kermit

Description: The neighbor's drill charger ceased functioning, and it was discovered that the primary wires in the transformer were defective. The charger featured a straightforward design, comprising a transformer, a diode bridge, and a larger resistor. It was rated to deliver 17.4 volts DC at 210 mA. The simplicity of this charger for a Ni-Cd battery was unexpected.

Many methods exist for charging Ni-Cd batteries that aim to detect the end-of-charge state. One approach involves monitoring the voltage increase during the charge cycle and then identifying a subsequent voltage drop, which signals the end of the charging process. Another method is to measure the battery's temperature, as Ni-Cd batteries typically generate heat near the end of the charging cycle. In this case, it is presumed that the objective is to supply a minimal amount of current toward the end of the charging process. Given the original charger's specifications of 17.4 volts DC at 210 mA, an attempt was made to replicate these parameters. Future enhancements may involve controlling this circuit with a PIC microcontroller, which would facilitate current regulation, voltage monitoring, temperature measurement, and time tracking.

A common 12-0-12 center-tapped transformer was utilized, which, when rectified to DC, produced approximately 35 volts. Initially, an LM317 voltage regulator was considered to manage the voltage, with a resistor used for current control. However, the anticipated heat dissipation was estimated to be around 4 watts, which was deemed excessive. Therefore, a pulse-width modulation (PWM) circuit configured as a buck converter was designed using an LM3524 chip. A potentiometer was employed to select an arbitrary output voltage, which was set to 17.4 volts. This configuration proved to be more efficient than using the LM317 regulator, although the power MOSFET did generate some heat. To mitigate this, a large heat sink was added to the MOSFET, along with a cooling fan.

The power supply included a connection from the 35-volt source to the input of an LM7812 voltage regulator, with the ground connected to the center tap of the transformer. The 12-volt regulated output was then utilized to power the cooling fan.

The overall schematic design encompasses the following components: a center-tapped transformer to step down the voltage, a diode bridge for rectification, an LM3524 PWM controller for efficient voltage regulation, and an LM7812 regulator to provide a stable 12V supply for cooling. This setup not only ensures effective charging of the Ni-Cd battery but also incorporates thermal management to enhance reliability and performance.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. 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. 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.