithsolderlugconnections.Youcanalsoget themwithvarioussortsofpressurefitconnections,whichareusually lessreliable. Youcangetbananajacksinpairs,exactlyspacedtoallowfor standarddual-bananacabling,whichmaybeconvenient.Alotoftest equipmentusesthisspacing,asdoesthecablingmadeforit. TheonlythingI’dlookoutforisthatyouwanttoavoidthe sortwithanexposedmetalsnout.Again,we’reworkingwithAC linewiringhere.Thepossibilityofaccidentalshortsistoogreat withexposedmetalcontacts. MeterCabling Yourmeternodoubtcamewithtestleads,typicallywithsharp probesononeendandbananaplugsontheother.Youcanusethese withthebrokencircuit,butabettersortisapairofdual-ended bananajumpers.Oneendgoesinthemeter’scurrent-measurement sockets,andtheotherendintothebrokencircuit. ImademyfirstsetofbananajumpersfromRadioShackparts, andtheyworkfine.YouneedasetofreplacementDMMprobes,anda pairofbananaplugs.ThehandlesontheRadioShackprobesetunscrew fromthetips,andthenyoucanjustheatupthesolderjointsonthe tipstoreleasethecable.Cutoffthesolderedendsofthesecables, stripbacktoexposefreshcopper,andputthebananaplugson. Alternately,Pomonamakesallkindsofweirdtestequipmentcabling, includingbananajumpers.YoucangetPomonaleadsatmanyelectronic partsdistributors,includingDigi-KeyandMouser. Assembly There’sreallynotmuchtosurpriseyouinthisproject.The mainthingistorememberthatyou’reworkingwithACline wiringhere,whichcancausedamageorevenkillyouifyoudoit incorrectly.ThisprojectissosimplethatwhatI’vedoneshould beimmediatelyobvioustoanyexperiencedelectricalorelectronics DIYerjustbylookingatthepictureatright.It’sprobably cleareventomanyaDIYnovice.Still,youshouldreadthroughthese stepscarefullyjusttobesafe,andbecauseImighthavefigured outsomethingthatwillhelpyoutodoabetterjobofbuildingit: Cuttheextensioncordabout6 infromthe female/receptacleend.Thiswillputmostofthelengthof thecablebetweenthebrokencircuitandthewallplug,so thattheprojectboxwiththebrokencircuitconnectionscan beconvenientlyplaceduponyourworkingsurface. IfyouusethesortofcableIshowabove,thereare twolayersofinsulation.Stripbacktheouterlayerhalf thelengthofyourprojectbox,orabitmore.Forinstance, ifyouhavea3 inlongprojectbox,somewherebetween about1-½ inand2 inwillserveyouwell. We’renotexposinganycopperatthispoint.That comeslater. Drillfourholesinyourprojectbox,twoforthe cableends,andtwoforthebananajacks.TheplanIshow inthepicturesworksbest,Ifind,asitreducestherisk ofelectricalshortsbykeepingeverythingaboutasfarfrom everythingelseasitcanbe,exceptwherecircuitconnections arenecessary. Mountthebananajacksintheirholes. Youmightwanttotakesomepainstoensurethatthe jacksdon’tbecomeloose.Ifthescrewthreads aremetal,IsuggestLocTite.Fortheplastic-bodied jacksintheplasticcaseIused,IdribbledabitofMEKonto themountingsurfacebetweenthecaseandthejackfromthe backside,whichdissolvestheplasticslightly,causing theplasticsurfacestoactuallymeldtogether,creatinga strongmechanicalbondwhentheMEKevaporatesaway.MEKand othersolventslikexyleneandtoluenearewhat’sin plasticmodelcements.Iimaginethebestchoiceofsolvent dependsontheplasticsbeingfused,butMEKjustseemsto work,forme.Youcanfinditatmostanywell-stockedhobby store. Sticktheextensioncableendsfarenoughintothe enclosuretogiveasecurefit,thenfigureouthowfar backtocuttheACline(“hot”)wire.(Black, typically.)Youwantittobejustshortenoughthatit reachesthebananajackwithenoughexposedwiretomakegood contact.Theotherwireswillremainlong,sotheycanbow outandawayfromthebananajacks. Stripbackthehotwire’sinsulationanywherebetween about¼inand½in.Ifyourbananajacksuse solderconnections,youwon’tneedasmuchexposed copperasifitusessomesortofpressureconnection.Make theseconnectionsnow. Stripbacktheinsulationfromtheremainingwires, exposingabout¼ inofwire.(I’mbeing deliberatelyvagueonthenumberofwires,asyoucouldbe usingeithera2-wireor3-wireextensioncable.Itreally doesn’tmatterwhich,forourpurposeshere.)Youmight needtore-trimthesewirelengthsabitfirst,toensure thattheybowoutwardawayfromthebananajacks,butnotso muchthatyou’regoingtohavetofindawaytostuff excesswireintotheenclosure,riskingshorts. Slideheatshrinkoverthesewireends,solderthem together,andshrinktheshrink.Youcouldusewirenuts instead,butthey’rebulkyanddon’tworkwellon thestrandedwiretypicallyusedinextensioncables. Addsomesortofstrainrelieftotheareaswherethe extensioncablesentertheprojectbox.Iusedhotglueonthe insideoftheenclosurearoundthecable.Youmightbealittle classierthanI,andusestrainreliefbushings. Closeitup,andexerciseit,asbelow. UsingtheBrokenCircuit Therearetwowaystodothis,primarilydependingonwhichmeter cablingoptionyouchoseabove. TheonlycommonstepistosetyourmeteronitshighestACcurrent measurementsetting,andfindoutwhichjacksonthemetercorrespond tothissetting.It’scommonforthehighcurrentsettingtohave adedicatedmeasurementjack,separatefromthoseusedforvoltage, resistanceandothermeasurements.Sometimestherearetwodedicated currentmeasurementjacks,withthesecondbeingforalowcurrent range,whileonothermetersthelowcurrentrangesharesajack withotherfunctions.Ifyou’renotsurewhichjackiswhich, RTFM! BananaJumperMethod Useyourbananajumperstoconnectthemetertothebroken circuit’sjacks.Thebrokencircuitasdescribedhere isdesignedforACmeasurements,soyoucan’thookitup “backwards.”ACcurrentdoesn’thaveapolarity anymorethanACvoltagedoes. (YoucouldbuildaDCbrokencircuitonthesameprinciplesasthe oneIdescribehere,perhapsusingbarrelconnectorsinsteadofanAC extensioncord.Inthatcase,polaritywouldbeanissue.) PlugtheDUTinto thereceptacleendofthebrokencircuit’scable,thenplugthe otherendintothewall.TheDUTshouldpowerup,andyoushouldget acurrentreadingonyourmeter.Ifnot,gobackovertheassembly steps! Thisorderofoperationsandtheuseofbananajumpersensures thatyourhandsandfaceareawayfromboththebrokencircuitand theDUTwhenyoumakethefinalconnection,applyingwallpower.If something’swrongenoughtocausesomethingtoexplodeorcatch fire,youareunlikelytobeharmedwhilecoweringunderthedesklike that.Getup,findafireextinguisher,fixtheimmediateproblem, thenyoucanworryabouttheproximateproblem. TestProbeMethod Thestepsforusingplainoldtestprobeswiththebrokencircuit areverysimilartothebananajumpermethodabove,exceptthatyou can’treallyhealthebreakinthecircuitwiththemeter’s testleadsuntilaftertheACwallandDUTconnectionsaremade. Thedownsideofthisisthatifsomethinggoesbadlywrong, yourhandsandfaceareupabovetheworksurface,exposedtothis badness. Standardtestprobesdohaveoneadvantageoverbananaplugs, however,whichisthatthey’redesignedtokeepyourfingers wellbackfromthepointofcontact.Ifyouarestartled,you’re likelytojerktheprobesback,breakingthecircuitagain. Becarefulanyway,anddoreconsiderbuildingorbuyingyourself asetofbananajumpers! IGetNoReading! IfeverythingappearstobeconnectedproperlybuttheDUTstays poweredoffandthemeterreads0 A,you’veprobablyblown thecurrentmeasurementfuse,ifthemeterhasone,orthemeteritself otherwise.TherearesomanywaystoblowafuseorfryameterthatI won’tbothertolistthemallhere.Bottomline,youscrewedup somewhere,andnowhavetofigureouthow. It’spossiblethatyoublewthefuseinthemeterprior toembarkingonthisproject,andareonlynowrealizingit.So, takethefuseoutofthemeter,ifithasone,andmeasureits resistance.Thevoltageandresistancemetersectionsaretypically protectedbyadifferentfuse,whichiswhyyoucana)notrealize thecurrentsection’sfuseisblownimmediately,andb)usea metertotestitsowncurrentmeasurementfuse. Ifthefuse’sresistanceislow,theproblemisnotthe fuse. Ifit’shigh,replacingitcouldfixtheproblem,oryou couldblowanotherfuse,whichareexpensiveinthebetterclassof meter.Ifyoublowtwofusesinarow,you’veeithergotabad connectionsomewhere,ortheDUTdrawsmorecurrentthanyourmeter cantolerate.Inthelattercase,whatyoureallyneedisashuntor clampmeter,notabrokencircuit,butthat’satopicIcover elsewhere. AftertheFirstSuccessfulReading Above,Irecommendedsettingthemetertoitshighestcurrent measurementrange,atleastatfirst.Manymetershavetworanges, ahighonewithoutverygreataccuracy,andamuchmoreaccurateone whichwillonlytoleratesomehundredsofmilliamps,typically.If yourmeterisofthissortandyourinitialmeasurementsaysthe deviceisonlypulling,say,0.02 A,youwillnowprobablywant toputthemeterintoitsmoreaccuratemode,whichtypicallygives milliamp-levelprecision,orbetter. Ifthemeasurementisrightontheborderofwhatyourmeter’s lowcurrentrangewilltolerate,youshouldsticktothehigh range.Almosteveryelectricaldevicemadehassomesortofpeak inrushcurrent,orsomeoperatingmodewhereittakesmorecurrent thanittypicallydoes,eitherofwhichcoulddamagethemeteror blowitsfuse. Whenyou’redonetesting,unplugeverythinginreverseorder. Inthebananajumpercase,that’swallplugfirst,DUTsecond, bananajumperslast.Forthetestprobecase,it’stestprobes first,wallplugnext,andDUTlast. Thisarticleiscopyright©2016 byWarrenYoung,allrightsreserved. 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