Help me make a small charging circuit for Siemens phone

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Charging a cell phone

I want to make a small charging circuit for a siemens phone. The charger that came with the phone is rated 7.5VDC 300ma min to 500max. The output is not regulated and I see a lot of ripple, variation and pulses on the waveform of the DC voltage.

Would it be OK if I just gave the phone 7.5VDC from a liear regulated. Would I be running a risk of damaging anything.

Do cell phones have internal charging circuits that regulate current and shut off charging when the battery is full charged.
 

Charging a cell phone

Hi techie,

every 2nd generation and up phone has charger circuit that not only regulates charging, but also measures various parameters like in and out current. Batteries also have their own chips inside, that for example give equivalent battery temperature.

Regulated DC supply will not do any damage to the phone. It is cheaper to make charger without regulated output.
 

Charging a cell phone

The charger says 7.5VDC. If I give it 8.0V DC, would it damage the phone?. Regulation is not a concern I can use 7808 which is very cheap.
 

Charging a cell phone

To be exact, you should consult mobile phone or charger IC specs on absolute max rating.

Anyway there should be some level of safeguard to protect phone from overvoltage. If you want to be on a safeside, you can always connect a diode in series with the phone.
 

Charging a cell phone

7.5Volt? It's so high. Many mobile phone's charger give 5.5 Volt charging level, Nokia , Samsung...

7.5V will not be the voltage to be added directly on the battery. So the charger circuit is on the phone, It's safe to use a normal regulated voltage as you have planned.

But it is an old style phone.
 

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