I was rather surprised to find such a simple charger for a Ni-Cd battery. 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.

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. But, the power MOSFET generated a little heat, so I added a large heat sink to the MOSFET and a cooling fan for the unit. The Electronic Goldmine usually has a wide variety of fans at a good price. All the fans I have operate from 12 volts. I connected the 35 volt source to the input of an LM7812 regulator. I connected connected the ground to the center tap of the transformer. I used the 12 volt regulated output to power a cooling fan.

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The LM3524 chip incorporates a simple current limiting feature. Using pins 4 and 5, the chip measures the voltage drop across a small resistor placed in the path of the main power supply. When the voltage drop across this resistor is greater than 200 mVolts, the chip reduces the pulse width. When I tested a 1.8 Ohm resistor, I noticed that a short circuit generated about 200 mAmps. I used two of these resistors in parallel to make a 0.9 Ohm current limiting resistor. A short circuit results in about 500 mAmps. When connected to a discharged battery, the current supply is around 200 mAmps. (To reduce the current, the user could clip out one of the two 1.8 Ohm resistors.)

I packaged all this inside of a Radio Shack project enclosure. I used a three wire computer power cable for power. The ground wire, a 1 Amp fuse, the fan and the transformer are bolted to the metal backplate. I drilled a few holes in the plastic box for air flow across the MOSFET and added a few adhesive rubber feet. I also added a little LED as an "on" indicator.

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Here is the general schematic. The inductor for this project is an small hand-made toroidal inductor. I decided I did not need a large output capacitor. The two resistors at the base of P-MOSFET keeps the Vgs value within specifications for the IRF9530. The diode before the MOSFET prevents any battery current escaping back into the circuit when the unit is not powered. This is not a perfect solution since a very small amount of current will be consumed by the resistors that detect the voltage level. I added the 4 Ohm resistor to provide a little resistance to surge current and to allow the source voltage of 17.4 to be achievable. This resistor was cannibalized from a old computer power supply. The original charger had a base unit that included the electronics and special connectors for the battery to snap into when charging. I reused this enclosure. I removed most of the components from the original circuit board and rewired it to work with the new charger. I also removed the old LED and added a new one with a 5k resistor. I tested this with my 18 Volt drill battery and with the 14.4 Volt battery. It seems to gently charge the battery and not produce much heat.

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