The charger described in this article can charge packs of 4 to 7 cells with capacities ranging from 600mAh to 2Ah. The charger automatically begins charging when a pack is connected. It charges at a nearly constant current and terminates the charge when the pack begins to get warm up when it has reached full An LED indicates that charging is in progress.
The circuit for the charger is simple. Z1A is a comparator which compares the voltage on its two inputs and produces a high output when the “+” input (pin 5) is higher than the “-” input (pin 6), and a low output otherwise. R1 and R2 form a voltage divider, presenting a fixed voltage (about 7V) to pin 5. A pair of thermistors (TR1 between the PAK+ and TEMP terminals, and TR2 between the TEMP and GND terminals; see Figure 2) form another voltage divider which presents a voltage to pin 6. This voltage is proportional to the temperature difference between TR1 and TR2. When TR1 is within 10°C of TR2, this voltage is below 7V, and the output of Z1A turns on Q1. This causes current to flow through Q2, R5, and R4, turning Q2 on. This in turn causes Q3 to conduct, resulting in current flow through the NiCd pack connected between the PAK+ and PAK- terminals. The amount of current flowing through Q3 (and thus the pack) is determined by the current flowing through Q2, which in turn is determined by the setting of R5. The current can range from about 2A to 5A. While Q1 is on, current will also flow through LED1 and R6, thus illuminating LED1 to indicate that charging is in progress.
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