Chng 4: O IN P C S O LNG IN T

Nhm 9

Nhm 9:1. Nguyn V Bo Quc 2. Nguyn Hoi Phng 3. Nguyn Qu Phc 4. Nguyn Hu Phc (1990) 5. L Vn Qun 6. Nguyn Hu Phc (1988)

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Chng 4: O IN P C S O LNG IN T

Nhm 9

Chng 4 : O IN PI. Gii thiu chungTng t o tn s, in p cng l mt trong nhng tham s rt quan trng ca tn hiu. Khi cn kim tra, xc nh ch lm vic ca thit b in t th bin php xc nh bng cch o in p kh thi hn nhiu so vi o dng in. T thng s in p, ngi ta cn c th xc nh nh o cng in trng, h s phm cht, tr khng ca mch Yu cu c bn v quan trng nht ca mt my o in p l my o cn c tr khng vo rt ln. Ty theo cc di in p o khc nhau m cu to cc my o s khc nhau. II. Cc Tr S in p Cn o Bin in p nh l bin in p cao nht ca cc bin in p tc thi xt trong khong thi gian nht nh thng l 1 chu k ca tn hiu. Bin in p hiu dng l bin in p trung bnh phng ca cc bin in p tc thi xt trong mt khong thi gian nht nh thng l mt chu k ca tn hiu:1 = u (t ) 2 dt T 0T

U hd

Bin in p 1 chiu l bin in p trung bnh ca cc bin in p tc thi xt trong mt khong thi gian thng vn l 1 chu k.U0 = 1 u (t ) dt T 0T

Trong trng hp chnh lu 2 na chu k, th n bng tr s trung bnh cng ca tr s tuyt i cc gi tr tc thi.U0 = 1 | U (t ) | dt T 0Trang 2T

Chng 4: O IN P C S O LNG IN T

Nhm 9

III.

Phn Loi Voltmeter

Cc Voltmet dng trong o lng in t c phn loi cn c vo cc tnh nng sau : Dng ch th: Voltmeter ch th bng kim hay voltmeter ch th bng s. Thng s ca in p o: Voltmeter o in p nh, in p trung bnh hay in p hiu dng. Di tr s in p: Micro Voltmeter Mili Voltmeter Kilo Voltmeter. Mc ch s dng: Voltmeter mu ( lm chun), Voltmeter xoay chiu, Voltmeter 1 chiu, Voltmeter xung

IV.Volt K Ch Th Kim 1. S khi h thng:

Thit b vo gm cc phn t bin i in p o u vo, nh b phn p, suy gim v ph ti katt tng tr khng vo ca Voltmeter. Gip phi hp tr khng to ra tn hiu n nh a ti thc hin o. Khi tch sng: lm nhim v chuyn i tn hiu xoay chiu thnh 1 chiu. C bn c th phn loi chng nh sau: Theo tr s in p hay theo dng in ra ca b tch sng: Tch sng nh (bin ), tch sng hiu dng hay tch sng trung bnh.

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Chng 4: O IN P C S O LNG IN T

Nhm 9

Theo ch tch sng: ch A, B hay C . Theo mch in tch sng: tch sng mch Diode, tch sng anot hay tch sng dng Transistor. Khi khuch i 1 chiu: lm nhim v khuch i tn hiu 1 chiu to ra tn hiu n nh iu khin kim quay, thng l cc tng khuch i vi sai. Khi ch th kim: lm nhim v ch th kt qu o dng kim quay.

2. Cc c tnh tch sng a. Tch sng nh Tch sng nh (hay tch sng bin ) l tch sng m in p u ra trc tip tng ng vi tr s bin ca in p u ra. T in c np in ti tr s nh ca in p o thng qua phn t tch sng ghim gi li tr s bin ca in p. V d cc mch tch sng nh v nguyn l:

Khi a vo in p ux=Umsint th in p t ln it. Trong na chu k dng, c xung dng in ln qua it lm cho t C np nhng cha y. Na chu k m, t C phng in qua R rt t v R rt ln (khong 50-100M); hng s thi gian phng =RC ln hn so vi chu k ca in p o ux.

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Chng 4: O IN P C S O LNG IN T

Nhm 9

Na chu k dng khc, t C li c np. V =Rt.C (Rt l in tr trong ca it) rt b so vi =RC. Nh vy t np rt nhanh m phng rt chm, nn sau mt vi chu k, in p ca t Uc c gi tr khng i v gn bng tr s bin Um ca in p u vo ux.

Gin thi gian in p np cho t hnh a. Khi Uc tng th in p trn it gim: u =uxuc n tch sng c thin p t cp Uc , v Uc < Um (do C phng qua R), nn na chu k dng, tr s tc thi ca u x > Uc th c dng np cho t C. Dng in ch qua it trong mt phn nh ca chu k, vi gc ct l (hnh b). in p Uc trn t in c o bng mt vn-mt ch th kim. Tr s ny cng gn bng Um ca in p ux khi gc ct cng nh. T hnh b: Uc Umsin(90-) = Umcos. = 3Ri R

Theo l thuyt tch sng it, ta c quan h gia gc ct v cc thng s ca mch: Trong , Ri l in tr ca it, R l in tr ti. Mun Uc=Um th =0, thc t l khng c, v Ri0 v R.Trang 5

Chng 4: O IN P C S O LNG IN T

Nhm 9

Song nu cng b cng tt, nn trong mch tch sng nh thng chn it c Ri b.

Mun gim Ri/R bng cch:

Tng R ln qu cng khng c, lm tng hng s thi gian phng p, b tch sng tr thnh c qun tnh, khi gim Ura th Uc vn khng bin i kp. Khuyt im ny tng t nh mch tch sng dng t C c tr s qu ln, v nh vy lm tng hng s p v n.

Xt mch tch sng nh nh hnh b.

Gi s ux = Umsint. na chu k dng th t C c np qua it, sau mt vi chu k, tr s ca Uc gn bng tr s Um. in tr R ln, nn hng s thi gian phng ln, do vy Uc bin i t v coi nh l khng i. C th coi t C nh mt ngun in p mt chiu Uc Um.

Nu in p o khng iu ho, ux = U0 + Umsint, th in p m ng h o c gm Um v c U0 ca ux.

b. Tch sng trung bnh C nhim v bin i in p xoay chiu thnh in p mt chiu c gi tr trung bnh t l vi tr s in p trung bnh ca in p vo.Trang 6

Chng 4: O IN P C S O LNG IN T

Nhm 9

Thng dng cc mch chnh lu na chu k hay c chu k. Mch chnh lu c chu k

Nguyn l: Mch 5-11a dng in i qua ng h ch th trong c hai na chu k u theo mt cng chiu. Na chu k dng dng qua 1 , ng h v 3. Na chu k m dng qua 4, ng h v 2.

lch kim ca ng h micro ampe khi dng phn ng thng ca c tuyn tch sng t l vi t s trung bnh ca in p u vo ca b tch sng.1 U 0 = | u (t ) | dt T 0T

(Tr s ny khi in p c dng khc nhau th khc nhau).

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Chng 4: O IN P C S O LNG IN T

Nhm 9

Mch 5-11b: Na chu k dng, dng in qua 1, R1 v sinh ra h p trn R1, in p ny c o bng vn-mt bi micro ampe v in tr ph R( na chu k ny UR2 gn bng 0).

Na chu k m, dng in qua R2 v 2, vnmt li o in p trn R2. m rng di tn cng tc ta mc thm b phn b tn s

in tr ph R c ni song song vi mch b tn s R1C1 . Khi f ca in p o tng, th tr khng gia a, b gim. Tr khng ca b chnh lu gia im b, c b gim do in dung bn thn C0 gy nn. Do vy s phn b in p trn cc phn t ab v bc l khng i khi tn s in p o thay i. c. Tch sng hiu dng Tch sng hiu dng l bin i in p xoay chiu thnh dng in mt chiu t l vi bnh phng tr s hiu dng ca in p o.

Biu thc xc nh tr s in p hiu dng:Trang 8

Chng 4: O IN P C S O LNG IN T

Nhm 9

B tch sng ny phi bao gm ba phn t thc hin ba thao tc: bnh phng, ly tr trung bnh v khai cn. Dng in tch sng t l vi bnh phng tr s in p hiu dng, khng ph thuc dng in p o.

3. Khi Khuch i Trong Voltmeter in t

C th l b khuch i xoay chiu. B khuch i dng in mt chiu ca vn-mt l khuch i cng sut, n to ra mt cng sut tc ng ln h thng c in iu khin kim dch chuyn ca b phn ch th. Ngoi ra, n cn nhim v phi hp tr khng gia cc b ch th c tr khng vo nh vi tr khng ra tng i ln ca b tch sng hay ca b phn p. Mch khuch i mt chiu thng gp nht l mch kiu cu c hi tip m.

Mch khuch i kiu cu c hi tip m

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Chng 4: O IN P C S O LNG IN T

Nhm 9

Yu cu ca b khuch i mt chiu: H s khuch i c n nh cao. t c hin tng tri im lm vic.

S d chn mch ny v n c nhiu u im : N c th iu chun im khng ca thit b ch th mt cch d dng m

khng cn ngun ph. N t ng cn bng c cc can nhiu ca tp m vo cc B cc n. Nng cao c n nh cng tc v phng nga qu ti v c hi tip m,

Hn na, c tuyn khuch i ca n li thng trong mt di tn tng i rng. Trng hp khuch i mt chiu rt nh, trnh hin tng tri, ngi ta dng mili vn-mt mt chiu.

B khuch i mt chiu thc hin bin i in p mt chiu thnh in p xoay chiu , ri khuch i in p xoay chiu, sau li tch sng ly ra in p mt chiu. B bin tn: bin in p mt chiu thnh in p xoay chiu, c bin t l vi ln v pha tu thuc vo du ca in p mt chiu.

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Chng 4: O IN P C S O LNG IN T

Nhm 9

Sau khi khuch i xoay chiu vi h s khuch i cn thit, th tn hiu c tch sng. B tch sng l b tch sng nhy pha, tch ra in p mt chiu c du sao cho tng ng vi in p 1 chiu u vo b khuch i. B khuch i ny trnh c hin tng tri im lm vic. Cc ch tiu k thut ca b khuch i xoay chiu: h s khuch i n nh, nhy cao, mo khng ng thng nh, di thng tn rng. m bo tnh nng trn, b khuch i in p xoay chiu phi thc hin phn hi m nn s lm cho h s khuch i gim do phi thc hin khuch i nhiu tng, thng dng ba tng hay su tng khuch i. Vi mt s loi vn-mt in t c thang khc l dB th phi dng b khuch i logarit.

4. Khi ch th kim

B ch thi kim l micr ampe mt t in(10-1000A). S d dng ng h t in v c nhy v chnh xc cao, ng thi tha mn dng in nh. Cc b ch th c khc theo n v V(hay mV, V, kV..). Tr s khc l tr s hiu dng ca in p hnh sin.

V. Volt k ch th s S khi h thng

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Chng 4: O IN P C S O LNG IN T

Nhm 9

Nguyn tc o: dng cc b m theo in p chun t xc nh in p cn o.

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