Product Manual

12 WELCH ALLYN UNIVERSAL DESK CHARGER SERVICE MANUAL 711380 REV. D
Yellow Charge indicator LEDs
Each yellow LED is meant to show that a particular well is
charging.
The yellow LED is on when the current limit is active. To do this,
the base – emitter junction of a PNP transistor is placed across
the battery charger current limit sense resistor. Whenever the
voltage drop across the current limit sense resistor exceeds .6V,
the transistor is turned on, and power is supplied to the LED.
Coincidentally, .6V is roughly ½ of the LM317 adjust voltage
(1.25V). Therefore, the LED starts conducting current when the
battery charge current is around ½ the current limit value. How-
ever, the LED becomes viewable at a somewhat higher charge
current.
Since the yellow LED receives its power on the high side of the
battery charger sense resistor, the LED current does not affect the
current limit to the battery.