LED from dynamo

[lance house]

Hi,

Ive just got a front-hub dynamo for my bike. I want to hook up some
white leds on the front and some reds on the back, preverably with a
battery as well which gets charged from the dynamo output as well as
protecting the leds when the voltage gets to high.


I read an article on a bike website
http://www.sheldonbrown.com/dynohubs.html

which suggests a bridge rectifier to make it DC and then charge a
battery from there.


At general cruising speed it outputs 8-12V DC (via rectifier) and I
reckon down a big hill I could easily top 20V. My multimeter only goes
to 200mA and it overloads it going pretty slow. I blew a standard bike
front led flasher hoing pretty slowly but the dynamo only gets a faint
glimmer out of a 2.5v0.5A halogen bulb.

So what do I need to do to be able to protect the LEDS from blowing
while still putting out a reasonable amount of light at lower speeds?

cheers

Lance

 

[Gene]

First of all, bicycles do not use dynamos but alternators. There are many
ways to solve the problem once the AC from the alternator has been converted
into a filtered DC. These are the most simple ones:

1. Put in series to the series combination "resistor - LED" a voltage
limiter. It can be build using a simple Zener diode, resistor and a NPN BJT
power transistor.
2. LED's are actually current devices; as such they "like" to operate in
constant current mode, therefore a current source could be good solution.
You can google and find how to build a current source.
3. Use a voltage regulator

It is evident that the "surplus" power generated when you are going
downhill, for example, will be "wasted" in the devices that are limiting the
voltage, therefore they must be able to dissipate the heat with a suitable
hitsink. To avoid this issue, more sophisticated circuitry is required.

Gene

 

[Dr Engelbert Buxbaum]

lance house wrote:

Hi,

Ive just got a front-hub dynamo for my bike. I want to hook up some
white leds on the front and some reds on the back, preverably with a
battery as well which gets charged from the dynamo output as well as
protecting the leds when the voltage gets to high.

You are dealing with an 6 V AC source and a 2 V DC consumer. The easiest
way to deal with that situation is to use 6 superbright red LED as
follows:

6 V ~ o----->|--->|--->|--------o Gnd
| |
---|<---|<---|<----


This simple circuit served me well for several winters. Because each LED
carries current only during half the cycle, the slight overvoltage does
not cause any problems, and no current limiting is necessary. The light
is much brighter than a standard light bulb, and lasts much, much longer
(I lost mine after several years of operation due to theft of the bike).

For the front light I still prefere halogen bulbs, but in principle a
similar circuit with 2 instead of 3 LED in each branch should also work.

Note however the legal situation in your place: You may be required to
carry an approved light source on your bike and to operate it while
riding in the dark. Thus such homebrew construction, although better
than anything you can buy, may be technically illegal.