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Unformatted text preview: t he rest of t he c ar's loads).
As is t he c ase wit h mult iple- range volt met ers, ammet ers c an be given more t han one
usable range by inc orporat ing several shunt resist ors swit c hed wit h a mult i- pole swit c h: Not ic e t hat t he range resist ors are c onnec t ed t hrough t he swit c h so as t o be in parallel
wit h t he met er movement , rat her t han in series as it was in t he volt met er design. T he
f ive- posit ion swit c h makes c ont ac t wit h only one resist or at a t ime, of c ourse. Eac h
resist or is sized ac c ordingly f or a dif f erent f ull- sc ale range, based on t he part ic ular rat ing
of t he met er movement (1 mA, 500 Ω).
Wit h suc h a met er design, eac h resist or value is det ermined by t he same t ec hnique,
using a known t ot al c urrent , movement f ull- sc ale def lec t ion rat ing, and movement
resist anc e. F or an ammet er wit h ranges of 100 mA, 1 A, 10 A, and 100 A, t he shunt
resist anc es would be as suc h: Not ic e t hat t hese shunt resist or values are very low! 5.00005 mΩ is 5.00005 milli- ohms,
http://www.allaboutcir cuits.com/vol_1/chpt_8/4.html 4/8 1/22/2014 Ammeter desig n : Dc M eter ing Cir cuits or 0.00500005 ohms! T o ac hieve t hese low resist anc es, ammet er shunt resist ors of t en
have t o be c ust om- made f rom relat ively large- diamet er wire or solid piec es of met al.
One t hing t o be aware of when sizing ammet er shunt resist ors is t he f ac t or of power
dissipat ion. Unlike t he volt met er, an ammet er's range resist ors have t o c arry large
amount s of c urrent . If t hose shunt resist ors are not sized ac c ordingly, t hey may
overheat and suf f er damage, or at t he very least lose ac c urac y due t o overheat ing. F or
t he example met er above, t he power dissipat ions at f ull- sc ale indic at ion are (t he doublesquiggly lines represent "approximat ely equal t o" in mat hemat ic s): An 1/8 wat t resist or would work just f ine f or R4, a 1/2 wat t resist or would suf f ic e f or R3
and a 5 wat t f or R2 (alt hough resist ors t end t o maint ain t heir long- t erm ac c urac y bet t er
if not operat ed near t heir rat ed power dissipat ion, so you might want t o over- rat e
resist ors R2 and R3), but prec ision 50 wat t resist ors are...
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- Winter '11