Charging of the mobile phone battery is a huge issue while traveling as power supply source is not usually available. In case you keep your mobile phone switched on continuously, its battery will go flat within to six hours, making the mobile phone useless. A fully charged battery becomes necessary when your distance from the nearest relay station increases. Here is a simple charger that replenishes the mobile phone battery within to hours. Fundamentally, the charger is a current-limited voltage source. Usually, mobile phone battery packs need three.6-6V DC & 180-200mA current for charging. These usually contain NiCd cells, each having one.2V rating. Current of 100mA is for charging the mobile phone battery at a slow rate. A 12V battery containing eight pen cells gives sufficient current (one.8A) to charge the battery connected across the output terminals.
Diagram of cellphone charger
The circuit also monitors the voltage level of the battery. It automatically cuts off the charging system when its output terminal voltage increases above the predetermined voltage level. Timer IC NE555 is used to charge & monitor the voltage level in the battery. Control voltage pin five of IC1 is supplied with a reference voltage of five.6V by zen-er diode ZD1. Threshold pin 6 is supplied with a voltage set by VR1 & trigger pin two is supplied with a voltage set by VR2. When the discharged mobile phone battery is connected to the circuit, the voltage given to trigger pin two of IC1 is below 1/3Vcc & hence the flip-flop in the IC is switched on to take output pin three high.
When the battery is fully charged, the output terminal voltage increases the voltage at pin two of IC1 above the trigger point threshold. This switches off the flip-flop & the output goes low to terminate the charging method. Threshold pin 6 of IC1 is referenced at 2/3Vcc set by VR1. Transistor T1 is used to enhance the charging current. Value of R3 is critical in providing the necessary current for charging. With the given value of 39-ohm the charging current is around 180 mA.
The circuit can be constructed on a tiny general-purpose PCB. For calibration of cut-off voltage level, use a variable DC power source. Connect the output terminals of the circuit to the variable power supply set at 7V. Fine-tune VR1 in the middle position & slowly fine-tune VR2 until LED1 goes off, indicating low output. LED1 ought to turn on when the voltage of the variable power supply reduces below 5V. Enclose the circuit in a tiny plastic case & use suitable connector for connecting to the cell phone battery.
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