Electronics > Beginners

Help with a dynamo

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trevorethier:
I was planning on building a bike light setup based around a hub dynamo.
The exact hub is this:http://techdocs.shimano.com/media/techdocs/content/cycle/SI/HubDynamo/SI_2YH0A/SI_2YH0A_001/SI-2YH0A-001-ENG_v1_m56577569830673555.pdf
So i assumed that it would produce ac power between 0-6 Vpp.
Upon getting the hub, i discovered that it will actually produce 0-100 vpp. As the load approaches closer to 3W it drops to around 6 Vpp.
Getting to my question, the load is not constant (want to do different modes of light), how do i make a circuit that will keep the voltage steady around say 5-10 volts? keep in mind i wish to keep this as power effective as possible.
Thanks for the suggestions.

Simon:
The simplest solution would be a zenner of say 8V 5W to cap the voltage. Although this is a wasteful method. Have you considered a battery setup ? Are you saying that if the dynamo is loaded the voltage comes down to 6V ? do you have a load/V plot/figures ? it might be a good idea to use a battery with a regulator, you could use a SMPS by rectifying the voltage with a doubling rectifier you would have about 12V so would still have the 6-7.2v needed for a lead acid battery

Floyo:
What I would do is make an sepic type smps. This would ensure that you always have a stable output voltage and current, as long as your dynamo can handle the current. There are IC's available that handle all the switching for you and only need an external coil and a small hand full of components to make it work. Of course you would first rectify the AC voltage, possibly with Schottky diodes for the lowest voltage drop, and higher switching speeds (I think normal diodes will be just fine in this application though, it depends on the output frequency of the dynamo.)

The rest of the solution depends a bit on what type of lights you want to use, I assume LED's. If so then you could opt for a converter IC witch has the ability to act as a constant current source, this way you ensure that you have the highest efficiency because there are no linear circuits wasting power, like drop resistors or linear regulators.

http://en.wikipedia.org/wiki/Single-ended_primary-inductor_converter

trevorethier:
Zener , out of the question as only 3 watts total and i wish to maximize power going to the led's (want to do a nice headlamp).
i was hoping to stay away from batteries and only use super caps with just low power led blinkers going when the hub is not moving and high power led on when it is rolling.
and yes the dynamo goes down in voltage as the current draw goes up, there is no data sheet other then the link above, i can provide scope screen shots under different loads.
i already have a circuit set up that will work with voltages from 2 to 15 volts getting the voltage to this range was the part i was unsure of, as i didnt think a SMPS would work for pulsating dc or not.
Thanks for the help, will post the finished project in the project area when/if i make it work

RayJones:
Ahh, the good old hub dynamo.

I've got a SON on one of my bikes and it has happily driven two CREE LED's in series for years, The only other component is a full wave rectifier.

The secret?
They essentially behave as constant current devices, typically limited to around 600mA, which works out to your 6V under full load for the nominal 3W rating.
I say nominal because you can actually pull more power if you let the volts rise.
Note that two CREE LED's in series are closer to 8V than 6V :-)

The best solution is a shunt regulator, and this is the classic element in a light fitted with a bulb. Without it you can pop bulbs on fast downhill runs.
A suitably rated zener will get the job done.

Or as you already want to use LED's, simply use the LED's as the voltage limiters, and hang your taillight over the LED pair with the supercap and a blocking diode?.