Posted on Feb 5, 2014

My neighbor`s drill charger stopped working. I found that the primary wires in the transformer were non-functional. This charger had a very simple design. It used a transformer, followed by a diode bridge and a larger resistor. The power supply indicated it delivered 17. 4 volts DC @ 210 mAmps. I was rather surprised to find such a simple charger for a Ni-Cd battery.

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I understand that many methods for charging Ni-Cd batteries attempt to detect the end-of-charge state. One method is to observe the voltage rise during the charge cycle and then looking for a drop in voltage. This drop in voltage indicates the end of the charge cycle. A second method would be to measure the temperature of the battery. NiCd batteries tend to generate heat at the end of the charge cycle. In this application, I suspect the goal is to provide only a small amount of current toward the end of the charge cycle. Since the original charger indicated it delivered 17. 4 volts DC @ 210 mAmps, I decided I would try to replicate these parameters. A future project might be to control this circuit with a PIC Microcontroller. This would include a method to control the current, observe the voltage, measure the temperature and track the time. I began with a common Radio Shack 12-0-12 center tapped transformer. When rectified to DC this produced about 35 Volts. I originally considered using an LM317 regulator to control the voltage and a resistor to control the current. However, the heat dissipation would be significant. I would say about 4 Watts for a rough estimate. Instead, I made a pulse-width modulation circuit in a buck configuration using an LM3524 chip. I used a potentiometer to select an arbitrary voltage output. I set this for an output of 17. 4 Volts. This configuration is more efficient than the LM317 regulator....

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