Converting AC to DC power from a stepper motor

One of the projects I want to do, is to build a flashing LED for my son’s bicycle. The idea is to have a stepper motor run off his wheel, which charges up a battery / capacitors and ultimately makes the light flash.

To use all the electricity generated by the stepper motor we need to convert it from AC to DC current.

Bridge rectifier stepper motor ac to dc circuit
Image 1: Bridge rectifier stepper motor AC to DC circuit

Here is an example of how you would do that. It is called a bridge rectifier and makes use of a few diodes to control the flow of current.

In Image 1 I am using 2 LED’s as placeholders for the coils in the stepper motor, so you just need a bit of imagination 😉

The current flows from the motors coils, through the purple wires, and through the diodes. Diodes serve a purpose here in that they only allow current to flow in one direction.

As we know stepper motors output AC  (alternating current) when turned, and diodes only allow current to flow through in one direction, so we can use this to convert the AC current from the stepper motor into DC.

Back to Image 1, when you look at a diode, such as in the circuit, you will see it has a silver line on the one side. This shows the direction of flow through the diode. Current coming from that side will be blocked, whereas the current flowing through the diode from the other side will be let through.

You can see from the circuit that for each coil, there are two sets of diodes, one set where the lines are facing one another, and another set where they are facing away from one another.

The wires from the coil, each connect to a side of the sets of diodes, so that there is a diode whose line is on the side of the coil and another diode where the line is away from the coil. What this does is only lets the current through when it is higher than the other side of the diode.

This will lead us to the following;

By Jjbeard (Made in Inkscape from scratch.) [Public domain], via Wikimedia Commons
Image 2: Wave rectification By Jjbeard (Made in Inkscape from scratch.) [Public domain], via Wikimedia Commons
In Image 2 the first wave depicts AC current. Using a single diode would give us the second graph, which is half wave rectification. We would have DC current, but we are throwing half of the current generated in the motor away.
The third graph shows the output of a bridge rectifier.

Lets take another look at this circuit;

Image 3: Full rectification Circuit By Wdwd (Own work) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons
 Image 3 has a different representation of the same circuit. Here you can see the diodes act in pairs to only allow the current to flow in one direction, regardless of the output from the stepper motor.

I am using a stepper motor with two coils, so I have an additional bridge rectifier to handle the output of the other coil. The converted DC current is then taken to a common rail, through a pull down resistor and into an LED.

Please leave a comment if you enjoyed this, or have anything to add 😉

One thought on “Converting AC to DC power from a stepper motor”

  1. Hello! I also have the same interest regarding using step motors as power source and converting the AC to DC. With that circuit on image #1 you don’t actually transform AC to DC, right? The leds would still flash, even with the full wave rectification (as shown on image #2).

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