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SECTION I V
Theory of O p e r a t i o n
4 . 1
I n t r o d u c t i o n
T h i s s e c t i o n c o n t a i n s t h e t h e o r y of o p e r a t i o n of t h e DRC-7/70 series
of i n s t r u m e n t s and c o n t a i n s a d e t a i l e d d e s c r i p t i o n of t h e m i c r o p r o c e s s o r
t e m p e r a t u r e d e t e r m i n a t i o n and c o n t r o l .
4.2
General D e s c r i p t i o n
R e f e r t o F i g u r e 4 . 1 , 4 . 2 and F i g u r e 6 . 1 f o r t h e f o l l o w i n g d i s c u s s i o n .
A p r e c i s i o n 1 0 microampere c o n s t a n t c u r r e n t s o u r c e i s used t o e x c i t e t h e
d i o d e s e n s o r (Model DT-500-DRC
o r DT-500CU-DRC-36).
The v o l t a g e g e n e r a t e d
i s f e d i n t o a n A/D c o n v e r t e r , where
i t
i s c o n v e r t e d t o a d i g i t a l s i g n a l .
The m u l t i p l e x e d BCD o u t p u t s of t h e A/D are sampled by a m i c r o p r o c e s s o r .
The
program f o r t e m p e r a t u r e conversion, and t h e DRC c u r v e , which i s broken up
i n t o 32 b r e a k p o i n t segments, are s t o r e d i n a PROM.
The m i c r o p r o c e s s o r
e x e c u t e s a program which samples t h e s e n s o r v o l t a g e and, u s i n g b r e a k p o i n t
v o l t a g e , t e m p e r a t u r e , and s l o p e i n f o r m a t i o n , c a l c u l a t e s t h e c o r r e c t K e l v i n
t e m p e r a t u r e .
The m i c r o p r o c e s s o r t h e n o u t p u t s t h e t e m p e r a t u r e i n f o r m a t i o n
which i s decoded and d i s p l a y e d by a n LED d i s p l a y .
The m i c r o p r o c e s s o r i s a n F8 w i t h a 3850/3853 c o n f i g u r a t i o n .
The m i c r o p r o c e s s o r a l s o c o n t r o l s the BCD o u t p u t of t e m p e r a t u r e and
BCD i n p u t of s e t p o i n t ( f o r c o n t r o l l e r s ) .
For t h e DRC-7C/70C Thermometer/Controllers, t h e F8 d e t e r m i n e s i f
c o n t r o l i s n e c e s s a r y and c a r r i e s o u t t h e c o n t r o l by a d d i n g power o r r e d u c i n g
power a p p l i e d t o a s y s t e m through a h e a t e r .
4.3
D e t a i l e d D e s c r i p t i o n
A d e t a i l e d d e s c r i p t i o n of t h e o p e r a t i o n of a DRC i n s t r u m e n t i s o u t -
l i n e d i n t h e f o l l o w i n g s e c t i o n s .
Refer t o F i g u r e s 6 . 1 and 6 . 2 f o r t h e
f o l l o w i n g d i s c u s s i o n .
4 . 3 . 1
Power S u p p l i e s
There are s i x d i f f e r e n t power s u p p l i e s i n c o r p o r a t e d i n t h e DRC
i n s t r u m e n t .
The main power t r a n s f o r m e r , TX 1, h a s two s p l i t p r i m a r i e s f o r
115 o r 230 v o l t AC o p e r a t i o n .
The s l i d e s w i t c h , S 1 , selects t h e p r o p e r l i n e
v o l t a g e .
There a r e t h r e e d i f f e r e n t s e c o n d a r i e s which p r o v i d e t h e r a w v o l t a g e s
f o r t h e s u p p l i e s .
The f i r s t s e c o n d a r y , which i s o u t p u t e d t h r o u g h l e a d s 5 , 6 and 7 of t h e
t r a n s f o r m e r , i s r e c t i f i e d by CR5-8.
The 5 V r e g u l a t e d s u p p l y t h a t i s used by
t h e
I.C.'s
i s formed by C7, C 8 and a 5 - v o l t p o s i t i v e v o l t a g e r e g u l a t o r , U34.
A 5 - v o l t u n f i l t e r e d ( b u t r e g u l a t e d ) s u p p l y f o r t h e LED d i s p l a y i s provided
by C6, C9 and t h e 5 - v o l t p o s i t i v e v o l t a g e r e g u l a t o r U31.
The r e a s o n f o r
s p l i t t i n g t h e 5 - v o l t s u p p l i e s i s t o avoid t h e LED d i s p l a y from l o a d i n g down
t h e main 5 - v o l t s u p p l y .
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