Stepping down a power supply

patterson7019

I don't remember my power equation. If I want to charge a 4.8 nicad pack at 100-500mA with a 7.5V 1A power supply, what will I need for a resister (resistance and power) and what is the equation to figure out?

I plan to use the power supply when plugged into my stamp board to power the stamp, the servo controller, and charge the 4.8V battery pack.

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Mike Green

Most NiCd battery packs are made to be charged at constant current (1/10 capacity) so they're charged overnight. If you need to charge faster than that, you need a charge controller that monitors the battery voltage and temperature as it charges. Your easiest solution is to use a voltage regulator in constant current mode. Pretty much any regulator's datasheet will show a sample circuit for a current regulator and the formula for the particular regulator to choose a resistor value to get the current you want. RadioShack carries the LM317 which is a variable voltage regulator that just needs one resistor to set the charge current (see www.national.com/ds/LM/LM117.pdf bottom of page 18).