A & A Scales PT300, Service Manual, Page: 10 / 52

PT300, Service Manual
Rev B, October, 2000
9
Theory of Operation
For the following discussion, please refer to the schematics in appendix B.
Power supply:
The power supply section provides a source of ±5 volts, a nickel-cadmium battery
charger, and the detector for low battery condition. The input is designed to accept any
voltage between 10.5 and 14.6 volts.
Q 5
D 4
D 5
D 2 0
C o n s t a n t
C u r r e n t
S o u r c e
C u r r e n t
L i m itin g
R e s i s t o r
C h a r g e
S e n s o r
B a tte r y
S w i t c h
S c a le
G n d G n d G n d
G n d
1 0 . 0 v
9 . 3 v
8 . 6 v
R 2 0
+ 1 4 . 6 v - + 1 0 . 5 v
Figure 2 Battery Charger
Input protection:
Diode D1 and fuse F1 provide protection from incorrectly connected power or a
circuit fault.