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anasimtiaz
Joined: 21 Oct 2006
Posts: 17
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 30 Oct 2006 10:46 usb charger beyond |
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| Im interested to make a charger for my nokia phone which can get its power from the usb port on the pc. I need some guidelines regarding USB port, and what do I need to know to get started.
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budhy
Joined: 21 Oct 2006
Posts: 836
Helped: 185
Location: Indonesia
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anasimtiaz
Joined: 21 Oct 2006
Posts: 17
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30 Oct 2006 12:36 how to make 5v to cell phone |
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| thanks for the link....i want more info regarding usb port....i wonder if anyone has tried that...although its available in the market, cost is high so i thought making one for myself, need more guidance!
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turttle
Joined: 31 May 2001
Posts: 45
Helped: 5
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30 Oct 2006 15:53 Re: USB charger for cell phone |
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To make a charger you don't need to know about USB beyond pinout, Vcc=5V and Imax. (500mA nominal). Data pins remain unconnected. Since Nokia phones refuse to charge with Vin beyond some threshold (about 3.X volts, don't remember exact value) you need a DC-DC converter with a current limiter to avoid overload USB port. That limitation will make charge probably slower than it would be with a std charger with greater current limit. Note that battery charge is managed by internal phone electronics, then don't worry beyond to build power supply.
I hope this can be helpful.
Cheers 
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30 Oct 2006 15:53 Ads |
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anasimtiaz
Joined: 21 Oct 2006
Posts: 17
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30 Oct 2006 18:01 Re: USB charger for cell phone |
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I got the first part of it but Im not sure I entirely understand the part about the threshold voltage of nokia phones. A standard charger for the phone has 5v/350mA written on it, so Im confused here about the threshold of 3.x volts.
How can I protect the USB overload?
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turttle
Joined: 31 May 2001
Posts: 45
Helped: 5
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30 Oct 2006 19:38 Re: USB charger for cell phone |
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Time ago I tried to connect a 5V PS (measured 5V) to a Nokia phone and it said incorrect power supply or something similar. I've just checked an original Nokia charger and is specified at 3.7V. I checked time ago a car adapter for Nokia and it was well below 5V as well. If you connect a charger with 5V nominal output either it has a lower voltage when loaded by the phone or if it's an original Nokia charger then your phone/charger it's different from Nokia phones I have notice of.
I suppose the reason to refuse 5V is from heating concerns: the higher the step between V-needed and V-supplied, the higher the dissipation the phone has to withstand.
USB overload can be avoid via a simple I-limiter below 500mA nominal threshold of USB port supply. It can be done with a simple LM317 and a resistor (see app.notes). Nevertheless if it's for personal use and your nominal load it's always (even with battery discharged) well below that point, you can obviate it since each USB port must have inside PC a protection polyswitch to protect PC from accidental overloads/shortcircuits.
Cheers
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