How do you keep an iPhone running for long periods of time when you are off the grid? You can’t change the battery in the phone, so carrying a spare isn’t an option. Devices like LadyAda’s excellent MintyBoost use AA batteries and can keep you going indefinitely, but unfortunately the MintyBoost and several other similar products don’t work with the iPhone 3GS. And anyway what I really wanted was a device that would take power from the high capacity 12V NiMH rechargeable pack that I use to power my portable ham radio gear. I could have re-purposed a charger meant for use with cars, but many of them use linear regulators and are therefore very inefficient, or aren’t compatible with the 3GS; plus they are overpriced. So instead I built a charger to suit my needs: the MintyBuck.
The circuit is a simple step-down DC to DC converter, based on the National Semiconductor LM2674-5.0 switching regulator. The PCB is one of the little square perfboards from RadioShack. The USB connector was cut from a USB extension cable. The red and black power inputs are 30 amp Anderson PowerPole connectors. These are neat genderless connectors widely used in the ham radio community. The case is a random mint tin I had lying around. There’s not much to it, really, but it’s still quite handy.
I haven’t measured the output current, but it draws about 200ma when charging, and holds a steady 5v on the output. Supposedly the iPhone draws 500ma when charging, which would put the efficiency at around 95%. I bet it’s actually less efficient than that, but I’m sure it is still much better than an LM7805 would have been.
If you want to make one yourself, you can follow this schematic. If you make a cigarette lighter plug adapter, you can use it in your car too. Note that the input capacitor I used for C1 is technically only rated for 10V, which is not enough for this application. I’m going to swap it out for a higher-rated part at some point. So don’t follow my example on that part. A few more photos are available on my Flickr page.
73, Dave, W2VV