Product Manual

SERVICE MANUAL 711380 REV. D
WELCH ALLYN UNIVERSAL DESK CHARGER 11
LED
The LED section detects the current limit condition to indicate
that a charge is in process. This requires one Yellow LED per
charge well. There is also a green LED’s that simply indicate that
power is on.
Green Power On Indicator LED
The green LED is powered from a constant current source.
The constant current source uses three PNP transistors, two pass
transistors (required to handle worst case power) and one sense
transistor. The sense transistor limits the current by robbing the
pass transistors of their base bias. The sense transistor has its
emitter connected to +VRAW. There is a resistor between the
emitter and base of the sense transistor that provides a path for
the LED current. Whenever the LED current exceeds approxi-
mately 0.65V / Rsense, the sense transistor turns on. The collec-
tor of the sense transistor is tied to the bases of the pass transis-
tors, which are normally pulled to ground (to turn on the pass
transistors). Hence, whenever the pass transistor begins to turn
on, the current is limited.
The pass transistors each have a 20 ohm resistor in series with
their base to avoid current “hogging” by one of the pass transis-
tors should the pass transistor be mismatched. These resistors
typically drop only .13V .
Some of the older handles (still in the field) have no charge
polarity diodes installed. Since the LM317T’s used in the charg-
ing circuit are not reverse bias protected, the green “power on”
LED’s will be powered by one of these batteries, when the AC
power is disconnected. This could confuse someone to think that
their charger is connected to an AC source when it is not. There-
fore, there are two diodes connected between the base of the
pass transistors (diode cathodes), with each anode connected to
one of the charging wells positive contacts. In this manner, should
either battery voltage ever exceed the raw DC voltage, the
corresponding diode will forward conduct, and the pass transistor
will not be biased (and the green “power on” LED’s will be shut
off).
APPENDIX A