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  1. #1
    Senior Member flammenwurfer's Avatar
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    Dynamo hub powered headlight wiring advice needed

    I'm working on a diy dynamo powered light for my Raleigh Sports with a somewhat vintage look. I saw somewhere online a guy who used a triple led mr16 bulb hooked to a dynamo hub as his headlight and seemed like it worked pretty well.

    I don't know much about circuits so I'm trying to keep it inexpensive and simple. I grabbed some small chrome bullet auto/motorcycle running lights at walmart that take an mr16 bulb. It seems that the bulb I have only accepts AC. I had a bridge rectifier in my wiring and it wouldn't light up. I would also like to eliminate some of the flickering at low speeds, which I believe is done with a capacitor. But, doesn't a capacitor need DC?

    Should I be trying to find a bulb that accepts DC or is there any other way to smooth out the flickering while still running the AC from the hub straight into the light?

    I'm even able to use my headlamp bracket!

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    Dynamo hub powered headlight wiring advice needed

    Quote Originally Posted by flammenwurfer View Post
    I'm working on a diy dynamo powered light for my Raleigh Sports with a somewhat vintage look. I saw somewhere online a guy who used a triple led mr16 bulb hooked to a dynamo hub as his headlight and seemed like it worked pretty well.

    I don't know much about circuits so I'm trying to keep it inexpensive and simple. I grabbed some small chrome bullet auto/motorcycle running lights at walmart that take an mr16 bulb. It seems that the bulb I have only accepts AC. I had a bridge rectifier in my wiring and it wouldn't light up. I would also like to eliminate some of the flickering at low speeds, which I believe is done with a capacitor. But, doesn't a capacitor need DC?
    Should I be trying to find a bulb that accepts DC or is there any other way to smooth out the flickering while still running the AC from the hub straight into the light? I'm even able to use my headlamp bracket!
    www.google.com; define: rectifier
    Rectifier - Wikipedia, the free encyclopedia ; en.wikipedia.org/wiki/Rectifier
    "A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction..."

    Your current circuit has a rectifier to generate DC output from AC input. Remove it or bypass it to regain the AC your LED needs. Might want to read up on wiring bike lights on sites such as Peter White's or Sheldon's.

    Hope this helps
    /K

  3. #3
    Senior Member flammenwurfer's Avatar
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    I think I at least understand the basics of dynamo bike light wiring. I know the hub is supposed to put out AC 6V 3W 500ma. I did some more tinkering last night and I think I have come to the conclusion that my Novatec hub either is not working correctly or just sucks. I hooked up a multimeter to the Novatec hub and spun the wheel with my hand as fast as I could. I would get a peak of about 8 volts, which at first I thought was normal. Then I did the same thing with my other dynamo hub (sram i-light) and was getting 9-10 volts.

    That doesn't seem like a huge difference but when I hook the same light up to both hubs, it is at least 3 times brighter when connected to the sram hub. Could that big of a difference in the power output of the two hubs be right? They both technically have the same specs of 6V 3W so I wouldn't think it would be that big of a drop in output if they were both working correctly.

    My fear is that the Novatec is just malfunctioning and I'll have to shell out for a new hub, spokes and truing of a wheel.

  4. #4
    Senior Member flammenwurfer's Avatar
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    PS. The MR16 bulbs I'm using have a little circuit board that I'm assuming has at least a rectifier on it because leds have to have DC current. So the bulb itself is taking the AC of the hub and converting it to DC. I dismantled one and disconnected the circuit board and wired just the leds up to the hub with my own bridge rectifier and the leds lit up. The bulbs just don't want to accept DC because it has the rectifier already built in i guess?

  5. #5
    You gonna eat that? Doohickie's Avatar
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    Stupid question, especially based on your most recent posts, but have you tried reversing the wiring between the rectifier and the light? LEDs only work for one direction of current.




    Yes, I know it is a stupid question, but ya gotta start somewhere.
    I stop for people / whose right of way I honor / but not for no one.



    Originally Posted by bragi "However, it's never a good idea to overgeneralize."

  6. #6
    Senior Member flammenwurfer's Avatar
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    Haha, yes. I switched everything around at one point or another while I was playing with it last night. Switched the wires going into the rectifier around, switched the wires from the rectifier to the light around.

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    I caution that the testing discussed of this dynohub vs the other brand dyno hub is not detailed enough to draw a conclusion that your original hub is broken. Each dyno is designed to produce a set amount of voltage at a set RPM. At other RPMs, the voltage will vary quite a bit. And some are designed for primary use at lower RPMs, some for Higher. You should try to hold some variables constant and retest. As a start, I would recommend using/clamping a small motor to a nearby desk and running a belt or piece of surgical hose around the wheel. This will allow you to get control of the RPM variable. Then a meter will tell the story for that RPM. Could play with pulley size on the motor. The overall goal would be to get a enough values to plot a curve from a low RPM to a high RPM and then compare the two dynos...

  8. #8
    Senior Member flammenwurfer's Avatar
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    Quote Originally Posted by ksisler View Post
    I caution that the testing discussed of this dynohub vs the other brand dyno hub is not detailed enough to draw a conclusion that your original hub is broken. Each dyno is designed to produce a set amount of voltage at a set RPM. At other RPMs, the voltage will vary quite a bit. And some are designed for primary use at lower RPMs, some for Higher. You should try to hold some variables constant and retest. As a start, I would recommend using/clamping a small motor to a nearby desk and running a belt or piece of surgical hose around the wheel. This will allow you to get control of the RPM variable. Then a meter will tell the story for that RPM. Could play with pulley size on the motor. The overall goal would be to get a enough values to plot a curve from a low RPM to a high RPM and then compare the two dynos...
    That is a good suggestion. I don't know if I have the supplies to rig something up like that though.

    The difference is very apparent though when I have the light hooked up. The light output of the Novatec is SO much lower than the SRAM that it is unmistakable. Doesn't matter how fast or slow I spin the wheel. The light is correspondingly an order of magnitude dimmer when connected to the Novatec.

  9. #9
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    I think it's made for 12v, so it might have some circuitry in there that you don't really want. 3 leds in series would tend to flicker at low speeds if there wasn't a capacitor holding the voltage a little. Not sure how you would reverse engineer the circuit without taking it apart. You don't want the cap before the rectifier. Not sure why a replacement for an MR16 would have a rectifier in it except for protection

    Was that bulb in the automotive section of Walmart?

  10. #10
    Senior Member flammenwurfer's Avatar
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    Yes, the MR16 bulbs are 12V. I'm assuming the rectifier is to provide the leds with DC current since the bulbs are intended for use with AC current.

    I have 5 different bulbs, which all came from either ebay or tmart.com. The housing is the only thing I got from the automotive section of walmart.

  11. #11
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    MR 11 and MR16 bulbs are built to run on AC or DC.They have a rectifier built in the back.The good MR led bulbs have a muffin fan inside them!There are 2 types of MR bulbs,low voltage and high voltage,both run on AC or DC,and both have a huge range of voltages they will run on.

    You can hook them up directly to your hub.3 leds work best,2-1 watt in front and 1 in back or 1-3 watt in front and a battery blinky in back.Hub dynos work best at 3 watts,that's what they are built for.

    You can daisy chain 6,7,8 together if you want but your going to be riding at a good pace to keep them all lit up.

    They are NOT the most efficient way to have leds but it's one of the easiest.I like them,I can buy a bulb anywhere if need be.

    I run a cree 3 watt MR16 in front and wired a Planet bike 5 led blinky in back.I thought the Planet Bike blinky would burn up but it's hanging in there good so far.They do blink at walking speed but above about 3MPH you don't see it.Actually they blink all the time on AC,it just blinks too fast.
    Last edited by Booger1; 11-19-12 at 10:18 AM.
    Everything should be as simple as possible...But not more so.---Albert Einstein

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