Epson PX-8 Technical Manual page 39

REV.-A
2.2.2.3 Auxiliary Battery Charging Circuit
As shown in Fig. 2-9, this circuit allows the
user to select, via switch SW3, whether or
not to enable charging and discharging (i.e.,
backup by the auxiliary battery). (SW3 is nor-
mally jumpered.)
When SW3 is jumpered, the battery is
charged by one of the following three modes:
• When the AC adaptor is connected:
(1) The auxiliary battery is charged from
VCH through R18.
(2) The auxiliary battery is charged from
VCH through 013.
• When the computer is turned on:
(3) The auxiliary battery is charged from
the LCD power supply.
• Mode (1) is a trickle (low current) charge,
which is enabled when the main battery
voltage is 5.0V or below.
• Mode (2) is a normal charge which is ena-
bled after low battery voltage (VB) condi-
tion is detected. When low voltage is de-
tected, P63 of the sub-CPU 7508 is held
low for 8.0 hours, forcing the normal (high
current) charge.
The low level at P63 causes a potential dif-
201
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10"
r----'
16V
0.1
OL06151 1
I I L4
1 I
TR6Y-O~---- ..J
G
v
N C
o
H
(from AC adaptor)
, . . - - - - - - - - { ) 750.
71108
(P63)
( P42)
750.
(P70)
'----r---{)
LCD power supply
Main
battery
v
B
Fig. 2-9 Auxiliary Battery Charging Circuit
ference of approximately 6V (VCH voltage), which is AC adapter voltage, to appear across zener
diode Z07, breaking it down (Z07 is a 3.3V zener diode). This lowers the base voltage of tran-
sistor 013 below the collector voltage, putting the transistor in conduction, and providing the
normal charging path from the VCH line through 013, 012, and R2.
• Mode (3) constantly maintains the auxiliary battery in a fully charged state for emergency (the
backup operation from the auxiliary voltage when low voltage is detected). Thus, the battery is
continuously charged as long as the LCD power supply is available (whenever power is on) re-
gardless of whether the ac adaptor is connected or not. See 2.2.5.2, LCD Voltage Regulator for
details.
2.2.2.4 Charging Timing Detection
The sub-CPU program is designed to control auxiliary battery charging from the AC adaptor using
the built-in clock. Connection of the AC adaptor. Port 61 of the sub-CPU goes high whenever the
AC adaptor is connected and the main battery is being charged. Since port 61 is connected to the
anode of the diode 04 in the charging circuit through the resistor R17, the presence of the VCH
voltage can be detected.
2-15