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Any electronics UFies know about voltages and char | by glitch | 2009-02-10 23:12:49 |
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by OddParity |
2009-02-11 11:59:19 |
I would expect all car adapters to be switch mode devices. Linear regulators would get too hot when delivering full load current for a phone or PDA. Think about it: The voltage from the car can be anywhere from 11.5 to 14 volts, with 13.8 volts being typical. In order to output 1 amp at 5 volts (1 x 5 = 5 watts), a linear deice draws about 1.3 amps (maybe more) at 13.8 volts (1.3 x 13.8 = 18 watts). The difference between input and output power (18 - 5 = 13 watts) must be dumped as heat. Compare this to my soldering iron, which uses 25 watts.
Gravitech's site doesn't give detailed specs on their 3 pin switching regulator, but since the part is designed for general purpose use, I would expect the input to be happy with anything the 7805 would accept. That easily includes 9 volts. But as with all switch mode devices, the output voltage is going to have a fair amout of high frequency ripple to it that you wouldn't get from a linear device. Unless the load (your phone) has adequate filtering built in, you'll need to filter it externally. The recommended 330 uF (microfarad) capacitor does just that. For electrolytic capacitors, you generally want a withstand voltage rating in the neighborhood of 2 to 3 times the highest voltage you would ever expect the part to experience. In this application, a 10 or 16 volt rating would be fine. Add a female USB connector and you should be good to go.
The other route you could go is to take a phone-specific car power adapter and feed it 9 volts from a battery. If it works, this is the better solution. Unfortunately, there's a chance it won't work. The problem is that the phone-specific car power adapter is designed for a specific application. As such, there is strong incentive to optimize the design within a very narrow range of input, possibly sacrificing its ability to perform outside that range. The only way you will find out for sure is to try it. Note that a 9 volt battery only puts out 9 volts when it's fresh, so you should test with lower voltage too. |
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s/linear deice/linear device/ (n/t) | by OddParity | 2009-02-11 12:05:45 |
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