bike light circuit advice

[sillyfly]

Hello
I want to build a light for my bicycle, all from salvaged components.
I have a DC motor that I want to use as the generator, and some LEDs mounted on a PCB with a resistor, accepting 5V.

I want the circuit very simple, and I have come up with the circuit in the attached image.
As the motor might put out more than 5V I put a Zener (5.1V) in backwards voltage.
The 1N4004 is there in case the wheel rotates in reverse, in which case the voltage will be reversed. since I don't expect this to happen much, I don't think a rectifier bridge is necessary.

the cap is actually 3 electrolytic caps connected in parallel. they are rated 25V, two at 1000uF and one at 2200uF.

my questions are - does the circuit look OK?
should I incorporate a resistor in series with the zener?
is the zener enough protection from voltage surge? keep in mind that I will only connect LEDs to it, and the caps are rated at 25V, so they can probably take some extra.

thanks a lot!

 

[duke37]

1. I think that you should measure the voltage coming from the generator to see if things are of the right order.
2. The capacitors are not necessary. LEDs can take a rough supply.
3. The zener may overheat if you have too much breakfast.
4. A resistor instead of the diode will limit the current through the zener and the zener will then prevent reverse polarity. I don't suppose you ride very fast in reverse.
5. A constant current circuit could perhaps be benefitial, allowing constant brightness with a supply of 7V to the limit of the chip (25V?)

Duke

 

[sillyfly]

thanks for your reply.

I think you are right - a resistor would be sufficient instead of the diode.
as for the capacitors - I want them so that the LEDs can stay on for a few seconds even if I stop.

I will check the voltage coming from the generator as soon as I find a way to mount it on the bike. I will take a few measurements at average cycling speed, and then a few at downhill ride, to get an idea of the voltage levels.

 

[(*steve*)]

I did something similar for my bike once...

I basically used a bridge rectifier, a large capacitor and powered the LEDs from that.

To regulate the voltage, I had a simple regulator made from a series pass transistor with the base tied to the junction of a resistor/zener (controlling voltage on the capacitor).

I used flashing LEDs, and the ones I used worked on voltages from about 3V to 12V and needed no external resistor.

I only ever got a few seconds whilst stopped (maybe 5 to 10) so the only thing it did was prevent the light going off if I slammed on the brakes and skidded to a halt.

These days, I'd probably go a similar route, but I would have a large-ish capacitor charged up to about 35V or so and use this voltage to drive a constant current switchmode LED driver.

Oh, at the time I measured the open circuit voltage of my alternator at something more than 60VAC, hence a discrete regulator rather than a 3 terminal regulator (which would not have survived).