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Miller Dynamo and a LED?

Captain TurokCaptain Turok Posts: 686
edited November 2010 in Road general
I have a Miller dynamo stamped "Miller made in England, 6 volt, 3.24 watt, model 53R" on the dynamo and "Miller made in England 598" on the lamp. 1693rxh.jpg I've been using this for a couple of months as a secondary light, but annoyingly its blown a fair few bulbs.

My question is would converting it to run an LED bulb solve this? I had in mind simply soldering an LED onto +/- wires and making a holder for the LED in the existing lamp body.

Although before butchering the the lamp body I thought I'd ask if this is possible and what LED should I be looking for?

Can any one help?..
*Rock Lobster Team Tig SL (22lb 14oz)
*C. Late 1950's Fixed Gear
*1940 Raleigh Dawn Tourist with rod brakes


  • laelae Posts: 555
    The problem is that the rear bulb is only rated 0.6w, and it's a 3w dynamo! It's designed to use a 2.4w front bulb and a 0.6w rear, so by either a)attaching a front 2.4w light, or b)using a 3w rear bulb, it should stop blowing (and if you attach a 3w rear bulb it'll be a hell of a lot brighter too).

    But anyway, if you wanna go for LEDs, I've been thinking about it lately too:

    There's a few ways of doing it, depending on how much work you wanna put into it. FYI there's no negative wire, they just use the frame as the ground.

    Simply plugging a 3w LED into it will work, but because bicycle dynamos (unlike true dynamos) put out AC instead of DC (as in, alternating current, the current goes positive-negative then negative-positive), the LED will flicker very annoyingly at low speeds (because LEDs, like all diodes, only allow current to pass in one direction). A couple of big 3000uF capacitors (found in an old computer motherboard, for example) wired in series should smooth out the worst of the flickering and provide illumination for a minute or so after stopping.

    Because these old dynamos are unregulated, the voltage can climb massively if you're going down a hill quickly, so a Zener diode or resistor is a good idea, to keep things under 6 volts.

    A better way of doing it would be to wire in a bridge rectifier (which converts the AC current into DC, so the LED won't flicker at all), smoothed by a 100uF capacitor in parallel. The problem with this is that it'll need to have a proper negative wire, so it'll have to be isolated from the frame (I was thining about making a plastic mount rather than a metal one).
  • Initialy many thanks for your time and very informative reply. There's more to it than I'd imagined!..

    Although in light of my surprise delivery from the bike fairy yesterday afternoon (a "fancy" lugged frame, Airlite hubs, Chater Lea pedals, Freius? an GB brake parts, numerous Campagnolo Gran Sport bits plus various other parts) I think time dictates I go with the 3 watt bulb option!

    Many thanks again.
    *Rock Lobster Team Tig SL (22lb 14oz)
    *C. Late 1950's Fixed Gear
    *1940 Raleigh Dawn Tourist with rod brakes
  • laelae Posts: 555
    No probalo. You might have to fiddle about with bulb holders and whatnot to get a 3w bulb to fit in the rear, but it should be alright.
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