Step 3: Build the Electronics

There are three major parts to the electronics of the USB Bike Generator, the stepper motor, the rectifier and the voltage regulator. 

Stepper Motor

The USB Bike Generator uses a stepper motor as a generator to produce the electricity.  In general any electric motor can be used as a generator but not all motors are well suited as generators.  The stepper motor I used in my last two instructables came from an old printer and was rated at 24 volts.  Through testing I found that this motor provided up to 48 volts when unloaded and spun at 3100 rpm.  People new to electronics should understand that high voltage doesn't always mean high power.  In order to reduce this voltage to the 12 volts needed for the BikeGen instructable the regulator just burned off the extra voltage as heat.  This meant the regulator was inefficient.  

In my searching for a new stepper motor I looked for two important aspects.  First, the voltage rating of the motor need to match the 5 volts required by the USB ports. Second, the amperage of the motor needed to be higher, meaning there was more power potential in the motor.  I found a somewhat local electronics surplus store that sells stepper motors and searched their website, http://www.electronicsurplus.com/home.cstm.  They had a stepper motor listed at 5 volts and 3.3 amps, this seemed perfect.  I went to the store and after looking at everything they had I got the motor.  I would recommend finding a local surplus store in your area if you plan on building anything electronic, they are a great resource.  


In basic terms a rectifier changes Alternating Current, AC, to direct current, DC.  The coils inside the stepper motor are energized as the motor spins causing the current in the coils to alternate.  This is the alternating current.  The 5 volts need for the USB port need to be direct current.  The rectifier, which is just 4 diodes, changes the alternating current from the stepper motor to the direct current needed for the voltage regulator.    
After doing the testing on the BikeGen regulator circuit I realized that the zener diodes I was using were getting very hot.  One of them even failed because it overheated.  I wanted to use a more suitable diode for this project and after checking back to the instructable that inspired this whole project for me, http://www.instructables.com/id/personal-powerPlant/, I decided to go with a 1N4001 diode which is rated at 50V and 1 Amp.  I got the diodes in a variety pack from Radio Shack.

Voltage Regulator

The voltage regulator is a switching voltage regulator as opposed to linear regulator.  At first my searching led me to the LM2575 switching regulator and I was planning on building the circuit myself.  Just search "LM2575" and the data sheet will show up.  On the data sheet the recommend circuit for a 5 volt output is shown with recommendations for all the components.  I continued searching and found the exact circuit I needed from Lightobject, http://www.lightobject.com/LM2575-High-Input-6V60V-Switching-5V-Power-Module-Regulator-P417.aspx.  This seemed to be a better option for me because I didn't want to have to buy all the components in much higher quantities than I needed from a electronics distributor.  

As a last resort I went to the local dollar store because they always seem to have the things I need.  To my complete surprise they had 12 volt car adapters with power for two USB ports.  So I bought two and went home to take them apart.  They had a switching regulator circuit already!  It uses a MC34063 regulator IC and has all the supporting components.  All of this for a dollar, you can do much better than that.

Follow through the notes on the pictures to see how I connected all the parts together. 

jwelbes4 years ago
"The rectifier, which is just 4 diodes..."

you're using 8 diodes. are you using 4 sets of two diodes connected in series, effectively making them one diode?
jwelbes jwelbes4 years ago
wait hold on. you made two seperate circuits, one for each USB charger inside your 12v car adapter. am I right? this makes more sense than my last comment. that would explain why your diagram you drew has only two wires going to your car adapter, but in your photograph there are 4 wires going to your adapter.
Doug Costlow (author)  jwelbes4 years ago
There are 2 rectifiers,each consisting of 4 diodes. A "rectifier" is just the combination of the 4 diodes in the pattern shown in the picture, nothing more than that. Two rectifiers are needed because there are two separate coils in the stepper motor. The positive/negative outputs of both rectifiers are connected to the 12v adapter and the adapter converts the voltage to 5v.
jwelbes4 years ago
"Follow through the notes on the pictures to see how I connected all the parts together."

I don't see any notes on the pictures.
jwelbes jwelbes4 years ago
i guess I need to be a pro member...
varunmehta5 years ago
Rather than dismantling the whole male plug system (car style) on the USB charger and connect the wires, how about making a female receiver, like that in the car. You can plug this USB drive charger into it + if tomorrow you make a new enhancement or want to connect something new to it, you can reuse the same stepper motor and circuit to achieve it!
tomtortoise5 years ago
can you please make a simple schematic to better explain the rectifier wiring.