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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!


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

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...

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.

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

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.

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.

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?

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

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