Stepper Generator

T.I.T.

www.tomsinstitute.com

This page describes how to use a small stepper motor as a generator to power LED lights.  Advantages of using a stepper motor as a generator include:

No Brushes -  Relatively high voltage for low RPM  - Relatively low torque required

The engine shown below is a PM Research mill engine. As shown, the engine turns the stepper motor at about 1000 RPM, producing about 10 Volts, and enough current to light several LED’s.  Lower speeds can be used and still generate enough voltage.

Stepper motors come in sizes small to large. The one I used was pulled from an old security camera. It was used to drive the pan and tilt axis.  It is less than 2” square and has a 5 MM shaft. Each winding has a resistance of about 16 ohms. Many old disk drives and printers used stepper motors of similar size.  They can be purchased from surplus electronics vendors for very little money.

A stepper motor has two windings and a multi-pole permanent magnet rotor.  It may have either 4, 5, or 6 leads depending on how it’s wired inside.  Turning the rotor generates an alternating current in each winding.  The circuit shown here is a simple bridge rectifier that turns the AC into DC.  No matter which direction the shaft is rotated, the positive and negative outputs will maintain the same polarity.  The circuits show how to wire either a 4, 5, or 6 wire motor to obtain the highest output voltage.  A small electrolytic capacitor is added to smooth out the output voltage.

I added a simple 5 volt voltage regulator to my circuit to maintain a constant voltage supplying the LED’s in the lamp post.

All parts in the circuit I built were used and pulled from old circuit boards. I milled a simple circuit board on my cnc mill to engrave away the copper cladding on a piece of circuit board.  The circuit could just as easily be built on a small piece of pre-drilled breadboard.

The circuit on the left above has a non-regulated output.  The circuit on th right above is being used with the engine above. The 4 wires from the motor are connected on the right edge. The white and white/red wires are supplying regulated 5 volts to the lamp post.  The resistor is used to limit the current to the LED bulbs in the lamp. Without a resistor, the LED’s would burn up. The red and green wires on the left are supplying unregulated voltage to a strip of LED’s on the front of the base. The strip contains its own current limiting resistors.

The parts I used are listed below. Prices shown are from a catalog to give an idea of cost (and the mark up Radio Shack gets).  Many substitutes are possible and the circuit is very forgiving. (Except for the current limiting resistor required for the LED’s) You may have to experiment with various values depending on the stepper motor you use and the speed you turn it. Higher speed generates higher voltage.

Diodes = 1N4007 $0.06

Capacitor = 2200 uF $0.69

Regulator = 7805 $0.25

A demonstration motor and circuit. A small red LED is easily lit by rotating the motor shaft back and forth.

LED’s are not easily available for all old sockets, like the ones in the lamp post I have.  The lamp post uses a screw in flashlight type bulb with a 1/2” diameter frosted glass bulb that looks like a globe.  Impossible to find as an LED. The bulb on the left is a 6.3V LED used to replace old incandescent bulbs in a pinball machine. These are still relatively expensive and sell for about $1. 

To make my lamp post bulbs, I broke out the glass from a flashlight bulb, used a diamond burr to grind out the glass and epoxy from the base, and then unsoldered the lead going through the end of the bulb. I cut a small slot in the edge of the metal can where the old wire was soldered on.  The negative lead of the LED was bent 90 degrees at the base of the LED. The positive lead is inserted into the can so it sticks through the hole in the end.  The negative lead is then soldered in the cut slot centering the LED in the can. The positive lead is soldered to the end/tip and trimmed flush.

It’s important to remember that these bulbs must be used with a current limiting resistor.  It is possible to fit a small resistor into the can if you know what voltage the bulb will be used with.  If you google “LED resistor” you will easily find the simple formula for calculating the value of the resistor.

To simulate the 1/2” diameter frosted glass globe, I found some 1/2” diameter plastic balls, drilled a hole in them and glued them onto the LED. An almost perfect replacement!

Here is a picture of the engine running at about 85 RPM, powering the lamp post and a strip of 6 blue LED’s. The brightness of the blue LED’s varies with the engine speed.  The brightness of the bulbs in the lamp remain constant because the are supplied with the regulated 5 volts. (Globes were not yet added in this picture)

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