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Old 12-03-09, 01:11 PM   #1
jimn
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Taillight's a drag - use a resistor?

Hi,

I have a Shimano Nexus generator hub (DH-3N72) powering a Lumotec N2. It's been great. There is drag but barely noticeable, and the light is bright enough to be seen in the daytime.

Last night I made a small taillight - 3 red LEDs in parallel, and wired it parallel to the Lumotec.

Now there is a noticeable resistance when I ride. What's worse, the Lumotec is about half as bright as before.

Would a resistor between the generator and the taillight solve this problem? If so, what ohms?

Would using a single LED be a better solution, and if so, why?

I just pulled the LEDs out of my parts bin. No idea of their specs. I think they are maybe around 8 years old if it matters. All three are oriented the same way toward the circuit. That is, given DC they will all be either on or off, and under AC they flicker in unison.

I'd thought that since LEDs are so efficient, the difference in drag would be miniscule. I was quite wrong.

Thanks,
Jim
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Old 12-03-09, 07:03 PM   #2
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LED's are still diodes. They will draw any amount of current, in the forward direction with little forward voltage drop. You need a series resistor to limit the current draw. The rear light on a standard 6 volt 3 watt dynamo was designed to draw only 100 milliamps (2.4 watts for the front and 0.6 watts for the rear). That should give you some indication as to how to size your current limiting resistor.

If you use an even number of LED's in the back and alternate their direction, then half the LED's will be emitting light at all times.
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Old 12-04-09, 09:36 AM   #3
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Right!

Thanks. Tried it last night. It seems to be working quite well with one 220 ohm resistor in series with the taillight.

The tip about alternating the direction of the LEDs is a good one, and I wish I'd thought of it before gluing the whole thing together.

I used this formula (and then did some filddling)


R = (Vs - Vf) / If


Where:
Vs = your source Voltage (usually 6 Volts for a bike dynamo).
Vf = the typical LED forward Voltage, find that in the LED's data sheet.
If = the typical LED forward Current, find that in the LED's data sheet.
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Old 12-04-09, 12:17 PM   #4
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Some bike dynamo taillights are designed to only be fitted to headlights that have a power takeoff from the headlight to the taillight, normally DC. Examples are the Light On! and Supernova taillights. The documentation that came with my Supernova taillight noted that it would decrease headlight output by about 10%.

Both the Supernova and Light On! taillights are vey small 3 LED units that appear to be too small to have any electronics in them other than possibly a resistor. I have no idea whether the LEDs are set up in series or parallel. As these are designed for use with specific headlights the current limiting could well be in the headlight itself.

The blinking effect suggested by SBinNYC would only be noticeable at relatively low speeds I suspect as once at any reasonable speed the frequency from the Dynamo and vision persistence of the viewer should make the flicker unnoticeable. After all the simplest DIY LED bike light circuits do not include any capacitors and once at any appreciable speed the blinking of the headlight is not noticeable.
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Old 12-04-09, 12:36 PM   #5
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BTW a resistor/diode taillight run directly off of the dynamo has the disadvantage of no standlight function. For night safety I want a standlight on any dynamo headlight or taillight that I am using. On most dynamo taillights driven by a power takeoff from the headlight the standlight is provided by the headlight circuitry I believe. That is certainly true of the Light On! and Supernova taillights.
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