Keen cyclists will no doubt have sometimes thought that it would be nice to have some kind of on-board power supply to charge a mobile device such as a phone or a satnav while on the road. The circuit described here shows how this can be achieved quite simply, using power from the bicycle’s dynamo and a switching regulator.
The alternating voltage delivered by the bicycle’s dynamo is converted to DC using a full-wave rectifier (comprising diodes D1 to D4) and a smoothing electrolytic (C1). We have chosen Schottky diodes for the bridge rectifier as their forward voltage drop is only about half that of normal silicon diodes (approximately 0.3 V rather than 0.75 V per diode at 1 A). This is all the more important because the LT1076-CT5 switching regulator requires an input voltage of at least about 8 V to provide a regulated 5 V output suitable for powering or charging mobile devices using their USB connector. Smoothing capacitor C1 is charged up to the peak value of the alternating voltage delivered by the dynamo; for most hub dynamos the typical peak output voltage is in excess of 10 V. Under load the typical voltage is of course lower, but still enough for the LT1076-CT5 as long as Schottky diodes are used and C1 has a value of at least 1000 µF.
The LT1076-CT5 is an integrated 2 A step-down converter whose output voltage is automatically fixed at 5 V if the feedback connection FB (pin 1 on the IC) is connected directly to the output voltage at electrolytic C3. As with all switching regulator designs, C3 should have a low ESR (equivalent series resistance): the Panasonic FC series of capacitors is suitable. The circuit arrangement generally reflects the recommendations in the Linear Technology datasheet . The 100 µH inductor used for L1 should be rated for a DC current of at least 1 A (DC resistance less than 0.3 Ω).
The circuit can easily be constructed on perforated prototyping board. The USB output cable can be fashioned by cutting a USB extension cable in two and soldering the bare ends of the part with the USB socket to the output terminals on the board. It is, of course, important to observe correct polarity! To protect the circuit against the elements it is a good idea to pot the whole thing in resin once the USB cable has been soldered and secured using a strain relief.
Internet Link www.linear.com/product/LT1076-5