Product Manual

SERVICE MANUAL 711380 REV. D
WELCH ALLYN UNIVERSAL DESK CHARGER 9
Detailed Description Theory
Input
The Input provides a power entry module, transformer, bridge
rectifier, and sufficient bulk capacitance to assure 200 mA opera-
tion under low line conditions.
The full wave bridge consists of four 1N4004 rectifier diodes.
The bulk filter capacitor is 1000 uF, 50V. Since the rectified
voltage cannot exceed 40V (LM317 limitation), and the trans-
former cannot exceed 25VACRMS (per agency) the design has a
safe margin. Given the expected 200 mA constant current load,
the droop in 10 mS (50 Hz, full wave rectified) should be 2V per
half cycle.
Current Limit
The Current Limit limits the output current to 65 mA typical.
The current limit circuitry in its most basic form employs a LM317
adjustable voltage regulator and a current viewing resistor. The
resistor is placed between the adjust pin and the output pin. The
input to the current source is the Vin pin. The output is at the
adjust pin end of the viewing resistor as shown:
By design, a voltage regulator will shut down its output current as
the voltage between the output and the adjust pin approaches
regulator’s set voltage. (In the case of the LM317, the set
voltage is nominally 1.25V.) This condition gets satisfied when the
output current approaches 1.25V/Rlimit. Therefore, simply
picking the correct resistor value can choose any desired current .
APPENDIX A