LX2
plus / advanced and LX4 plus / advanced
B2270901
Content is subject to change without notice!
12
3.4
Charging Unit (PCB No. 228.06.xx)
Charging Unit with PCB No. 228.06.xx
3.4.1
Power Supply
The Charging Unit is powered by an external AC/ DC Adapter plugged into the wall power
socket (U
AC
= 230V, f = 50Hz). The adapter delivers an unstabilized DC voltage U
=
= 6V at
I
L
= 0,4A to supply the Charging Unit.
The adapter is connected to the Charger using socket BU1 of the Charger.
The electrolythic capacitor C12 removes residual noise and buffers the supply voltage so
that mechanically caused short connection interrupts cannot lead to malfunction. Addi-
tionally components R22 (Varistor) and D7 (Supressor Diode) can be placed to prevent
over-voltage problems.
To supply the internal microcontroller an integrated voltage regulator (IS2) produces a
stable 3V supply. Capacitors C7, C8 und C9 are necessary for a stable operation of the
regulator. Diode D3 prevents supply via the output whenever the cable of the adapter is
removed.
C13 is a decoupling capacitor placed near to the microcontroller. Resistor R49 discharges
the capacitors quickly if the Charger is turned off by removing the adapter cable. This is to
prevent the microcontroller from working with supply voltages outside the specified range.
A decoupled voltage to supply external devices is available on socket BU2. That supply is
current- limited by R20 and protected by fuse SI1.
3.4.2
Charge Current
The chargeable battery inside the pager gets charged whenever one of the three current
paths is turned on (switching transistors T1, T52 or T53 eneabled by the microcontrol-
ler). The respective base resistor R50, R51 or R52 is drawn to ground by an open-drain
port of the controller.
Charging is done by a more or less constant current. The current is set by the respective
limiting resistance. The charge currents are:
I
L1
≈
(U
=
-U
batt
)/(R57+R7) = (6V-1,4V)/(2,2
Ω
+10
Ω
) = 375mA
I
L2
≈
(U
=
-U
batt
)/(R58+R7) = (6V-1,4V)/(33
Ω
+10
Ω
) = 105mA
I
L3
≈
(U
=
-U
batt
)/(R59+R7) = (6V-1,4V)/(120
Ω
+10
Ω
) = 35mA
The current paths are chosen according to the type of pager and the programmed bat-
tery capacity. For LX2 and LX4 having battery capacities below 1000mAh only currents
I
L2
and I
L3
are used. In case of LX4 with larger battery capacity I
L1
is used additionally.
Resistor R7 is used as an additional “sense“ resistor for current measurement. Voltage
dividers R54/ R55 and R34/ R35 reduce the voltage taken from the two terminals of R7
to be inside the specified range of the microcontroller. Inductors L4, L5 and capacitors
C1 and C10 are used for smoothing and preventing RF interference. The measured
voltage difference is used to calculate a charge account
.
Transistors T50 and T51 are current sinks, activated during self-diagnostic or discharge of
the battery. They are turned on via resistors R45 and R53 by means of “active high“.
