MintyBoost 2 AA USB Charger
Ladyada has a new howto on building a USB device charger based on two AA batteries, instead of previous, similar devices based on 9V batteries.
Another problem with the 9V+7805 scheme is that a 7805 is a linear regulator. That means if you want 100mA at 5V (basically, USB power) then you’re taking 100mA at 9V and then losing the 4V*100mA = 400mW (.4W) difference as heat. As the battery wears down to 7V the heat loss goes down to (7-5V)*100mA=.2W but you’re still getting bad efficiency. At best the efficiency is 72% (5V/7V) and at worst its 55% (5V/9V) That means you’re losing about a third of the battery power to heat!
- Link to announcement
- MintyBoost page
- A copy of the tutorial has also been posted on Instructables.
