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Switchless NiCd/NiMH Charger Schematic Circuit Diagram

Upgrading Battery Chargers with Enhanced Current Regulation

This schematic provides a solution for replacing the commonly used single current limiting resistor found in inexpensive battery chargers. The presented alternative proves advantageous in the long run. Preventing the premature disposal of NiCds due to inadequate charging methods. In this circuit, an LM317 is employed in a constant-current configuration. Notably, it deviates from the typical setup by lacking the customary fixed or variable resistor at the ADJ pin to regulate the output current. Additionally, there is no switch with an assortment of resistors catering to different charge currents for the three types of cells or batteries under consideration: AAA, AA, and PP3 (6F22).

Switchless NiCd NiMH Charger Schematic Circuit Diagram

Constant-Current Mode Activation

When an empty AAA cell is connected, the voltage developed across R1 biases T1 through voltage dropper D1. This action allows about 50 μA to flow from the LM317’s ADJ pin into the cell, activating the circuit into constant-current mode. D4 is incorporated to prevent battery discharge when the charger is switched off or lacks a supply voltage. The charging current (I) is determined by R1/R3/R3 according to the equation:

R(n) = (1.25 + Vsat) / I
where Vsat is 0.1 V.

Where Vsat equals 0.1 V. Ideally, the current should be one-tenth of the nominal battery capacity; For example, 170 mA for a 1700-mAh NiCd AA cell. It’s crucial to note that ‘PP3’ rechargeable batteries typically contain seven NiCd cells, making their nominal voltage 8.4 V, contrary to the common belief of 9 V. For applications requiring higher currents, managing power dissipation in R1/R2/R3 becomes vital. As a guideline, the charger’s input voltage should exceed three times the cell or battery pack voltage to account for the LM317’s dropout voltage and the voltage across R(n).

Important Considerations and Safety Measures

It is essential to fit the LM317 with a small heatsink for effective heat dissipation. Prioritizing electrical safety, it is recommended to use a general-purpose mains adapter with a DC output over a dedicated mains transformer/rectifier combination for the charger’s power supply. This choice ensures safer and more efficient operation while charging batteries.


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