On-Board Power Supply for Cyclists
For avid cyclists, the idea of having an on-board power supply to charge mobile devices like phones or satnavs during their rides has likely crossed their minds. This circuit offers a straightforward solution to this need, utilizing power from the bicycle’s dynamo and a switching regulator. The following description explains how this can be accomplished easily.
Converting Dynamo AC to Regulated DC
To convert the alternating voltage generated by the bicycle’s dynamo into a steady DC supply, a full-wave rectifier employing diodes D1 to D4 is used, followed by an electrolytic smoothing capacitor (C1). For the bridge rectifier, Schottky diodes are chosen due to their lower forward voltage drop, typically around 0.3 V per diode at 1 A, in contrast to the approximately 0.75 V drop of regular silicon diodes. This choice is crucial as the LT1076-CT5 switching regulator necessitates an input voltage of at least 8 V to deliver a regulated 5 V output suitable for powering or charging mobile devices through their USB connectors. C1 is charged up to the peak value of the dynamo’s alternating voltage, which usually exceeds 10 V, although the voltage is lower under load. Still, with Schottky diodes and a minimum 1000 µF for C1, it remains sufficient for the LT1076-CT5.
The LT1076-CT5 Regulator and Feedback Connection
The LT1076-CT5 is an integrated 2 A step-down converter with automatic regulation to 5 V if the feedback connection FB (pin 1 on the IC) is directly linked to the output voltage at electrolytic C3. As with all switching regulator designs, C3 should possess low ESR (Equivalent Series Resistance). The Panasonic FC capacitor series is a suitable choice. The circuit configuration closely adheres to the recommendations in Linear Technology’s datasheet . The 100 µH inductor (L1) should be rated for a DC current of at least 1 A, with a DC resistance of less than 0.3 Ω.
Construction and Waterproofing
The circuit is easily assembled on a perforated prototyping board. To create the USB output cable, you can cut a USB extension cable in half and solder the exposed ends of the section with the USB socket to the output terminals on the board. Correct polarity must be adhered to. To safeguard the circuit from environmental factors, it is advisable to encase the entire assembly in resin after soldering the USB cable and securing it using a strain relief.
Internet Link www.linear.com/product/LT1076-5