Service manual

PT300, Service Manual
Rev B, October, 2000
10
Charger:
The battery charger works on the constant voltage charging principle. The diode
D4 is a 10 volt zener diode. This diode is biased by the constant current source
formed by R1, R2, Q8, D2, and D3. Current is sourced to D4 until the voltage
across R2 is equal to the voltage dropped across D2. D3 cancels the voltage
drop across the Vbe of Q8. R1 is used to draw a bias current through D2 and
D3. Q7 is configured as a common base amplifier, with 9.3 volts appearing on
emitter Q7 (10 volts from D4 minus 0.7 volts Vbe of Q7). R3 limits the charging
current to safe value. D5 is used to establish 8.6 volts across the string of cells.
The charge indicator circuit senses the current through R3. When 0.7 volts is
developed (about 28 milliAmps) across R3, Q10 turns on, lighting D20.
R4
R?
R6
D6
Q9
Q5
Off
On
Logic
Auto
Off
V Bat
V Switched
Gnd
Gnd
Gnd
Figure 3 Switch
Switch:
Q6 acts as the main power switch. R4 provides bias to hold Q6 off when in the
off state. Current is drawn through R5 to turn Q6 on. This current may be
provided by the ON switch contact, or through the transistor Q5. Q5 derives its
bias from the low voltage sensor of R6 and zener diode D6. While 6.8 volts is
present on the collector of Q6, D6 will conduct through the current limiting
resistor R6, and bias Q5 on. The OFF switch starves the base current of Q5.
This cuts off the base drive for Q6, turning the unit off. An output from the U12
(74HC191) may also hold D6 below turn on voltage, again, depriving Q5 of base
drive and indirectly turning off Q6 (auto off feature).