2.
TYPESOFSWITCHES
2.1
Classificationby
USES
Theycanbeclassifiedintothree;power,selectoranddig-
italswitches.
a)PowerSwitch
PowerswitchconnectsordisconnectsanACpowerline
totheprimarywindingofa powertransformer.Thisswitch
hasbeensodesignedtostandhighheattodealwitha large
currentwhichgeneratesarcsandheatwhenswitched.
Thecontactpiecesshould,therefore,bemadeofa mate-
rialwhichhasa highmeltingpointandshouldmovequick-
lytominimizesparks,andthegapbetweenthecontact
pointsshouldbewideenoughtopreventthepiecesfrom
meltingandsticking.Ifthegapisnarrowortheopera-
tionisslow,thearcwillcontinuefora longperiod.Fig.
3showsa typicalpowerswitch.Thecontactpointsare
madeofsilvercadmiumoxide(AgCdO)andareoperated
quicklywiththehelpofa spring.AllPioneerproductsem-
ploypowerswitcheswhichhavepreliminarybeenapproved
bytheauthoritiesofsafetystandardsineachcountry.
MajorsafetystandardsareUL(USA),CSA(Canada),
SEMKO(Sweden),DEMKO(Demmark),NEMKO(Nor-
way),
BS(GB),VDE(WestGermany),EI(Finland)and
SEV(Switzerland).
Bs(Ag-plated)
BsorCu
AgCdO
BsorCu
-Bs(Ag-plated)
Terminals
Fig.
3 ContactPointofPowerSwitch
b)SelectorSwitch
Selectorswitchisa multipositionswitchthatpermitsone
ormoreconductorstobeconnectedtooneofothercon-
ductors.Thisisusedinthefollowingcircuitstothesecon-
darywindingofa powertransformer.Mostswitchesare
forselectinga signalandsomeforswitchinga DCcur-
rent.Herewewilldiscusstheswitchesforselectingsignal
channels.AstotheswitchesforopeningandclosingDC
circuitsandforsensingvariousconditions,refertotheitem
"2.2SwitchesClassifiedbyFunctions".
Switchesforselectingsignalsusuallyemploythesliding
mechanismwhichscrubsandcleanthecontactsurfaceby
itselfbecausethesignalstobeselectedareverylowinvolt-
age,
smallincurrentandarehighlyaffectedbytheresis-
tivedustfilm.Figure4 showsthemechanismofa linear
slidingsignalselector.
Theclipormovingpieceismadeofsilver-platedphos-
phorousbronzeandthefixedpieceissilver-platedbrass.
Lowtenacitygreaseisappliedonthecontactsurfacesto
protectsilverfrombeingaffectedbythesulphurintheair.
Thesulphuraffectssilver,formsa resistivefilmonthesil-
versurfaceandcausespoorcontact.
Terminals
Fig.
4
LINEAR
SliderTypeSignalSelector
Thereareshorting(make-before-break)andnonshorting
(break-before-make)typesinthisswitch.Fig.5showsthe
difference.Type(a)onceconnectsandtype(b)onceopens
allterminalsofA,B andC whenswitching.Generally,
shortingtypeisusedinmuters,attenuators,mode-
selectors,etc.andnonshortingtypeisusedinfunction
selectors,tapemonitors,etc.
Fig.
5 ShortingandNonshortingTypesofContacts
c)DigitalSwitch
Theswitchexclusivelyusedina digitalcircuitiscalled
digitalswitch.Thisisalsoa kindofsignalselectors.
Generallytheyareusedinkeyinputanddatacircuits.
3
Clip
Conductivepiece
