Wall Cube Replacement Circuit for Cordless Devices Gerhard Dekker copyright 1991 Gerhard Dekker Here's a nicad charging circuit that's cheap, simple and automatic. It will also tell you at a glance how the charging is proceeding. I've built one of these for every cordless gadget in my home (flashlight, vacuum, drill). The LM 317T voltage regulator (Radio Shack #276- 1778) is carefully adjusted to maintain a tiny trickle charge current (2 to 5 milliAmperes) upon filling the nicad batteries. To adjust, it is necessary to have the battery fully charged; this can be done by adjusting the regulator up to maximum voltage using the 10 k½ potentiometer. Then only the current limiting resistor (Rlim) is holding down the current. Charge the battery until its capacity in Ampere-hours have been replaced. Then lower the voltage slowly by turning the 10 k½ pot until the LED indicator just dims out. This will set the trickle charge current at between 2 to 5 milliAmperes. After setting the circuit, return the appliance to the charger after each use. The indicator LED will show that connection has been made and the device is charging. At first, maximum charge current will be supplied to the battery. Maximum charge current is determined by the resistance of Rlim. When the battery is fully recharged the indicator LED will dim and finally go off. This circuit has worked very well for me. My flashlight, which I use every day, still has the original nicad battery after six years and it has not diminished in capacity. To determine the resistance value of Rlim: INSERT EQUATION1 For my flashlight which used two AA cells (maximum charge rate 200 mA.), this works out to: INSERT EQUATION2 INSERT SCHEMATIC Access Gerhard Dekker, Box 689, Steinbach, Manitoba, R0A 2A0 Canada ¥ 204-434-6143