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USB Charger Question... Answered

Ok, I have a (4) AA battery holder which is wired in series so that I get 6V. I was then going to use (3) LM7805 regulators in parallel with heatsinks because a AA battery has approximately 2600mAH and each 7805 is rated @ 1A... I tried using (1) 7805 and it was so hot it burn my finger (BAD!) I'm making this device to power a Motorola V3 Razor. I guess my question is if this will work or not, the more I've been reading the USB standard is 5V @ 500mA... Just because the batteries are rated @ 2600mAH does that mean that they will put out that much power? Or will the phone only draw as much power as it needs? I can't afford to use this phone as a test! Please only respond if you know for sure - someone with an EE degree would be nice ;) Thanks in advance!

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LasVegas (author)2007-06-25

There are a number of Instructable about building USB chargers. A simple search should show you a number of ways to accomplish it.

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02gsxr600 (author)LasVegas2007-06-30

The point of this one though is to be able to put 4 AA batteries in and be able to get multiple charges from the same batteries, rather than a 1 time use...

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LasVegas (author)02gsxr6002007-06-30

And there's no reason you can't use it for multiple recharges. Personally, I would use the batteries in series/parallel, doubling up each pair to supply 3v to the MintyBoost that would effectively double the charging capacity and waste very little power to heat. Using a 7805 with 4 batteries you would be throwing away 1/6th of the battery's power.

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NachoMahma (author)2007-06-25

. You're comparing apples and oranges (mA and mAH).

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02gsxr600 (author)NachoMahma2007-06-25

I know I am - that's why I'm asking... I've read the other instructables, and they either use a 9V with an LM7805, or (2) AA's and multiple other parts. As far as comparing mAH and AH I know they're different - that's why I'm asking if it will ruin the phone or if it will only use what it needs.

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02gsxr600 (author)02gsxr6002007-06-25
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LasVegas (author)02gsxr6002007-06-25

Your phone will draw no more than it can handle given a regulated 5v supply. There should be no concern for the safety of the phone. You only need one LM7805, but make sure that it does have a sufficient heat sink.

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02gsxr600 (author)LasVegas2007-06-26

I realize 7805's don't regulate current, but wouldn't you need 3 if they are rated at only 1A?? I would need (3) 1A 7805's so they don't get too hot... right?

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LasVegas (author)02gsxr6002007-06-26

No. Your cell phone will only draw about 700mA under full charge, so you don't need anymore than 1A.

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trialex (author)2007-06-25

The 7805's regulate voltage, not current, so as Las Vegas says you only need one, with a sufficient heatsink.

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