Tandy 1000 SL Technical Reference Manual page 207

5V Output Voltage D e t e c t i n g C i r c u i t
The c i r c u i t detects t h e c h a n g e of o u t p u t load c u r r e n t compared
w i t h t h e o u t p u t v o l t a g e and A C l i n e i n p u t v o l t a g e , which feeds
b a c k t o t h e c o n t r o l c i r c u i t t h r o u g h a p h o t o c o u p l e r PHCl t o k e e p
t h e o u t p u t v o l t a g e
s t a b l e .
The P h o t o c o u p l e r i s o l a t e s t h e
primary a n d s e c o n d a r y c i r c u i t s .
Over-Voltage P r o t e c t i o n
When t h e +5 o u t p u t v o l t a g e r i s e s , b e t w e e n 5.8V t o 6.8V, a c o n t r o l
s i g n a l t u r n s o n
t h e
p h o t o c o u p l e r PHC2 ( P h o t o T h y r i s t o r ) w i t h t h e
c u r r e n t of z e n e r
diode ( D l 1 1
a n d s t o p s o s c i l l a t i o n by t u r n i n g o n
Q3,
which t u r n s o f f
Q 1
i n t h e s w i t c h i n g c i r c u i t .
I n t h e case of s t o p p e d o s c i l l a t i o n , correct t h e c a u s e of t h e
f a i l u r e , a n d r e i n p u t t h e power. The o v e r v o l t a g e p r o t e c t i o n
c i r c u i t w i l l reset a u t o m a t i c a l l y u n d e r good c o n d i t i o n s .
The P h o t o T h y r i s t o r i s o l a t e s t h e p r i m a r y and s e c o n d a r y c i r c u i t s .