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current limiting battery charger circuit

2016-05-10 10:28  
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current limiting battery charger circuit
Figure 1 is a current limiting battery charger. VT transistor and resistor R3 form the limiting network. According to the figure, resistor R3 is a base-emitter power resistor of the transistor VT. It is connected in series with rechargeable batteries. Charging current flows through resistor R3. When the charge current is too large, the voltage drop on R3 exceeds 0.6V, the transistor VT can be conducted. As resistor R2 is in parallel with VT, the conduction of VT leads to the reduction of the equivalent parallel resistance. The output voltage Uo reduces and the output current decreases. So the purpose of limiting charging current is achieved. When taking R3 = lΩ), the maximum charge current is Iom = 0.6 / 1 = 0.6A. Select the R3 of a smaller resitance, so charging current can be larger. But the charging current can not exceed the maximum output current value of the integrated voltage regulator.