Putting LED bulbs into Dynamo Lighting Systems

For discussions about bikes and equipment.
Posts: 97
Joined: 27 Jan 2007, 11:08am

Putting LED bulbs into Dynamo Lighting Systems

Postby javatime » 27 Jan 2007, 11:44am

There are several "make your own LED bulb" projects on the internet, where basically you use a big bright and low current LED that is put into a conventional bulb holder.

Using a suitable resistor you can drop the voltage to the lower voltage required by an LED compared to a filament bulb.

The questions are :

A. What is the effect on a dynamo lighting system, which typically has an output of 3 Watts.

B. What would happen if you had an LED bulb at the front, and conventional filament at the year. Would the rear bulb neeed to uprated, and to what value.

C. How does the actual brightness compare. The halogen filament bulb is 2.4 Watts, The LED is 20,000 Mcd. 3.2V 20 mA

User avatar
Mick F
Posts: 50940
Joined: 7 Jan 2007, 11:24am
Location: Tamar Valley, Cornwall

Postby Mick F » 27 Jan 2007, 11:59am

If I cast my mind back to my Ohm's Law days ...

Is the dynamo AC or DC? If AC, you won't run a LED, they need DC. Also, LEDs, I think, require a rather specific voltage.

If it is DC, a 'suitable resistor' should be ok. You'd have to pick your resistor very carefully, as you could over run it downhill and fast. When I ran a dynamo setup, I often blew bulbs, so I installed a voltage limiter in the circuit.

Rear lights on dynamo sets are in wired in parallel with the front, so there should be no effect with mixing lights.

Hope that helps! But try it, and see what happens!

Mick F. Cornwall


Postby Jimbo » 27 Jan 2007, 12:46pm

If it is AC, though, a bridge rectifier is a very cheap and easy way to convert to DC (you just need a few diodes) - if interested, Google for it, it isn't anything like as complicated as it sounds.

Posts: 131
Joined: 11 Jan 2007, 5:13pm

Postby David » 27 Jan 2007, 1:30pm

No need for a rectifier at all with a dynamo, just use two LED's in reverse parallel, one lights on +ve cycles, the other lights on -ve cycle. Just choose the correct resistor to limit the current. I'm currently looking at modifiying Emily's lights in this way. Must do some surfing :-)


Postby Jimbo » 27 Jan 2007, 2:07pm

I suppose you could use one LED per direction, but 3W LEDs are a lot more expensive than simple diodes, so it seems a bit wasteful.

User avatar
Posts: 5316
Joined: 7 Jan 2007, 9:29pm
Location: Gloucestershire

Postby andrew_s » 28 Jan 2007, 2:27am

C: Brightness

It (20cd, 20 mA) wouldn't compare with a halogen bulb. It would only be any good as a "be seen" light, in cylcling terms.
The 1W or 3W LEDs used in riding lights are more like 200-800cd

Homebrew light comments:

Powering the bright LEDs from a dynamo is easy, provided that you don't mind them not running at full power.
You just need a bridge rectifier (available as single devices, no need to make one from separate diodes) with the AC connections going to the dynamo. and the DC connections to the LED(s). The one I used cost the princely sum of 18 pence fom Maplins. No resistors or anything else apart from a switch is required, as the 3W luxeons can be driven at higher currents than the 0.5A that the dynamo will supply.
Note also that there are now brighter LEDs than the 3W luxeons that most projects have used. Google for "Cree XR-E" or "Seoul P4". (Same power, double the light)

The hard bit is physically constructing the light, so that it's neat, robust, and will stop the LEDs overheating.
Putting one in an existing dynamo light wouldn't work too well, as (a) they emit light forwards rather than onto the reflector, so the light is spread all over, and (b) high power LEDs get hot, and would burn out if run in an enclosed light with a plastic body that didn't conduct the heat away.
There are various lenses that fit on the front of the LED and direct the light forwards. These are 20mm in diameter, and people have used 22mm plumbing fittings as the housing.

I ended up getting a Solidlight after about 8 months of using my home-made LED light. It has lots of fancy electronics inside to drive the 2 LEDs at full power at higher speeds, and has a much more solid body than anything I could make.