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  1. #1
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    Help a motorcyclist in need?

    I'm trying to convert a 1980 trail bike to an led headlamp.

    Nominal 6vac magneto.
    Recommended bulb is 6v 21w incandescent.
    Measured 4vac ~ 18vac per RPM.
    Maximum 28watts.

    So far I've collected these parts. Prematurely perhaps, since I'm learning as I go.

    Three LEDs
    Invalid Request

    Reflector
    3*Cree Reflector - Orange Peel Aluminium (18mm x 50mm) - Free Shipping - DealExtreme

    Power Supply
    Invalid Request

    Bridge Rectifier
    Invalid Request

    Not sure if I need a smoothing cap or not. Some say the power supply will serve the purpose itself. Don't know.

    I'm stumbling on that "Voltage - Forward (Vf) Typ 2.9V"

    The rectifier, and smoothing cap, will drop the magneto's output to near 2v at idle. So the dc/dc converter/regulator suggested most often is buck/boost. I can't find one with adequately wide input and output range that delivers more than 2A.

    Also should have ordered Schottky diodes and made my own rectifier bridge, since they are twice as efficient where I need it most. Packaged Schottky rectifiers, for some reason, are astonishingly expensive.

    I haven't connected or tested anything yet. I'm trying to get as reasonably close to proper parts as I can, and learn a bit more of what I'm doing before experimenting. Still trying to get my head around current/voltage to LEDs and whether to drive them in series or parallel. I've got a drawer full of small heatsinks I can mount the LEDs onto.

    I'm mounting the 3*reflector inside the existing headlamp housing and glass lens.

    Any ideas on the buck/boost regulator in particular would be very very much appreciated.

  2. #2
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    Strange. The links say "invalid request" but they actually work.

  3. #3
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    You should search this forum for dynamo light circuits. I think your application could apply to them. The DC-DC converter you linked may not work like you want. Typically those are constant voltage devices but LEDs are best driven by a constant current device.

    The bare LED emitters you linked will be tough to work with unless you are making a custom MCPCB to mount them on. You may want to consider getting ones pre-mounted on star MCPCBs.

    The reflector you linked may lack the throw needed for a moto application. Depends on how fast you are riding. I believe you would be outrunning your light at 25mph or so with that triple reflector. I would look for individual reflectors in the 30-45mm diameter range and as deep a parabola as you can find. More like this...

    Cheap Aluminum Smooth Flashlight Reflector - Silver

  4. #4
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    My advice would be to go to ebay and look for pre-
    manufactured lights with Cree Led's (which is what you are ordering).

    These lights will have internal circuitry to regulate the LED current, but typically use batteries aka constant voltage source. Single LED lights use one or two 18360 batteries, li-ion batteries typically used in computer battery packs. Their fully charged voltage is 4.4 volts and work down to somewhere around 3.5 volts. Other lights use 8.8

    I have a single LED head mount light that uses two batteries (in parallel). It is advertised as having 1800 lumen output. I've seen postings that suggest the actual output is more like 900. BUT I've seen sources that say that this is comparable to a car's low beams. I tested the light by holding it out the window and shining it into the headlight beam. The low and high beam of my headlight were comparable to the low and high beams of my car. That is to say, I could see the beam from my light in the middle of the car beam. The implication is that the light output is indeed about the same as the car's. FWIW, the lighting on my car is not very good, but I do drive at 55-60 at night.

    You can get lights with at least 6 Cree XML-T6 LED's, which I believe is pretty close to state of the art. This will be seriously bright; you may start fires and blind the wildlife. And all of the engineering has been done and tested, which may not be what you want. But prices are between $20 and $50, much cheaper than the LBS lights.

    CREE 6 LED @ EBAY

  5. #5
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    Despite being referred to as dynamos, bicycles actually use magnetos- hence vancbikers recommendation to look at dynamo circuits. Martin's website is a good place to start.

    The difference however is that your bike magneto appears to produce more current than the 0.5A available from a bike "dynamo"

    To figure out what happens with your circuit, I would start with a very simple circuit - bridge rectifier, 3 leds in series. I would try to avoid using a smoothing capacitor if you can. Set it up on the bench, run it & see what happens

  6. #6
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    The forward pattern is a complete guess at this point.

    I intend to begin experimenting with the "bucket" and glass diffuser lens that is OEM on the bike at present. The form of hyperbolic reflector and cast glass diffuser lens is already constructed, similar to automotive headlights, to cast a downward and side reflecting beam. It is, obviously, designed around an omni directive filament incandescent bulb placed about 1.5 inchs from the base of the parabola at or near the focal point.

    I'm going to experiment with placing that linked 3*reflector at the base of the reflector so that its spread fills the glass lens to the corners. The original 4" reflector serving very little funtion in this arrangement.

    That's a start. The CREE 6 LED @ EBAY is certainly an attractive alternative If I'm unable to get any satisfactory results with those first attempts.

    The bare LED emitters you linked will be tough to work with unless you are making a custom MCPCB to mount them on. You may want to consider getting ones pre-mounted on star MCPCBs.
    I'm pretty comfortable with building curcuits with SMDs; even micro miniature. That's not a worry. I have a drawer full of variously sized aluminum heat sinks that i'll mount the LEDs to, adding thermal paste, so that the heat sink is behind and outside of the headlamp bucket.

    I'm all for using off-shelf solutions. It's just difficult (do you have a source?) to locate pre-mounted star MCPCBs in the precise color temp and voltage that is needed for the project.

    Great ideas. Thanks VERY much for responding here. Please keep it coming.

  7. #7
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    Quote Originally Posted by frascati View Post
    The CREE 6 LED @ EBAY is certainly an attractive alternative If I'm unable to get any satisfactory results with those first attempts.
    I would very much stay away from these cheapo lights in this application. One of them could possibly make a start by providing housing and reflector/optic but the LEDs are almost always cool white, not the warmer tint you are looking for. The driver board in most of these type lights is pretty cheaply made with marginally spec'd components and widely varying assembly quality. You still have to do a circuit to rectify, condition and regulate the voltage from the magneto before connecting to the light. The U/I usually cycles through the brightness levels, a flashing mode, then off. The moto application should only switch between two brightness levels (if even that, one may be fine if off-road use only) without turning off.




    Quote Originally Posted by frascati View Post
    I'm all for using off-shelf solutions. It's just difficult (do you have a source?) to locate pre-mounted star MCPCBs in the precise color temp and voltage that is needed for the project.
    Cutter Electronics in Australia (www.cutter.com.au/index.php) has a great selection of LEDs on various MCPCBs. You should be able to find the bin and tint you are looking for. Led Supply (LEDs | LED Drivers, Optics, Heatsinks & LED Kits | LEDSupply.com) is another source I have had good dealings with. XMLs are all in the 3.3Vf range at max drive current so no choices there.

  8. #8
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    Help a motorcyclist in need?

    If i have this right, your magneto should, like a bike "dynamo" be a roughly constant current device, with voltage being rpm dependent.
    Quote Originally Posted by frascati View Post
    Measured 4vac ~ 18vac per RPM.
    Maximum 28watts.
    Your numbers suggest that the current =~1.5A

    That will mean you are looking at around 3v per led - iirc @ 1.5A xm-l ~3v while an xp-g is ~3.1v

  9. #9
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    Cutter Electronics
    35mm round LED Boards for best cooling technology by sinkpad

    The first one cost 18 dollars to my door.
    The second one I have yet to hear back from after sending an email asking for quote.

    Any other options to shop around for a triple XM-L board/star/disc?

  10. #10
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    Triple XM-L boards are looking a bit scarce at the moment

    The only triple board at dx and fasttech don't look like they are suitable for an XM-L.

    Assuming that I have the maths right, 3 x 16mm stars should fit in a 35mm circle, but it might not put the LEDs in the correct spot for your optics.

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