Epson PX-8 Technical Manual page 35

REV.-A
2.2.2 Charging Circuit
Two 4.BV, rechargeable, Ni-Cd batteries are connected to the MAPLE board. The main battery,
which is housed the bottom case and can be replaced by loosening a single screw, has the larger
capacity of 1100 mAH. Its charging circuit includes an overcharge protection circuit which pro-
tects the battery from overcharge by automatically discontinuing charge. The auxiliary battery,
which is mounted on the MAPLE board, has a capacity of 90 mAH. A switch is inserted in both
the charging circuit and backup line which can disable the backup by the auxiliary battery.
2.2.2.1 Main Battery Charging Circuit
2E
P41
7508
Main battery
VBof
Fig. 2-7 Main Battery Charging Circuit
ACAdaptor
GND
The charging circuit includes jumper A2 which allows two modes of charging:
When J2 is jumpered ..................... No Overcharge Protection Control
Jumpering J2 causes the base of transistor 09 to be always tied to ground, holding it in conduc-
tion. This effectively bypasses resistors R3 and R5, inserted in the charging circuit in series, and
causes the charging current to be supplied to the battery through transistor 09, current limiting
resistor R1, and reverse-current preventing diode 04. This setting ecuses the battery to be con-
tinually charged as long as the AC adaptor is connected. Because of the low current limiting resis-
tance in the mode of operation, the battery is highly liable to overcharge.
When J2 is open ............................ Shipment Setting
• When J2 is open, the charge current bypassing transistor 09 is controlled by the port 41 out-
put of the 4-bit sub-CPU 750B. This CPU has a clock feature built in and maintains port 41 at
the low level only for the first eight hours after it detects that the AC adaptor is connected, pro-
viding the same charging mode as when the jumper J2 is closed. With port 41 held low, tran-
sistor 033 is cut off, leaving its collector at the high level (The collector is pulled up to the ac
adaptor output through the resistor R39.) This maintains transistor 030 in conduction; the col-
lector is held at the low level, providing the same effect as if jumper J2 were closed.
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