Voltage, Current, Power, Series Resistance, Parallel Resistance, and Diodes

Transcription

1 Lctur 1. oltag, Currnt, Powr, Sris sistanc, Paralll sistanc, and Diods Whn you start to dal with lctronics thr ar thr main concpts to start with: Nam Symbol Unit oltag volt Currnt ampr Powr W watt oltag is dfind as th amount of work don or th nrgy ruird (in jouls) in moving a unit of positiv charg (1 coulomb) from a lowr potntial to a highr potntial. oltag is also calld potntial diffrnc (PD) or lctromotiv forc (mf). Whn you masur voltag you must hav two points to compar, on of thm bing th rfrnc point. Whn masuring th voltag drop on a circuit componnt it is somtims calld masuring th potntial across that componnt. 1 volt 1 joul/coulomb oltag may b considrd lctrical prssur. t is analogous to watr prssur in a municipal watr supply. A battry is to voltag as a pump is to watr prssur. Currnt is th amount of lctric charg (coulombs) flowing past a spcific point in a conductor ovr an intrval of on scond. 1 ampr 1 coulomb/scond Elctron flow is from a lowr potntial to a highr potntial. Wir Currnt flow is convntionally thought to flow from th positiv to th ngativ potntial in a circuit. This is du to blif of arlir scintists.

2 Powr is th rat at which nrgy is usd in an lmnt. 1 watt 1 joul/scond oltag can b considrd to b lctrical prssur. You can think of a pump and a battry as uivalnt. BATTEY sistor Pump Hat Exchangr Currnt Watr oltag is th prssur usd to push th currnt through th circuit. Currnt is th charg that movs through th circuit. Making voltag masurmnts ruirs two points of rfrnc.

3 Basic Elctrical Laws Ohm s Law Ohm s Law is th basis for which all th othr paramtrs may b dtrmind. Kirchoff s oltag Law 0 Kirchoff s oltag Law is drivd from th laws of th consrvation of nrgy. n a circuit if you trac a path from a known starting point and includ all th componnts ncssary for an analysis and nd at th starting point, this is calld a loop. f you sum all th voltags in that loop th sum will ual zro volts. Typically in a circuit thr is a powr supply, ithr AC or DC. For Kirchoff s Law to b valid th sum of th voltags across ach componnt must ual th powr supply valu. This is why it is important to mak sur th polaritis on th componnts ar accurat. Kirchoff s Currnt Law n Th point in a circuit whr componnts ar connctd togthr is calld a nod. Kirchoff s Currnt Law says that th valu of th currnt flowing into a nod is ual to th sum of th currnts flowing out of th nod. Again this prsrvs th consrvation of nrgy.

4 Circuits Sris 3 n A a r1 r2 1 2 B TOTAL TOTAL From Kirchoff s Law 0 From Ohm s Law 1 2 a a 1 2

5 Substituting in th abov uation givs 0 a a 1 a 2 Equivalnt Circuit b r b Sinc w hav th sam currnt in ach circuit w can say a b Substituting for a and b givs, sinc w hav an uality and both th numrators and dnominators ar ual.

6 Paralll sistanc n A 1 2 r r2 4 B By inspction you can s that 1 4. From Kirchoff s Currnt Law From Kirchoff s oltag, using a loop from th battry, through th 1 rsistor and back to th battry givs 0,

7 Taking a loop from th battry through 2 givs Substituting 2 and 3 in givs 2 1 Equivalnt Circuit b r b Sinc 1 b w can uat th two uations giving

8 Again, du to th uality, sinc th numrators ar ual th dnominators ar ual and w gt To simplify this fraction find th LCD 1 What you rally want is so just invrt th abov uation to gt th mor asily rcognizd uation for paralll rsistanc 1 2 DODES A diod can b considrd an lctrical onway valv. Thy ar usually mad from silicon (SD), grmanium (GE), or gallium arsnid (GaAs). Thy ach hav thir own oprating charactristics. For our discussion w will assum w ar working with silicon diods. A diod is a two trminal dvic. ANODE D1 DODE CATHODE t is mad from two typs of matrial calld P and N. Th anod is mad from P matrial, which has bn dopd to hav an xcss of positiv ions. Th cathod has bn dopd to hav an xcss of ngativ ions.

9 Currnt conducts asily from th anod to th cathod and not asily from th cathod to th anod. Th diod you will gt in th lab has a strip on on nd. This is th cathod. Curv As th potntial across th diod incrass it is asir for th currnt to flow from th P to th N rgion. At th point whr th currnt flows xponntially is calld T, th thrshold voltag. Currnt will flow in th rvrs dirction whn you rach th vrs Brakdown oltag, Z. n this rgion you ar litrally ripping lctrons away from th N ions and forcing thm to combin with th P matrial. Usually this occurs by applying 100 volts or mor to th diod. Diods can b usd to convrt AC voltag to DC voltag. This is calld rctification. You will study a half wav rctifir in lab.

10 DODE SGNAL AC load out HALF WAE ECTFE CCUT Sinc a diod can only conduct whn T is > 0.6 th output looks lik