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Thermal controlled Ni-Cad battery charger
This circuit uses an LT1006 to form a thermally controlled NiCad battery charger. The K type thermocouples sense the ambient and the battery temperatures. Assume a discharged battery pack in the transistors collector line. The battery and ambient thermocouples are at the same temperature. The battery thermocouple is directly mounted to one of the cells in the pack. The ambient thermocouple is thermally insulated and mounted to a mass, perhaps a frame member of the equipment. Under these conditions the sensors are phase matched, their outputs cancel and the LT1006 sees 0v. The offset adjustment deliberately introduces enough input offset for the LT1006 to swing positively, turning on the transistor. Current flows from the supply, through the battery pack and to ground via the 250 micro ohm shunt (this resistor is formed by 1.8 inches of #12 wire). The low impedance shunt minimizes losses, cost, and complexity. The voltage across the shunt rises to about 625 microvolts (the amount of offset forced by the potentiometer), and the amplifier servo controls about 2.5A through the battery pack. As the battery charges, it heats up generating a voltage difference between the thermocouples. As battery temperature rises, this small negative voltage (1°C difference equals 40uV) becomes larger. The amplifier gradually reduces the current through the battery to maintain its inputs at balance. The values given limit the battery surface temperature rise over ambient to about 15°C. Connect all grounded points directly to the supply.