Philips PM2517E Service Manual page 34

3.3.2.5. Besistance measurements (Fig.'s 16. 17 and 18)
- T h e r e s i s t a n c e m e a s u r i n g c i r c u i t i s d r a w n i n f i g u r e 1 6 .
- A k n o w n c o n s t a n t c u r r e n t w h i c h i s s u p p l i e d b V t h e p r o g r a m m a b l e c u r r e n t s o u r c e O O 0 0 6 3 f l o w s t h r o u g h
t h e u n k n o w n r e s i s t o r R X .
D e p e n d e n t t o t h e r a n g e ( m a n u a l o r a u t o m a t i c s e l e c t i o n ) t h e c u r r e n t s a r e s e l e c t e d b y t h e s i g n a l s R N G A ,
R N G B a n d R N G C ( s e e t a b l e ) .
r .
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I
26
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t r x
R N G C
R N G B R N G A R A N G E
l m A
100 r1A
1 0 g A
1 ,uA
1 0 0 n A
t x x
0 0 0
0 0 1
0 1 0
0 1 1
1 K
1 0 K
1 0 0 K
1 M
1 0 M
- 1 V
- 1 V
, 1 V
- 1 V
Forward R eversed
G e 1 0 0 . 0 . . . 3 0 0 . 0
s i 5 0 0 . 0 . . . 8 0 0 . 0
G e ) 0
s i > o
u v e r r a n g e
T h e m e a s u r i n g c u r r e n t s i n t h e O O 0 0 6 3 a r e d e r i v e d { r o m a r e f e r e n c e c u r r e n t s o u r c e w h i c h i s a d i u s t e d w i t h
r e s i s t o ( R 4 ' 1 1 a n d p o t e n t i o m e t e r R 4 0 3 . T h e o u t p u t c u r r e n t o f t h e r e f e r e n c e c u r r e n t s o u r c e i s s u p p l i e d t o
t h e c u r r e n t m u l t i p l i e r s { ] ) w f r i c f r g i v e s t h e c u r r e n t s a s s h o w n i n t h e a b o v e m e n t i o n e d t a b l e , d e p e n d e n t t o
t h e s i g n a l s R N G .
T h e v o l t a g e o v e r R x ( V x i i s m e a s u r e d b y A D C ( - 1 V a t e n d o f r a n g e ) .
- T o a v o i d i n c o r r e c t r e a d i n g , t h e i n p u t c u r r e n t o f t h e A D C h a s t o b e c o m p e n s a t e d .
T h i s i s d o n e i n t h e
f o l l o w i n g w a y :
T h e v o l t a g e o v e r ( R x V x ) i s a m p l i f i e d 2 x b y t h e c o m p e n s a t i o n a m p l i i i e r i n t h e O O 0 0 6 3 ( + V i n ) . T h e
a m p l i f i c a t i o n i s d e t e r m i n e d b y t h e r e s i s t o r s R 4 0 6 / R 4 0 8 .
T h e o u t p u t v o l t a g e 2 V x i s s u p p l i e d t o o n e s i d e o f r e s i s t o r R 4 0 9 . O n t h e o t h e r s i d e o f R 4 0 9 v o l t a g e V x
i s a v a i l a b l e ( f r o m t h e i n p u t t e r m i n a l s ) w h i c h m e a n s l h a t o v e r R 4 O g a v o l t a g e o f 2 V x - V x = V x i s
a v a i l a b l e . B y s e l e c t i n g t h e s a m e v a l u e f o r R 4 O g a s t h e i n p u t r e s i s t a n c e o f r h e A D C 1 1 0 l M f 2 ) , t h e i n p u t
c u r r e n t o f t h e A D C i s c o m p e n s a t e d .
l r x ' = l r x + | a d c - | c o m p . A s I a d c = | c o m p
l r x ' = l r x ( s e e f i g u r e 1 7 ) .
- Protection
B y m e a n s o f t h e P T C r e s i s t o r R 4 0 1 a n d t h e b r i d g e r e c t i f i e r s V 4 5 0 a n d V 4 5 1 ( B Y
' 1 6 4 )
t h e c u r r e n t s o u r c e
is protected.
The diodes start conducting at - 2.4 V voltage difference between inputs Vf)mAland
0. In case of a
h i g h v o l t a g e { c u r r e n t ) o n t h e i n p u t t e r m i n a l s P T C R 4 0 1 b e c o m e s h i g h o h m i c .
T o p r e v e n t t h a t a p a r t o f l r x ( r e f e r t o f i g u r e
' l B )
i s l e a k i n g t h r o u g h t h e p r o t e c t i o n d i o d e s , t h e ' v s i d e o f t h e
d i o d e s o f v 4 5 0 i s c o n n e c t e d t o v x . T h e _ s i d e o f v 4 5 0 i s c o n n e c t e d u i a R 4 1 2 t o t h e _ v i n i n p u t o f t h e
compensation amplifier. The common point of V 450 and V 451 is connected to the output of the
c o m p e n s a t i o n a m p l i f i e r v i a R 4 1 4 . I n t h i s w a y t h e l e a k i n g c u r r e n t t h r o u g h t h e d i o d e s i s c o m p e n s a t e d { r e f e r t o
fig. 18) in the same way as the compensation of the I adc.
Diode measurements (semi conductors )
I n t h e d i o d e m e a s u r i n g r a n g e t h e O O 0 0 6 3 i s s e t i n p o s i t i o n 1 m A . D i o d e m e a s u r e m e n t s
a r e p e r f o r m e d i n t h e
same way as resistance measurements in the 1 kQ range (refer to 3.3.2.5.).
The voltage drop over the diode is measured by the ADC.
t
5
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t
r
t
[ -
3,3.2.6.