Hacked Gadgets Forum

July 16, 2013

Bike Phone Charger

at 4:20 am. Filed under DIY Hacks, Electronic Hacks


Sean Michael Ragan shows us how he built the Bike Phone Charger  project that Alastair Bland wrote for Make magazine. It uses an interesting component construction inside a plastic housing. I think a great addition to this project would be cram a small USB battery system in the housing so that the system charges a battery so that it could be used when the bike is stationary.

“The charger circuit consists of a rectifier, a capacitor, and a voltage regulator.  The rectifier contains four one-way electrical gates called diodes which, working together, convert the back-and-forth wiggling of the charges in AC to a series of DC charge pulses.

The large capacitor connected between the DC terminals of the rectifier smooths out these pulses, charging up when there’s more energy in the system and discharging when there’s less.

Finally, the voltage regulator holds the incoming DC power down at a steady 5V, which is what most cell phones and other mobile devices are designed to accept.  Without the regulator in place, the charger might deliver more than 5V to your device, which could damage it.”



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5 Responses to “Bike Phone Charger”

  1. Polytech Says:

    Those wheel-coupled bike generators are a real drag, pun totally intended! I always wondered if there’s a better way – something like mounting magnets on the spokes and mounting the receiving coil on the frame or something of that nature. The friction of the generator against the wheel seems to waste more energy than it produces…

  2. Alan Parekh Says:

    As far as that goes I think the idea of a hub motor that normally powered electric bikes in reverse would be the optimal way of harnessing the bike power to power things.

  3. medix Says:

    Shimano sells hub generators, which produce around 6V (brushless I believe). You still get drag, but much less annoying. I have been planning to build something similar to keep my phone/GPS charged while bike touring here in the Netherlands.

    On another note, the amount of drag will partially depend on the amount of load on the generator, so something like charging a phone shouldn’t generate that much additional resistance. Besides, you’re on the bike to get exercise, so a little more resistance won’t kill you. 😉

  4. ACG Says:

    I have a better idea. Stop talking on the phone so much. Full proof plan.

  5. Dave Says:

    That will work, but is a bad, very inefficient design and prone to mechanical failures. LM7805 is a linear regulator with a minimum forward voltage that depends on the current, but let’s say around 1.5Vf loss. At 5V output, that is already 30% loss.

    What does this “loss” turn into? HEAT! It’s a poor design for not heatsinking the regulator. If you have some wimpy old phone that charges at 500mA or less, you’re still close to pushing it past 1/2W to 1W which can at the least make the plastic it’s screwed to, brittle and crack apart, but with more modern phones at higher charge current, let’s hope it has thermal protection.

    Next he chose an inefficient silicone bridge rectifier which loses another 1.4V or so. Now the cumulative loss is closer to 60% !! IF your design includes a separate rectifier, use schottky diodes instead of silicon.

    The capacitor should be secured, not dangling from its leads in such a high vibration environment. At least put a dab of sensor-safe RTV on it to fix it to the case wall, but that case is huge and the mounting of this giant thing is ugly and it’s not even waterproof.

    ALSO with that plug-adapter-cable hanging from the phone, odds are high that the vibrations during use will rip the phone’s mUSB connector right of the circuit board. Maybe not the first day but it won’t take long.

    For this last reason I would not do this project at all, but if it weren’t for that, at least use a switching voltage regulator so in instead of 60% loss you have more like 15% loss. If your regulator is designed to accept AC input then it will already have the diodes, you don’t need to add a bridge rectifier to that.

    This will result in either less drag from the generator and/or a faster charging phone, as well as avoiding the overheating issue I mentioned above.

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