Chng 6: Cm bin Nhit

Trang 179

Chng 6

CM BIN NHIT (Temperature Sensors)

6.1 Khi nim

Trong tt c cc i lng vt l, nhit l mt trong cc i lng c

quan tm nhiu nht v nhit ng vai tr quyt nh n nhiu tnh cht ca

vt cht. Nhit c th lm nh hng n cc i lng chu tc dng ca n.

Th d nh p sut, th tch ca cht kh. Bi vy trong cng nghip cng nh

i sng hng ngy phi o nhit . Dng c o nhit n gin nht l nhit

k s dng hin tng gin n nhit. ch to cc b cm bin nhit ngi

ta s dng nhiu nguyn l cm bin khc nhau nh:

o Phng php quang da trn s phn b ph bc x nhit do dao ng nhit

(hiu ng Doppler).

o Phng php da trn s gin n ca vt rn, cht lng hoc cht kh (vi p

sut khng i) hoc da trn tc m.

o Phng php in da trn s ph thuc ca cc in tr vo nhit .

o c tr s chnh xc ca nhit l vn khng n gin. Nhit

l i lng ch c th o gin tip trn c s tnh cht ca vt ph thuc vo

nhit . Trc khi o nhit ta cn cp n cc c tnh ca cm bin nhit

.

6.2 Cc c tnh ca cm bin nhit

a. Thang o nhit

Vic xc nh thang nhit xut pht t cc nh lut nhit ng hc.

Thang o nhit tuyt i c xc nh da trn tnh cht ca kh l tng.

nh lut Carnot nu r: Hiu sut ca mt ng c nhit thun nghch hot

ng gia 2 ngun c nhit 1 v 2 trong mt thang o bt k ch ph thuc

vo 1 v 2:

Chng 6: Cm bin Nhit

Trang 180

)(F)(F

2

1

= (6.1)

Dng ca hm F ch ph thuc vo thang o nhit . Ngc li, vic la

chn hm F s quyt nh thang o nhit . t F() = T chng ta s xc nh T

nh l nhit nhit ng hc tuyt i v hiu sut ca ng c nhit thun

nghch s c vit nh sau:

2

1

TT

1= (6.2)

Trong : T1 v T2 l nhit nhit ng hc tuyt i ca hai ngun.

b. Thang Kelvin

Nm 1664 Robert Hook thit lp im khng l im ng ca nc ct.

Thomson (Kelvin) nh vt l Anh, nm 1852 xc nh thang nhit . Thang

Kelvin n v l 0K, ngi ta gn cho nhit ca im cn bng ca 3 trng thi

nc nc hi mt tr s bng 273,15 0K.

c. Thang Celsius

Nm 1742 Andreas Celsius l nh vt l Thy in a ra thang nhit

bch phn. Trong thang ny n v o nhit l 0C, mt Celsius bng mt

Kelvin. Quan h gia nhit Celsius v nhit Kelvin c xc nh bng

biu thc:

T(0C) = T(0K) 273,15 (6.3)

d. Thang Fahrenheit

Nm 1706 Fahrenheit nh vt l H Lan a ra thang nhit c im

nc tan l 320 v si 2120. n v nhit l Fahrenheit (0F). Quan h gia

nhit Celsius v Fahrenheit c cho theo biu thc:

{ }95

32)F(T)C(T 00 = (6.4)

32)C(T59

)F(T 00 += (6.5)

Bng thng s c trng ca mt s thang o nhit khc nhau:

Chng 6: Cm bin Nhit

Trang 181

Nhit Kelvin (0K) Celsius (0C) Fahrenheit (0F)

im 0 tuyt i 0 -273,15 -459,67

Hn hp nc nc 273,15 0 32

Cn bng nc nc hi nc 273,16 0,01 32,018

Nc si 373,15 100 212

Nhit o c

Nhit o c nh mt in tr hay mt cp nhit, chnh bng nhit

ca cm bin v k hiu l TC. N ph thuc vo nhit mi trng k hiu l

TX v vo s trao i nhit trong . Nhim v ca ngi thc nghim l lm

th no gim hiu s TX TC xung nh nht. C hai bin php gim s

khc bit gia TX v TC:

o Tng trao i nhit gia cm bin v mi trng o.

o Gim trao i nhit gia cm bin v mi trng bn ngoi.

o nhit trong lng vt rn

Thng thng cm bin c trang b mt lp v bc bn ngoi. o

nhit ca mt vt rn bng cm bin nhit , t b mt ca vt ngi ta

khoan mt l nh ng knh bng r v su bng L. L ny dng a cm

bin vo su trong cht rn. tng chnh xc ca kt qu phi m bo hai

iu kin:

o Chiu su ca l khoan phi bng hoc ln hn gp 10 ln ng knh ca

n (L 10r).

o Gim tr khng nhit gia vt rn v cm bin bng cch gim khong cch

gia v cm bin v thnh l khoan. Khong cch gia v cm bin v thnh

l khoan phi c lp y bng mt vt liu dn nhit tt.

6.3 Nhit in tr vi

in tr ca kim loi thay i theo nhit .

dao ng nhit quanh cc v tr cn bng ca chng, c

ng t do trong khong khng gian gia cc ion b

Khi nhit cng cao

cc ion cng tng dn ti

nng va chm gia ion d

vi cc ion dng nm mt trt t trong mng tinh th l

lm cn tr s di chuyn ca io

Vy in tr kim lo

6.3.1 in tr kim loi thay i theo nhit

S chuyn ng ca cc ht mang in tch theo mt h

dng in trong kim loi. S chuyn ng n

trng gy nn v i

nhau. dn in c

kim loi c h s nhit d

Trong hnh 6.2

Nh th in tr kim loi c h s nhit in tr d

Chng 6: C

Trang 182

t in tr vi Platin v Nickel

n tr ca kim loi thay i theo nhit . nhit bnh th

ng nhit quanh cc v tr cn bng ca chng, cn cc electron th c th

ng t do trong khong khng gian gia cc ion bn trong vt th kim loi.

Hnh 6.1 Cu trc mng tinh th ca kim loi

ng cao lm cho cc ion dao ng cng mnh

n ti vn tc chuyn ng nhit ca electron tng

m gia ion dng vi electron cng ln. Chnh s va chm gia electron

m mt trt t trong mng tinh th l nguyn nhn gy ra i

n tr s di chuyn ca ion. Hnh 6.1

kim loi s tng ln khi nhit tng.

n tr kim loi thay i theo nhit

chuyn ng ca cc ht mang in tch theo mt h

n trong kim loi. S chuyn ng ny c th do mt lc c

n v in tch c th l m hay dng dch chuyn vi chiu ng

ca kim loi thun t l nghch vi nhit hay in tr ca

i c h s nhit dng.

2 ta c cc c tuyn in tr ca cc kim loi theo nhit .

in tr kim loi c h s nhit in tr dng PTC (Positive

: Cm bin Nhit

nh thng, cc ion ch

n cc electron th c th chuyn

t th kim loi.

u trc mng tinh th ca kim loi.

nh, nn s mt trt t

n tc chuyn ng nhit ca electron tng lm tng kh

n. Chnh s va chm gia electron

nguyn nhn gy ra in tr

ng hnh thnh mt

do mt lc c hc hay in

ch chuyn vi chiu ngc

l nghch vi nhit hay in tr ca

c tuyn in tr ca cc kim loi theo nhit .

ng PTC (Positive

Chng 6: Cm bin Nhit

Trang 183

Temperature Coefficient): in tr kim loi tng khi nhit tng. hiu ng

ny c th s dng c trong vic o nhit , h s nhit cn phi ln. iu

c ngha l c s thay i in tr kh ln i vi nhit . Ngoi ra cc tnh

cht ca kim loi khng c thay i nhiu sau mt thi gian di. H s nhit

khng ph thuc vo nhit , p sut v khng b nh hng bi cc ha

cht. Gia nhit v in tr thng khng c s tuyn tnh, n c din t

bi mt biu thc a cp cao:

R(t) = R0 (1 + A.t + B.t2 + C.t3 +) (6.6)

Hnh 6.2 Cc c tuyn in tr ca cc kim loi theo nhit .

o R0: in tr c xc nh mt nhit nht nh.

o t2, t3: Cc phn t c ch nhiu hay t ty theo yu cu chnh xc ca

php o.

o A, B, C: Cc h s ty theo vt liu kim loi v din t s lin h gia nhit

v in tr mt cch r rng.

Thng thng c tnh ca nhit in tr c th hin ch mt h s , n

thay th cho h s nhit trung bnh trong thang o (v d t 00C n 1000C.)

0 200 400 600 800 Nhit

in tr St

ng

Than

Chng 6: Cm bin Nhit

Trang 184

= (R100 - R0) / 100. R0 (C-1) (6.7)

6.3.2 Nhit in tr Platin

Platin l vt liu cho nhit in tr c dng rng ri trong cng nghip.

C 2 tiu chun i vi nhit in tr platin, s khc nhau gia chng nm

mc tinh khit ca vt liu. Hu ht cc quc gia s dng tiu chun quc t

DIN IEC751-1983 (c sa i ln th nht vo nm 1986, ln th 2 vo nm

1995), USA vn tip tc s dng tiu chun ring.

c 2 tiu chun u s dng phng trnh Callendar - Van Dusen:

R(t) = R0 (1 + A.t + B.t2 + C[t - 1000C].t3) (6.8)

R0 l tr s in tr nh mc 00C.

Standard [//C] R0 [] H s Nc sn sut

IEC751

(Pt100) 0.00385055 100

200C < t < 0C

A = 3.90830x10-3

B = -5.77500x10-7

C = -4.18301x10-12

0C < t < 850C

A &B nh trn,

ring

C = 0.0

c, o, B, Brazil,

Bulgaria, Canada, Cng

ha Czech, an mch, Ai

Cp, Phn Lan, Php, c,

Israel, , Nht, Ba Lan,

Rumania, Nam phi, Th

Nh K, Nga, Anh, USA

SAMA

RC-4 0.0039200 98.129

A= 3.97869x10-3

B = -5.86863x10-7

C = -4.16696x10-12

USA

R0 ca nhit in tr Pt 100 l 100, ca Pt 500 l 500, ca Pt 1000 l

1000. Cc loi Pt 500, Pt 1000 c h s nhit ln hn, do nhy ln

hn: in tr thay i mnh hn theo nhit . Ngoi ra cn c loi Pt 10 c

nhy km dng o nhit trn 6000C.

Chng 6: Cm bin Nhit

Trang 185

Tiu chun IEC751 ch nh ngha 2 Cp dung sai A, B. Trn thc t xut

hin thm loi C v D xem bng pha di. Cc tiu chun ny cng p dng cho

cc loi nhit in tr khc.

Cp dung sai Dung sai (C)

A t = (0.15 + 0.002.| t |)

B t = (0.30 + 0.005. | t |)

C t = (0.40 + 0.009. | t |)

D t = (0.60 + 0.0018. | t |)

Theo tiu chun DIN vt liu platin dng lm nhit in tr c pha tp. Do

khi b cc tp cht khc thm thu trong qu trnh s dng s thay i tr s

in ca n t hn so vi cc platin thun. Nh th c s n nh lu di theo thi

gian, thch hp hn trong cng nghip. Trong cng nghip nhit in tr platin

thng dng c ng knh 30m (so snh vi ng knh si tc khong

100m).

6.3.3 Nhit in tr nickel

Nhit in tr nickel so vi platin r tin hn v c h s nhit ln gn

gp hai ln (6,18.10-3 0C-1). Tuy nhin di o ch t -600C n +2500C, v trn

3500C nickel c s thay i v pha. Cm bin nickel 100 thng dng trong cng

nghip iu ha nhit phng.

R(t) = R0 (1 + A.t +B.t2 +D.t4 +F.t6) (6.9)

A = 5.485x10-3 B = 6.650x10-6 D = 2.805x10-11 F = -2.000x10-17.

Vi cc trng hp khng i hi s chnh xc cao ta s dng phng trnh

sau:

R(t) = R0 (1 + .t) (6.10)

= 0.00672 0C-1

T d dng chuyn i thnh gi tr nhit :

t = (Rt / R0 - 1) / = (Rt / R0 - 1) / 0.00672 (6.11)

Chng 6: Cm bin Nhit

Trang 186

Hnh 6.3 ng c tnh k thut ca cm bin ZNI1000

Cm bin nhit ZNI1000 do hng ZETEX Semiconductors sn xut s

dng nhit in tr Ni, c thit k c gi tr 1000 ti 00C.

6.4 Cm bin nhit vi vt liu bn dn silic

Cm bin nhit vi vt liu silic ang ngy cng ng vai tr quan trng trong

cc h thng in t. Vi cm bin silic, bn cnh c im tuyn tnh, s chnh xc,

ph tn thp, v c th c tch hp trong 1 IC cng vi b phn khuch i v cc

yu cu x l tn hiu khc. H thng tr nn nh gn, mc phc tp cao hn v

chy nhanh hn. K thut cm bin nhit truyn thng nh cp nhit in, nhit in

tr c c tuyn khng tuyn tnh v yu cu s iu chnh c th chuyn i chnh

xc t gi tr nhit sang i lng in (dng hay p) ang c thay th dn bi

cm bin silic vi li im l s nh gn ca mch in tch hp v d s dng.

6.4.1 Nguyn tc

Hnh 6.4 th hin cu trc c bn ca mt cm bin. kch thc ca cm bin l

500 x 500 x 200 m. Mt trn ca cm bin l mt lp SiO2 c mt vng hnh trn

c m kim loi c ng knh khong 20m, ton b mt y c m kim loi.

Chng 6: Cm bin Nhit

Trang 187

Hnh 6.4 Cu trc c bn ca mt cm bin.

Hnh 6.5 biu din mch in tng ng tng trng thay th cho cm bin

silic (sn xut theo nguyn tc in tr phn ri (spreading resistance)). S sp xp ny

dn n s phn b dng qua tinh th c dng hnh nn, y l ngun gc ca tn gi

in tr phn ri.

Hnh 6.5 Mch in tng ng cho cm bin silic.

o in tr cm bin nhit R c xc nh nh sau:

dR ./= (6.12)

R: in tr cm bin nhit.

: in tr sut ca vt liu silic ( l thuc vo nhit ).

d: ng knh ca hnh trn vng m kim loi mt trn.

Hnh 6.6 th hin loi kt cu th hai ca cm bin. Li im ca kiu kt cu

ny l in tr cm bin khng ph thuc vo chiu dng in. Tri li kiu kt cu

th nht, dnh cho dng in ln hn v nhit trn 1000C, s thay i in tr ca

cm bin nh.

Chng 6: Cm bin Nhit

Trang 188

Cm bin nhit silic vi nguyn tc in tr phn ri c h s nhit dng nh

trng hp cm bin nhit vi vt liu platin hay nickel. Hnh 6.6

Hnh 6.6 Kt cu gm hai cm bin mc ni tip nhng ngc cc tnh.

6.4.2 c trng k thut c bn ca dng cm bin KTY (hng Philips sn xut)

Vi s chnh xc v n nh lu di ca cm bin vi vt liu silic KTY s dng

cng ngh in tr phn ri l mt s thay th tt cho cc loi cm bin nhit

truyn thng. Cm bin KTY c cc u im chnh sau

o S n nh:

Gi thit cm bin lm vic nhit c gi tr bng mt na gi tr nhit

hot ng cc i, sau thi gian lm vic t nht l 450000 h (khong 51 nm), hoc

sau 1000 h (1,14 nm) hot ng lin tc vi dng nh mc ti gi tr nhit hot

ng cc i cm bin silic s cho kt qu o vi sai s nh bng.

Bng Sai s ca cm bin silic (do thi gian s dng)

TYPE Sai s tiu biu (K) Sai s ln nht (K)

KTY81-1

KTY82-1 0.20 0.50

KTY81-2

KTY82-2 0.20 0.80

KTY83 0.15 0.40

o S dng cng ngh silic:

Do cm bin c sn xut da trn nn tng cng ngh silic nn gin tip chng

ta s hng c li ch t nhng tin b trong lnh vc cng ngh ny, ng thi iu

ny cng gin tip mang li nhng nh hng ch cc cho cng ngh ng gi, ni

m lun c khuynh hng thu nh.

o S tuyn tnh:

Chng 6: Cm bin Nhit

Trang 189

Cm bin vi vt liu silic c h s gn nh l hng s trn ton b thang o.

c tnh ny l mt iu l tng khai thc, s dng. Xem hnh c trng k thut

ca KTY81 hnh 6.7.

Nhit hot ng ca cc cm bin silic thng thng b gii hn 1500C.

KTY 84 vi v bc SOD68 v cng ngh ni c bit gia dy dn v chip c th hot

ng n nhit 300 0C.

Hnh 6.7 c trng k thut ca KTY81.

6.4.3 Mch in tiu biu vi KTY81 hoc KTY82

Hnh 6.8 cho ta mt mch in in hnh c thit k cho cm bin KTY 81-

110 hoc KTY 82-110 (nhit o t 00C n 1000C). in tr R1, R2, cm bin v

cc nhnh in tr R3, chit p P1 v R4 to thnh mt mch cu.

Hnh 6.8 Mch o nhit s dng KTY81-110.

Gi tr R1 v R2 c chn sao cho gi tr dng in qua cm bin gn bng

1mA v tuyn tnh ha cm bin trong di nhit cn o. in p ng ra thay i

Chng 6: Cm bin Nhit

Trang 190

tuyn tnh t 0,2 VS n 0,6 VS (VS= 5 V th Vout thay i t 1V n 3V). Ta iu

chnh P1 Vout =1V ti 00C, ti 1000C iu chnh P2 Vout= 3V. Vi mch in ny

vic iu chnh P2 khng nh hng n vic chnh zero.

6.4.4 IC cm bin nhit

Nhiu cng ty trn th gii ch to IC bn dn o v hiu chnh nhit .

IC cm bin nhit l mch tch hp nhn tn hiu nhit chuyn thnh tn hiu

di dng in p hoc tn hiu dng in. Da vo cc c tnh rt nhy cm ca cc

bn dn vi nhit , to ra in p hoc dng in t l thun vi nhit tuyt i

C, F, K hay ty loi. o tn hiu in ta bit c nhit cn o. Tm o nhit

gii hn t -550C n 1500C, chnh xc t 10C n 20C ty theo tng loi.

S tch cc ca nhit s to ra in tch t do v cc l trng trong cht bn

dn bng s ph v cc phn t, bt cc electron thnh dng t do di chuyn qua cc

vng cu trc mng tinh th, to s xut hin cc l trng nhit lm cho t l in t t

do v cc l trng tng ln theo qui lut hm s m vi nhit . Kt qu ca hin

tng ny l di mc in p thun, dng thun ca mi ni p n trong diode hay

transistor s tng theo hm s m theo nhit .

Trong mch t hp, cm bin nhit thng l in p ca lp chuyn tip pn

trong mt transitor loi bipolar. Texinstruments c STP35 A/B/C; National

Semiconductor LM 35/4.5/50

Cm bin nhit LM 35/ 34 ca National Semiconductor

Hu ht cc cm bin nhit ph bin u kh s dng. Chng hn cp nhit

ngu c mc ng ra thp v yu cu b nhit, thermistor th khng tuyn tnh. Thm

vo ng ra ca cc loi cm bin ny khng tuyn tnh tng ng vi bt k thang

chia nhit no. Cc khi cm bin tch hp c ch to khc phc c nhng

nhc im . Nhng ng ra ca chng quan h vi thang chia Kelvin hn l

Celsius v Fahrenheit.

Loi LM35: Precision Centigrade Temperature Sensor.

Chng 6: Cm bin Nhit

Trang 191

Vi loi LM35 ta c in p ng ra t l trc tip vi thang nhit Celsius

(thang bch phn). Nh th mt mch in b tr im zero ca thang Kelvin (thang

nhit tuyt i) khng cn cn thit nh mt s IC cm bin nhit khc.

c im:

in p hot ng: VS= 4V ti 30V

in p ng ra tuyn tnh: 10mV/0C

Thang o: -550C n1500C vi LM 35/35A,

-400C n1100C vi LM 35C/35CA

00C n1000C vi LM 35D

S t nung nng rt nh: 0,08 0C (trong mi trng khng kh)

Mc khng tuyn tnh ch 1/40C

Cm bin nhit AD 590 ca Analog Devices:

Cm bin AD 590 (Analog Devices) c thit k lm cm bin nhit c tng tr

ng ra kh ln (10 M ). Vi mch c cn bng bi nh sn xut, khin cho dng

mA ra tng ng vi chun nhit tuyt i K. in p lm vic cng nh cng tt

trnh hin tng t gia nhit. Khi cp in p thay i, dng in thay i rt t.

Thang o: -550C ti 1500C

in p hot ng: 4 ti 30 VDC

Dng in ra t l: 1 A/0K

6.4.5 Cc mch o nhit

Mch o nhit bng LM35:

Chng 6: Cm bin Nhit

Trang 192

Hnh 6.9 Mch o nhit bng LM35.

Mch ng dng LM35 vi thit b khuch i m thanh:

Trong mch ng dng ny, nhit IC khuch i m thanh (IC1) l i

lng c quan tm. LM35 v IC1 c s gn kt v nhit. Tn hiu ng ra ca

b so snh s xung mc thp nu nhit vt qu gii hn (thng s ny c

chn bng R1, R2 v in p tham chiu). H thng c thit k qut hot

ng khi nhit vt qu khong gi tr 800C v tt qut khi nhit h xung

di 600C.

Hnh 6.10 LM35 vi b khuch i m thanh cng sut 60W.

6.5 Cch ni dy o

Nhit in tr thay i in tr theo nhit . Vi mt dng in khng

thay i qua nhit in tr, ta c in th o c U = R.I. cm bin khng b

nng ln qua php o, dng in cn phi nh khong 1mA. Vi Pt 100 0C ta

R2

V

5V

LM35

5V 12V

12V

R1

R3

R3 R4

R4

R5

R6 R7

VADJ

1k

1k

1k

1k 10k

10k

8,2k

1,8k

2,2k

c in th khong 0,1V. in th n

c cc k thut ni dy o.

Hnh

Tiu chun IEC 751 y

mu ging nhau ( hoc trng) v

6.5.1 K thut hai dy

Gia nhit in tr v

in no u c in tr, in tr n

tr ca hai dy o, mch in tr s nhn

o. Kt qu ta c ch th nhit k cao h

xa, in tr dy o c th l

V d vi dy ng:

Din tch mt ct dy o:

in tr sut: 0,0017

Chiu di: 100m

Chng 6: C

Trang 193

n th khong 0,1V. in th ny cn c a n my o qua dy o. Ta

thut ni dy o.

Hnh 6.11 Cch ni dy nhit in tr.

n IEC 751 yu cu dy ni n cng u nhit in tr phi c

ng nhau ( hoc trng) v dy ni n 2 u phi khc m

thut hai dy

Hnh 6.12 K thut 2 dy.

a nhit in tr v mch in t c ni bi hai dy. Bt c dy dn

u c in tr, in tr ny ni ni tip vi nhit in tr. Vi hai in

ca hai dy o, mch in tr s nhn c mt in th cao h

t qu ta c ch th nhit k cao hn nhit cn o. Nu khong cch qu

dy o c th ln n vi Ohm

vi dy ng:

n tch mt ct dy o: 0,5mm2

0,0017mm2m-1

100m

: Cm bin Nhit

n my o qua dy o. Ta

u nhit in tr phi c

i n 2 u phi khc mu.

c ni bi hai dy. Bt c dy dn

i ni tip vi nhit in tr. Vi hai in

c mt in th cao hn in th cn

t cn o. Nu khong cch qu

R = 6,8, vi 6,8

l 170C. trnh sai s ca php o do in tr ca dy o gy ra, ng

b tr in tr ca dy o bng mt mch in nh

ni vo mt trong hai dy o v

100. Mch in t

chnh chit p sao cho c ch

6.5.2 K thut 3 dy

T nhit in tr ca dy o

ny ta c hai mch o

chun. Vi k thut 3 dy, sai s cho php o do in tr dy o v

ca n do nhit khng c

v c cng mt nhit . K

6.5.3 K thut 4 dy

Vi k thut 4 dy ta

dng cho dng i

Chng 6: C

Trang 194

i 6,8 tng ng cho nhit in tr Pt 100 mt thay i nhit

trnh sai s ca php o do in tr ca dy o gy ra, ng

in tr ca dy o bng mt mch in nh sau: Mt

t trong hai dy o v nhit in tr c thay th bng mt in tr

ch in t c thit k vi in tr d phng ca dy o l

sao cho c ch th 00C: Chit p v in tr ca dy o l

Hnh 6.13 K thut 3 dy.

nhit in tr ca dy o c ni thm (hnh 6.13

ch o c hnh thnh, mt trong hai mch

n. Vi k thut 3 dy, sai s cho php o do in tr dy o v

a n do nhit khng cn na. Tuy nhin 3 dy o cn c c

t nhit . K thut 3 dy rt ph bin.

Hnh 6.14 K thut 4 dy.

i k thut 4 dy ta thu c kt qu o tt nht. Hai dy

in khng i qua nhit in tr. Hai dy

: Cm bin Nhit

ng cho nhit in tr Pt 100 mt thay i nhit

trnh sai s ca php o do in tr ca dy o gy ra, ngi ta

t chit p b tr c

c thay th bng mt in tr

a dy o l 10. Ta

n tr ca dy o l 10.

6.13). Vi cch ni dy

t trong hai mch c dng lm mch

n. Vi k thut 3 dy, sai s cho php o do in tr dy o v s thay i

n c cng tr s k thut

t qu o tt nht. Hai dy dng 1 c

n khng i qua nhit in tr. Hai dy dng 2 c dng

Chng 6: Cm bin Nhit

Trang 195

lm dy o in th trn nhit in tr. Trng hp tng tr ng vo ca mch

o rt ln so vi in tr dy o, in tr dy o coi nh khng ng k. in

th o c khng b nh hng bi in tr dy o v s thay i ca n do

nhit.

6.5.4 K thut 2 dy vi b bin i tn hiu o

Ngi ta vn c th dng hai dy o m khng b sai s cho php o vi b

bin i tn hiu o. B bin i tn hiu o bin i tn hiu ca cm bin thnh

mt dng in chun, tuyn tnh so vi nhit c cng t 4mA 20mA.

Dng in nui cho b bin i c ti qua hai dy o vi cng khong

4mA. Vi k thut ny tn hiu c khuch i trc khi truyn ti do khng

b nhiu nhiu.

o Nhit in tr vi k thut dy qun:

Nhit in tr vi v gm: Si platin c gi cht trong ng gm s vi

bt oxit nhm. Di o t -2000C n 8000C.

Nhit in tr vi v thy tinh: loi ny c bn c hc v nhy cao.

Di o t - 2000C n 4000C, c dng trong mi trng ha cht c n mn

ha hc cao.

Nhit in tr vi v nha: Gia 2 lp nha polyamid dy platin c ng

knh khong 30mm c dn kn. Vi cu trc mng, cm bin ny c dng

o nhit b mt cc ng hay cun dy bin th. Di o t -800C n 2300C.

Hnh 6.15 Cu trc nhit in tr kim loi dy qun (v ceramic).

o Nhit in tr vi k thut mng mng:

Chng 6: Cm bin Nhit

Trang 196

Cu trc cm bin gm mt lp mng mng (platin) t trn nn ceramic

hoc thy tinh. Tia lazer c s dng chun ha gi tr in tr ca nhit

in tr.

Hnh 6.16 Cu trc nhit in tr kim loi dng mng mng ( ceramic).

6.6 Phng php o nhit

C hai phng pht o nhit :

Phng php trc tip:

o Cp nhit in (Thermocouple)

o Nhit in tr kim loi (RTD)

o Nhit in tr bn dn.

o nhit gin tip:

o Phng php quang (Optical Pyrmeter)

o Phng php bc x in t (Photon Detector)

6.6.1 o trc tip

o Trng hp chn la o trc tip

Khi tm nhit cn o khng qu cao

Thng dng o cc vt tnh (khng di chuyn)

Vt cn o khng c yu cu cao v vic thay i cu trc, khi b gy nh

hng nhiu khi thc hin php o.

Phng php o khng qu phc tp d thc hin.

Gi thnh cho cc linh kin (nh cm bin, khi gia cng,) l khng cao

o Tm nhit o trong phng php o trc tip

c s dng tm o t: -190oC n 1063oC

Chng 6: Cm bin Nhit

Trang 197

Vi:

-190oC: l nhit m Oxy chuyn t th LNG th HI (Kh)

1063oC: l nhit nng chy ca vng (Au)

o Cu to:

Hnh 6.17 Cu to b o nhit phng php o trc tip.

Phng php o trc tip thng dng loi cm bin nhit platin v nickel

o Nguyn l hot ng:

in tr vt thay i theo nhit :

)1( TA

lR

o

+=

= (6.13)

L : chiu di dy dn

A: din tch tit din dy dn

: in tr sut

Khi nhit tng in tr tng

S ph thuc ny gn nh tuyn tnh:

( )TRRT += *10 (6.14)

( ) ( )[ ]000 1 TTbTTaRR += (6.15)

a,b,: hng s ph thuc kim loai

RT: in tr ti nhit cn o

Ro: in tr ti nhit T

a. Cp nhit in (Thermocouple)

Thermocouple (c

loi khc nhau c tip

, u ra in p.

Thermocouple ho

khm ph nm 1821)

Cu to Thermocouple

Chng 6: C

Trang 198

n tr ti nhit cn o

n tr ti nhit To

p nhit in (Thermocouple)

Thermocouple (cp nhit ) l cm bin nhit cu to bi 2 thanh kim

c tip xc nhit vi nhau. Cm bin tc ng: u v

Hnh 6.18 Nguyn l vn hnh cp nhit in

Thermocouple hot ng da trn hiu ng Seebeck. (Do Thomas Seebeck

khm ph nm 1821)

u to Thermocouple

Hnh 6.19 Cu to cp nhit in.

: Cm bin Nhit

m bin nhit cu to bi 2 thanh kim

m bin tc ng: u vo nhit

p nhit in.

(Do Thomas Seebeck

Nguyn l o:

o Thermocouple ch

chun so snh.

( 21 TCV =

o Quan h gia V vi T l

1 eeV +=

(/

/

TeV

ee

AB

BBA

=

=

o Hai u ni T1, T2. u nng l

li l u lnh hay u chun.

o o nhit chnh xc th

nh nhit chun n

c tuyn

Chng 6: C

Trang 199

Hnh 6.20 Nguyn l o cp nhit in

Thermocouple ch o c s sai bit nhit . Do cn phi c nhit

n so snh.

)(2) 212212 TTCT +

gia V vi T l phi tuyn. Nhng ta chp nhn cng thc

1/2/2 TeTee BAAB +=

)12

/

TTAB

u ni T1, T2. u nng l u tip xc vi nhit cn o. u c

u lnh hay u chun.

o nhit chnh xc th phi o c nhit u chun v

chun ny

Hnh 6.21 c tuyn cp nhit in.

: Cm bin Nhit

n.

c s sai bit nhit . Do cn phi c nhit

(6.16)

p nhn cng thc sau:

(6.17)

(6.18)

u tip xc vi nhit cn o. u cn

c nhit u chun v phi gi c

.

Chng 6: Cm bin Nhit

Trang 200

Cc phng php xc nh v gi c nh nhit chun

o Cch 1: C nh nhit chun

Hnh 6.22 Cch c nh nhit chun dng 1.

Nhit o c tnh nh sau

))(( Re fTipBAout TTSSV = )(Re BAout

fTip SS

VTT

= (6.19)

SA, SB l h s Seebeck ph thuc vt liu ch to

BABA SSe =/ (6.20)

o Cch 2: Dng mch in t to nhit chun.

Hnh 6.23 Cch c nh nhit chun dng 2.

o RT xc nh Tref, Tref s c chuyn sang Vref

V1= V-Vref ; T1=V1/e1 (6.21)

S v nguyn tc o

o in p o c ch ph thuc v

m khng ph thuc v

o Nu c mt vt liu kim loi th

nguyn l ny b

(/

1/

TeE

TeE

AB

AB

=

=

Hnh 6.25 Nguyn t

Ch : Tt c cc dy kt ni v

dy ng ni! Dy ni cho T/C l

o Nu ta ni 2 kim loi A v

khng nh hng

Hnh 6.26

Chng 6: C

Trang 201

Hnh 6.24 Nguyn tc o cp nhit in

c ch ph thuc vo s chnh lch nhit gia hai u ni

thuc vo nhng ni khc trong mch

u c mt vt liu kim loi th 2 chn gia th p ra khng b

nguyn l ny bo m cho vic i dy di

)21

2///1

TT

TeTeTeT BAjACiCA

+++

Hnh 6.25 Nguyn tc o cp nhit in khi c vt th 2 ch

t c cc dy kt ni vo TC phi ng loi dy, Khng s dng

ng ni! Dy ni cho T/C l loi dy cng chu nhit cao t tin!

u ta ni 2 kim loi A v B bi mt kim loi th 3 C th

ng

Hnh 6.26 Nguyn tc o cp nhit in khi ta ni kim loi A v

: Cm bin Nhit

c o cp nhit in.

ch nhit gia hai u ni

p ra khng b nh hng

(6.22)

c o cp nhit in khi c vt th 2 chn gia.

i ng loi dy, Khng s dng

i dy cng chu nhit cao t tin!

i mt kim loi th 3 C th in p o cng

c o cp nhit in khi ta ni kim loi A v B.

(/1/

TeE

TeE

AB

CB

=

=

V: CBAB ee // =

o Khi o nhit m

Hnh 6.27 Nguyn t

1213 EE =

o Hoc ta c th o gin tip nh

Hnh 6.28 Nguyn t

acab EE =

Vt liu ch to

Thermocouple

chn theo 4 tiu ch sau:

o nhy

o n nh

o Tng thch vi h thng o

o Chi ph hp l

o nhy: nhy nhit

Chng 6: C

Trang 202

)212/1/1

TT

TeTeT BAAC

++

ACe /

t m cha bit nhit chun ta c th o gin tip nh

Hnh 6.27 Nguyn tc o cp nhit in khi cha bit

23E+

c ta c th o gin tip nh sau:

Hnh 6.28 Nguyn tc o cp nhit in gin tip

cbE+

Thermocouple c ch to bi cc kim loi khc nhau. Cc kim loi

u ch sau:

i h thng o

nhy: nhy nhit c xc nh bi cng thc

: Cm bin Nhit

(6.23)

(6.24)

t nhit chun ta c th o gin tip nh sau:

t nhit chun.

(6.25)

c o cp nhit in gin tip

(6.26)

c ch to bi cc kim loi khc nhau. Cc kim loi c

Chng 6: Cm bin Nhit

Trang 203

dT

dEs BA /= CV o/ (6.27)

b. Nhit in tr kim loi Resistance Temperature Detector (RTD)

o RTD c ch to t nhng dy dn rt nhy vi nhit , vt liu ch to

thng l Niken, ng, Niken-ng, Platin

Hnh 6.28 Cu to cm bin nhit trc tip (RTD)

o o c nhit , RTD c mc theo cu in tr

+

+=

43

3

21

2

RR

R

RR

RVV inout (6.28)

Hnh 6.30 Cu in tr.

Chng 6: Cm bin Nhit

Trang 204

R1=R2=R4=R3=Rg (6.29)

Khi R3=Rg dR

dRR

dRVV

g

inout

=22

(6.30)

o RTD l mt in tr nhit kim loi, gi tr in tr thay i ph thuc vo s

thay i ca nhit . Khi nhit tng th in tr tng, khi nhit gim

th in tr gim.

o RTD c chnh xc cao: RTD trong cng nghip c chnh xc l

0.1C. Standard Platinum RTD c chnh xc l 0.0001C, thng dung

trong phng th nghim.

Hnh 6.31 Hnh dng RTD

Vt liu ch to:

o RTD thng lm bng cc loi kim loi c in tr thay i theo nhit

nh platin, ng, nikel

o Platin c tm nhit ln nht 184.44C (300F) n 648.88C (1200F),

v mt ha hc v s n nh trong cu trc tinh th, c th t chnh xc

cao 99.999%.

o Nikel c nhy nhit cao nht, tm nhit thp khong 250C, hot ng

ha hc cao d b oxi ha khi nhit cao.

Chng 6: Cm bin Nhit

Trang 205

o ng c tuyn tnh cao, tuy nhin do hot ng ha hc cao nn ch hot

ng trong tm < 180C.

Hnh 6.32 c tuyn ca cc loi kim loi thay i in tr theo nhit

RTD c 2 dng: RTD dy qun v RTD b mt.

o Cun dy Platin lm cho R c gi tr ln nn cho php chng do vi nhy

tng i tt

o Gm, mica t trong v bc chng rung ng v va chm mnh.

Cu to ca RTD b mt: RTD b mt thng dng o nhit b mt ca

vt rn.

Hnh 6.33 Cu to RDT loi b mt

Quan h gia in tr v nhit RTD

)......1( 2210N

N TTTRR ++++= (6.31)

Trong : R0 l in tr ca RTD 0C

Chng 6: Cm bin Nhit

Trang 206

o Cc l cc h s nhit in tr, hay nhy nhit T.

o H s nhit in tr c trng cho kim loi, ph thuc vo kim loi v nhit

.

o Trong tm o nhit ca kim loi, th R thay i tuyn tnh vi nhit

nn:

).(1 00

TTR

R=

(6.32)

o Khi nhit ngoi tm o th quan h R v nhit l khng tuyn tnh. C

th biu din theo quan h bc hai:

200

0

).(2).(1 TTTTR

R+=

(6.33)

Mch o RDT:

o Trng hp R tuyn tnh vi nhit

Hnh 6.34 o RDT trng hp R tuyn tnh vi nhit .

o Trng hp R khng tuyn tnh vi nhit

Hnh 6.35 o RDT trng hp R khng tuyn tnh vi nhit .

RTD Rshunt DVM

Dy 1 Dy 2

Dy 3 Dy 4

Vi

Ngun dng

RTD

Dy 1

Dy 2

Dy 3

Vi Ngun p in tr

ni tip

Chnh offset

R1

R2

+

_

Chng 6: Cm bin Nhit

Trang 207

o S dng thm Rshunt=3.RTD, lm tng tnh tuyn tnh (nhng khng hon

ton).

o Mch RTD c Rshunt hot ng trong tm -196C 260C

ng dng RDT

o Dng o nhit trong cc ng ng, kho cha, o nhit khng kh.

o Dng trong cc ngnh cng nghip thc phm, dc phm cn chnh

xc v nhit cao, v an ton (RTD khng gy hi vi mi trng).

c. Nhit in tr bn dn Thermistor

Nguyn tc: Ging nh RTD nhng c cu to t cc loi oxit ca nikel,

mangan, cobal

Tnh cht: C nhy nhit m v rt cao, khong 10 ln so vi cc in tr

kim loi.

o n nh ca nhit in tr ph thuc vo vic ch to v iu kin s

dng.

o Tm hot ng ca thermistor c th t 1 vi tuyt i n khong 300C.

o Thermistor c ch to t nhng vt liu bn dn, s thay i in tr ca

vt liu t l vi nhit trong gii o.

o Khi nhit tng th in tr gim, do vy Thermistor c h s nhit m.

Mc d vy cng c mt s Thermistor c h s nhit dng.

o Thch hp dng trong cc ng dng c gii nhit nh.

Quan h gia in tr v nhit :

))11

.((

00. TT

B

eRR

= (6.34)

o R0 l in tr nhit 25C

o Gi tr B nm trong khong 300o-500oK. Khi nhy nhit:

2T

B= (6.35)

Chng 6: Cm bin Nhit

Trang 208

o Quan h gia in tr v nhit ca thermistor l khng tuyn tnh.

Hnh 6.36 c tuyn ca RDT.

Mch o ca thermistor:

Hnh 6.37 Mch o RDT.

o Thermistor c ni vo mch cu H nh hnh v.

o Khi cu cn bng:

)).((

.

432

30

RRRRR

RR

E

E

TT

T

I +++

=

(6.36)

o Thng thng ngi ta chn R2=R3, Rt=R4. Khi :

TTTTT

TT

I RRRRRRRR

RR

E

E

////2

/

222

0

++++

=

(6.37)

Vo

Vi

R1

R3

RT RTD

R2

Chng 6: Cm bin Nhit

Trang 209

Hn ch ca phng php o trc tip:

Ngoi mt s u im c nu trn, phng php o trc tip ny cn

mt s hn ch nh sau:

o Khng th o nhit trn din tch ln

o Khng o c nhng vt ang chuyn ng

o Ch dng trong tm t -190oC n 1063oC

o C th nh hng n cu trc h thng cn o nhit v cm bin c th

tch ln

6.6.2 o gin tip (cch ly)

a. Cc trng hp cn o cch ly

o Xc nh nhit ca mt vt ang di chuyn.

o Xc nh nhit ca mt vt rt nng khng th o c bng dng c

thng thng.

o Xc nh nhit ca mt vt nm trong khu vc nguy him nh l phn

ng ht nhn, l nung, l luyn kim

o Xc nh nhit ca mt khu vc c din tch rng.

o Xc nh nhit m khng gy nh hng n i tng cn o.

o Xc nh nhit m khng lm thay i cu trc ca vt cn o.

b. Nguyn l o nhit cch ly

o Cc vt th nhit ln hn 0oK pht ra mt bc x in t trn b mt t

l vi nhit ca vt.

o Ta o nhit ca vt da trn bc x in t vt pht ra

o Lin h gia cng v bc sng bc x vi nhit

)1(

2/5

2

=

TkhcA e

hcW

(6.38)

W:cng quang ph bc x ca vt en

Chng 6: Cm bin Nhit

Trang 210

: bc sng bc x

h : hng s Planck

c : vn t nh sng

k : hng s Boltmann

o Ph pht x ca vt en theo bc sng

Hnh 6.38 Ph pht x ca vt en theo bc sng

c. Cc cch o nhit cch ly

o S dng phng php quang (optical pyrometer).

o S dng phng php bc x in t (photon detector).

d. o nhit cach ly dng phng php quang (optical pyrometer)

o Dng o nhit trong khong t 973 - 4273oK.

o Pyrometer hot ng da trn nguyn tc so snh sng ca dy tc bng

n vi sng ca vt cn o

o sng ca bng n c chun ha vi mt ngun en tuyt i pht x

mt nhit bit trc.

Chng 6: Cm bin Nhit

Trang 211

Hnh 6.39 o nhit cch ly bng phng php quang

o Khi cng sng ca nn v dy tc bng nhau th ta c phng trnh:

1

1

1 // 22 =

frr TCTC

(6.39)

l bc sng ca nh sng khong 0.63m

l h s pht x ca vt

Tf l nhit ca dy tc bng n

T l nhit ca vt

Khi T < 4273oK ta c:

1/2 TC v fr TC

T/1/)(ln

1

2 +=

(6.40)

e. o nhit cach ly dng phng php bc x in t (photon detector)

o C rt nhiu ng dng i hi phi xc nh nhit khng tip xc nh: o

lng nhit ta nh, h thng ng dn, khu vc c din tch rng, hoc

nhng l luyn kim

o Thit b o nhit s dng Photon Detector l gii php cho vn trn.

o Photon Detector l mt loi sensor m p ng ca n l in p t l vi mt

dng photon tp trung trn b mt cm bin.

Chng 6: Cm bin Nhit

Trang 212

Hnh 6.41 Hnh dng cm bin o nhit bc x in t

Nguyn l hot ng:

Hnh 6.42 Cm bin o nhit dng cch ly

Cng thc:

)(4 2

2

0 Tgf

DkE t = (6.41)

o kt : nhy ca h thng (bao gm h s truyn, li in p v nhy

ca cm bin)

o Do c nhiu ng dng thng mi v cng nghip s dng photon detector

nn tm o ca n rt a dng

o Vi 1 mt my o nhit quang c bn c th s dng o nhit trong

khong 253 n 1873oK

Chng 6: Cm bin Nhit

Trang 213

o nhy ca scanner trn 30oC l 0.1o

o H thng o lng nhit bng hng ngoi.

o Cc vt th nhit ln hn 0oK pht ra mt bc x in t trn b mt t

l vi nhit ca vt.

Hnh 6.43 H thng o lng nhit bng hng ngoi.

Hnh 6.44 S khi h thng o lng nhit bng hng ngoi.

Hnh 6.45 B iu khin nhit cm tay dng nh sng hng ngoi

o Ba bc chnh o nhit ca vt:

Bc 1: Chuyn tn hiu hng ngoi pht x ra t vt nhn c ra tn

hiu in

Bc 2: B nhit nn ca mi trng cn o.

Bc 3: Tuyn tnh ha kt qu nhn

cn o.

6.7 Mch o

6.7.1 Gii thiu

o IC (LM135/235/335) h

l tuyn tnh vi nhit v

o Khi mun c tm o ln h

cc sensor khc nh Thermocouple, RTD ho

o Vi sensor l RTD, Thermistor th

l in tr, cn v

i l khng tuy

o C thm cc m

n tm mong mun.

6.7.2 o dng mch c

Hnh 6.46 M

Vd: vi RTD 00C, R = 100

R

RVVout =

o R in p ra thay i rt nh

(RIV +=

Chng 6: C

Trang 214

c 3: Tuyn tnh ha kt qu nhn c v xut ra thng tin v

IC (LM135/235/335) hn ch v tm o t -55oC n +150

n tnh vi nhit v d s dng

n c tm o ln hn hoc mun c nhy cao h

cc sensor khc nh Thermocouple, RTD hoc Thermistor.

RTD, Thermistor th i lng thay i tng

n vi Thermocouple l in p, nhng rt nh (V). S thay

khng tuyn tnh vi nhit

C thm cc mch gia cng tn hiu bin i thnh i

n tm mong mun.

cu Wheatstone

Hnh 6.46 Mch o dng cu Wheatstone

C, R = 100 , TC = 0.385/0C

RR

RVV

RR

RR

+

=++

42

R in p ra thay i rt nh

) RIVR =+ *

: Cm bin Nhit

ra thng tin v nhit

n +1500C, tuy u ra (p)

c mun c nhy cao hn th phi s dng

c Thermistor.

ng ng vi nhit

t nh (V). S thay

nh in p, khuch i

(6.42)

(6.43)

Chng 6: Cm bin Nhit

Trang 215

o in p ra thay i khng tuyn tnh vi R.

o p ra ln khi I ln

o I khng th qu ln

o Gii hn cng sut

o To nhit trn phn t o.

o C mt phng php tt hn l s dng cu Wheatstone s dng ngun

dng/p

C 4 loi cu Wheatstone ngun p thng dng

Hnh 6.47 Cc dng cu Wheatstone ngun p

o Cu Wheatstone vi 1 phn t thay i

p ra khng tuyn tnh (c th gii quyt bng phn mm hoc k thut

mch)

nhy 0.5%/%

o Cu Wheatstone vi 2 phn t thay i (dng 1)

Khng tuyn tnh (0.5%/%)

nhy tng gp i so vi trng hp u

o Cu Wheatstone vi 2 phn t thay i (dng 2)

Vo:

Vo

R+R

VB

R+R

R-R

R-R

Vo

R

VB

R+R

R-R

R

Vo

R+R

VB

R+R

R

R

R

Vo

VB

R+R

R

R

VB

4

R

R+R/2

VB

2

R

R+R/2

VB

2

R R

VB R R

Sai s TT: 0,5%/%

0,5%/% 0 0

Chng 6: Cm bin Nhit

Trang 216

Tuyn tnh

nhy tng gp i so vi trng hp u

Cu ny thng dng cho cm bin p sut hoc lu lng.

o Cu Wheatstone vi 4 phn t thay i

Tuyn tnh.

nhy tng gp 4 so vi trng hp u

Cu dng ny thng dng vi Loadcell

Bn dng cu Wheatstone vi ngun dng

Hnh 6.48 Cc dng cu Wheatstone ngun dng

o u im:

in tr dy, tip xc khng gy sai lch kt qu

C th dng dy dn n gin, r hn.

6.7.3 Tuyn tnh ha khuch i tn hiu sau cu

o Tn hiu a ra cn nh (vi chc vi trm mV), do cn phi c

khuch i trc khi a vo b ADC hoc x l tip.

Khuch i n gin c th dng b OpAmp

Vo:

Vo

R+R

R+R

R-R

R-R

Vo

R

R+R

R-R

R

Vo

R+R

R+R

R

R

R

Vo

IB

R+R

R

R

IBR

4

R

R+R/4

IB

2 R

IB

2 R IB R

Sai s TT: 0,25%/%

0 0 0

IB IB IB

Chng 6: Cm bin Nhit

Trang 217

Hnh 6.49 Mch khuch i n gin dng Opamp

o Mch ny cho chnh xc khng cao

khuch i ph thuc vo in tr cu, RF

Tng tr khng cn bng ng vo.

p ra khng tuyn tnh do ch n thun l b khuch i

Hnh 6.50 Mch khuch i tuyn tnh c chnh li.

o Mch ny cho chnh xc cao

o u im:

chnh xc ln hn do khuch i ch ph thuc vo Rg.

Tng tr vo ln nn khng nh hng n cu o v in tr cu o

khng nh hng n mch s cn bng ca b khuch i.

o Khuyt im:

p ra khng tuyn tnh, khc phc bng software

R

REF

VB

R

R+R

R VOUT

+VS

-VS VOUT=

VB

4

R

R+R/2 GAIN

VREF=VB

RG

VS

R RF

VB

R

R+R R

2

+VS

Chng 6: Cm bin Nhit

Trang 218

Mch ny va khuch i, va tuyn tnh ho Vout so vi R

Hnh 6.51 Mch khuch i tuyn tnh dng 2 Opamp.

c tuyn ca cc loi cm bin

Hnh 6.52 c tuyn ca cc loi cm bin

6.7.4 Nguyn l x l tn hiu t RTD v Thermistor

Dng mch Opamp

Hnh 6.53 Mch x l tn hiu Thermistor dng opamp

R

VB

R

R+R R

VOUT

+VS

-VS

VOUT= VB

2

R R

R1 R2

1+

+VS

-VS

Dng mch cu cn bng

Hnh 6.54

Dng IC vi x l

Hnh 6.54

Chng 6: C

Trang 219

ch cu cn bng

Hnh 6.54 Mch x l tn hiu RTD dng cu cn bng

l

Hnh 6.54 Mch x l tn hiu Thermistor dng IC

: Cm bin Nhit

u cn bng

ng IC