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LED from dynamo

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

Reply to
Gene
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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

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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

Reply to
lance house

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 | | ---|
Reply to
Dr Engelbert Buxbaum

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