Service manual

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.
Q5
D4
D5
D20
Constant
Current
Source
Current
Limiting
Resistor
Charge
Sensor
Battery
Switch
Scale
Gnd
Gnd
Gnd
Gnd
10.0v
9.3v
8.6v
R20
+14.6v-+10.5v
Figure 2 Battery Charger
Input protection:
Diode D1 and fuse F1 provide protection from incorrectly connected power or a
circuit fault.