Want brighter LED flash from stepper motor, better circuit?

Hi there,

I have a small stepper, two windings, no centre tap. Says 4.2 Ohm on the nameplate. This stepper has an extra gear wheel to get a fairly fast turn speed.

I wired up two bridges with eight 1N4148 signal diodes direct to a white 14,000mcd LED. Can get a fairly bright flash out of the LED.

I'm wondering if there's a better way to connect the windings?

I put a picture of the contraption on:

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The circuit is simple, a diode from each of the four stepper lines to LED anode, and another four diodes going from each line to the LED cathode.

Wondering if some sort of series winding be better? This is just something I can give to a friend's kid to he can make some light and see the components. One may run the thing up one's leg or arm to create some light too.

Dunno if I should put any over current protection in there as well, trying to keep it simple.

I did have a capacitor in there but it reduced light output, seems the spikier the voltage, the better the light output.

Maybe a Joule thief type circuit? Ideas please?

Thanks, Grant.

Reply to
Grant
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Putting the windings in series should be a bit better, as then the output from one winding would not be (directly) driving the other - which is happening now.

Reply to
Robert Baer

I think they were isolated, working in parallel before with two bridge rectifiers.

And yes, much brighter with half the diodes and series connection :)

I tried a joule thief circuit on it but it needed a cap to get some brightness, tried a supercap as well, but that didn't absorb much of the voltage, tended to slowly build up to say 1.5V then drive the joule thief dimly until voltage fell to about .5V. The other supercap had more internal resistance, so the LED flashed dimly as the joule thief switched off, then the cap voltage rose, turning it on again. Quite a strange effect. So I thought about some current limiting LED driver and gave up, wired the LED straight across the bridge output, it's better than it was now, and simple too.

Grant.

Reply to
Grant

You might find it educational to allow the same circuit to be switchable drive an incandescent filament bulb or an LED. It is much harder work to get even a feeble glow out of one of them and very educational for children. A lot less likely to leave lights on after having to pedal a bike to power a 60W filament bulb for a while.

You already have enough voltage it is just a case of making it available to the load in an efficient manner.

Regards, Martin Brown

Reply to
Martin Brown

Interesting point, apart from me having no little lamps about...

Switch the globe on and the LED will die, leaving said feeble glow in lamp, nice :) Power the lamp from the AC side, so it gets all available juice and no diode drops in the way.

Grant.

Reply to
Grant

As a hand powered toy it's the way you want it now. It's simple and it works.

If this was for a bicycle you'd maybe want voltage doublers on the windings then an LED buck regulator module. How the motor works as a generator over a broad range of speeds will have to be tested.

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Reply to
Kevin McMurtrie

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Exactly, and the kid is quite happy with it, asked if I made it, so I explained where the stepper came from, how the gear makes it go faster for more light, and why I added the diodes make the current flow in the one direction through the LED. So he seemed absorbed with it for a while.

Yeah, I think he liked it :) At least there's no batteries to go flat.

Was always intended as a handheld toy, now I'm wondering how long the LED will last with no current limit resistor :) I didn't get around to putting say a 1 Ohm resistor in series and measuring voltage across R.

Grant.

Reply to
Grant

Observation, having nothing to do with current limiting: For something that had to be built with flying leads, it is pretty damn well built. For the intended purpose, it seems perfect to me. :-)

Ed

Reply to
ehsjr

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