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Old 02-10-08, 09:34 AM
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Here is how you power USB devices with a bicycle dynamo.

The 4 diodes are standard Schottky Barrier Rectifier types carrying a parts designation of 1N5818. Google 1N5818 and you will find page after page of information. The reason for using Schottky diodes is due to the lower voltage drop across the junction making them a little more efficient than standard reciter doides. These 4 diodes form what is called a bridge rectifier used to convert AC to DC. Here is a photo of these diodes soldered to a terminal board mounted to the front reflector and front rack bracket.

The 4 Ni-MH batteries regulate voltage and current and provide filtered and safe power for the USB connector and any devices plugged into it. The batteries start to impose a high impedance load on the dynamo as voltage approaches 1.25. Four series connected 1.2 volt Ni-MH batteries will limit voltage to 5.2 volts which is within the limits set for USB devices. It's also perfect when recharging them. The batteries also use excess current produced by the dynamo as recharging current. In essence they provide a place for excess current to go sort of like water filling a lake behind a dam. These batteries also provide power when the dynamo is not. Your USB powered devices will continue to operate and recharge while your stopped or climbing that steep hill. The LED lights are setup with the resistors shown so they draw 380mA of current from the batteries when all USB devices are unplugged from the USB connector and S1 is closed for riding after dark or in a tunnel. The reason 380mA was chosen was due to the fact that 120mA is the suggested slow recharge current for Ni-MH batteries. This allows your lights to continue to burn even while stopped in traffic or climbing steep hills and are slowly recharged when your moving again. The 470mF capacitor across the 1-watt LED is not required when the batteries are in the circuit but is included in the event the batteries are lost or damaged. That capacitor becomes a filter providing the headlight LED with smooth DC current eliminating slow speed flickering and damage if it were not there. It's location is in the headlight across the LED terminals. A dynamo without a load can produce very high voltages and that capacitor can easily store it for a long time without a load. If there is an intermittent or poor connection between the capacitor and LED causing the capacitor to store and then discharge high voltage across the LED consequently burning it out.

Here is a free online course in Basic electricity and electronics.
I attended this school in 1980 in San Diego right after Boot camp. It starts assuming you know nothing and at the end you can pretty much work on anything!

Last edited by n4zou; 02-10-08 at 09:53 AM.
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