Electronics Projects, USB Battery Charger Circuit Nimh Nicd article
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Charger Circuit LM393 op-amp based on the computer’s USB port with 5 volts on 2 AA NiMH, NiCd batteries can be recharged. Charging current of 470 pcb circuit drawing and detailed information have.
During charging, R1, R2, and R4 form a three-way voltage divider which yields about 1.26V at the non-inverting input of Z1a (pin 3, Vref). TR1 is a thermistor that is in direct contact with the cells being charged. It has a resistance of 10k at 25°C (77°F), which varies inversely with temperature by about 3.7% for every 1C° (1.8F°). R3 and TR1 form a voltage divider whose value is applied to the inverting input (pin 2, Vtmp). At a temperature of 20°C (68°F), TR1 is about 12k, which makes Vtmp about 1.76V.
The charger in this project is designed to charge two AA NiMH or NiCd cells of any capacity (as long as they are the same) at about 470mA. It will charge 700mAh NiCds in about 1.5 hours, 1500mAh NiMHs in about 3.5 hours, and 2500mAh NiMHs in about 5.5 hours. The charger incorporates an automatic charge cut-off circuit based on cell temperature, and the cells can be left in the charger indefinitely after cut-off.
Size: 3.8″L x 1.2″W x 0.7″H (9.7cm x 3.0cm x 1.5cm).
Cells: Two AA, NiMH or NiCd
Charging Current: 470mA
Charge Termination Method: Battery Temperature (33°C)
Trickle Current: 10mA
Power Source: Desktop, Laptop, or Hub USB port
Operating Conditions: 15°C to 25°C (59°F to 77°F)
Source: stefanv.com USB Battery Charger schematic pcb Alternative link: usb-battery-charger-circuit-nimh-nicd.rar