Build Your Own 12V Nicad Charger Circuit Diagram

WARNING: Exercise caution when working with this circuit. Utilize a multimeter to verify correct polarity. Nickel-cadmium batteries can present an explosion hazard if subjected to short circuits or improper polarity connections.

Circuit Diagram

Notes:

This uncomplicated charger employs a solitary transistor functioning as a constant current supply. The voltage across the set of 1N4148 diodes establishes the base bias of the BD140 medium-power transistor. The base-emitter voltage of the transistor, alongside the forward voltage drop across the diodes, maintains a fairly consistent level. The charging current is approximately 15mA or 45mA when the switch is engaged. This configuration is suitable for most 1.5V and 9V rechargeable batteries. The transformer ought to have a secondary rating of 12V AC at 0.5amp, whilst the primary should be 220/240volts AC for Europe or 120volts AC for North America.

Integrated Circuit Details

The circuit relies on a BD140 medium-power transistor, a commonly utilized switching and amplification device. These transistors are typically bipolar junction transistors (BJTs), meaning their current flow is controlled by a small base current. The BD140 specifically is known for its robust performance and is frequently employed in power supply circuits and general-purpose amplification. Its characteristic values, such as a collector current rating of 1A and a Vce(max) of 40V, make it well-suited for this constant current charger application.

The 1N4148 diodes are silicon diodes that allow current to flow in only one direction. They are frequently deployed as rectifiers, converting alternating current (AC) to direct current (DC). These diodes offer a forward voltage drop, which contributes to the biasing of the transistor's base, thereby establishing the constant current flow. Their key parameters include a typical forward voltage drop of around 0.7V and a current rating sufficient for the circuit’s needs.