Join Date: Sep 2005
Bikes: Windsor Fens, Giant Seek 0 (2014, Alfine 8 + discs)
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Yes, you do NOT want to use a linear regulator for this application. You'll be throwing away almost all of the power used, and many linear regulators can't even handle this level of drop. If you want 500 mA charge current at 5 volts off a 48 volt feed, the linear regulator would have to be able to dump (48-5)*500mA = 21.5 watts of heat. That's a WHOLE LOT of heat. You're talking a fist sized heatsink getting too hot to touch. And the efficiency at best can't be better than 5/48= 10%. 90% of the energy will just be dumped as heat.
I doubt most off-the-shelf linear regulators could even dump this much heat without cooking themselves.
A switcher wouldn't be that much harder to build, and could easily be 90% efficient. Buck regulators are dirt simple.
Keep in mind though, if your USB device is "well mannered" it won't draw more than 50mA, and in fact will probably refuse to charge if you just put power on the USB. You need to have a chip on board to "talk" with the device and tell it it's OK to pull more current and begin to charge.
The easiest and best thing to do might be to get a 12V to USB adaptor that you know works - it will already have the USB chip built in, and you can get these for $5. Then all you need to do is to get the voltage down to somewhere in the 8 to 15 volt range; those adaptors already have regulation built in and are designed to take a range of voltages from a car. THEY are not likely to be very efficient, but they'll be more like 40% efficient, which is a damn sight better than 10%.
You'd still want to build a switcher to feed that device though, or you'd be introducing a 25% efficient part in before you even got to the final regulation stage.
Besides, having a 90+% efficient regulator that can deliver a couple of amps at 12 volts from your battery will be useful for many other things.
Work: the 8 hours that separates bike rides.
Last edited by ItsJustMe; 03-24-10 at 08:43 AM.