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Wiring question for dynamo powered lights

Old 03-12-22, 10:15 AM
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le mans
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Wiring question for dynamo powered lights



I,ve always known this set up requires one wire

I needed an extra earth wire so the lights would work

can someone tell me why?


All the lights I removed from scrap bikes had one wire, and when I fitted lights to my road bike in the 70s it only needed one wire

The mind boggles
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Old 03-12-22, 10:22 AM
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Sometimes the dyno is grounded through the frame (one wire) and sometimes it isn't (two wires). Even if a 2 wire dyno is grounded to the frame (and I think most are), you would still need the second (-) wire if your light was not electrically bonded to the frame. I use coax for my dyno setups so I use one wire in every case.
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Old 03-12-22, 10:23 AM
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Originally Posted by le mans View Post


I,ve always known this set up requires one wire

I needed an extra earth wire so the lights would work

can someone tell me why?


All the lights I removed from scrap bikes had one wire, and when I fitted lights to my road bike in the 70s it only needed one wire

The mind boggles
I would think that the bike acts as the ground. It should not work on a carbon bike.
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Old 03-12-22, 11:32 AM
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Yes, the frame is the return ground wire. Look at the generator bracket on the fork blade and you'll see a screw sticking out. That screw has a pointed end that is meant to dig into the metal, under the paint, for that connection. The lights are grounded through their brackets too. And yes, this means that the current travels through the headset. Another reason to keep the headset in good shape.

BTW these bottle generator light systems can over power a bulb (some had 22mph max speed limits back in the day) and burn it out. A common story is the tail light burns out and the greater power the head light now sees burns that out soon enough after. Andy
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Old 03-12-22, 02:58 PM
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Originally Posted by andrew r stewart View Post
yes, the frame is the return ground wire. Look at the generator bracket on the fork blade and you'll see a screw sticking out. That screw has a pointed end that is meant to dig into the metal, under the paint, for that connection. The lights are grounded through their brackets too. And yes, this means that the current travels through the headset. Another reason to keep the headset in good shape.

Btw these bottle generator light systems can over power a bulb (some had 22mph max speed limits back in the day) and burn it out. A common story is the tail light burns out and the greater power the head light now sees burns that out soon enough after. Andy
this^^^^^^^^
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Old 03-12-22, 04:49 PM
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Yes, the ground screw on the fork blade clamp has to fully penetrate the paint and seat in the fork blade to provide a return path for the electricity.
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Old 03-12-22, 05:01 PM
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some newer headsets have something non-conductive in the way.
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Old 03-12-22, 07:25 PM
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It is relatively simple to attach a wire to jump around the headset, and insure continuity. Andy
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Old 03-12-22, 11:05 PM
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Originally Posted by Andrew R Stewart View Post
Yes, the frame is the return ground wire. Look at the generator bracket on the fork blade and you'll see a screw sticking out. That screw has a pointed end that is meant to dig into the metal, under the paint, for that connection. The lights are grounded through their brackets too. And yes, this means that the current travels through the headset. Another reason to keep the headset in good shape.

BTW these bottle generator light systems can over power a bulb (some had 22mph max speed limits back in the day) and burn it out. A common story is the tail light burns out and the greater power the head light now sees burns that out soon enough after. Andy
Ahh, I had forgotten what those little pointed screws are for

When testing I didn't screw it in, and assumed it's an added safety lock or something

Thanks Andy, I've always appreciated your help.
Now to get rid of that bulky wiring I got from an old kitchen appliance

Last edited by le mans; 03-12-22 at 11:12 PM.
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Old 03-12-22, 11:11 PM
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BTW these bottle generator light systems can over power a bulb (some had 22mph max speed limits back in the day) and burn it out. A common story is the tail light burns out and the greater power the head light now sees burns that out soon enough after. Andy


Interesting, I must have had decent bulbs back in the day, I used to reach 35MPH, those mornings when it was still dark heading to work.
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Old 03-12-22, 11:17 PM
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Originally Posted by daniell View Post
I would think that the bike acts as the ground. It should not work on a carbon bike.
Nor an aluminum frame I would imagine

I have a Raleigh beach cruiser - aluminum frame that i'll fit lights on as well, that will probably need a earth wire.

Love these old lights.. I have a box full

I heard about magnetic generators, great idea, but I suppose you'd need steel rims to make them spin
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Old 03-12-22, 11:20 PM
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We ran a rubber cap on our generator tire rollers to quieten and slow down the unit. No mountains around here but we do get over 40mph often enough. Andy
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Old 03-12-22, 11:29 PM
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Those things were so useless, I seldom bothered, with or without the cap. There was NO way they ever went 22 mph, with 30% drag. LOL. I don't think the cap lasted 200 miles anyway. That's the exact setup I had.
About all they did was dig into the sidewall or not turn at all. What usually happened anyway, was you hit a bump and the front bulb filament broke, then the tail fried 20 secs later.
I had this happen while I waited for new lights for my SA dyno drum. The LED bulb I bought melted the plastic tail housing too.

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Old 03-13-22, 03:02 AM
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Originally Posted by le mans View Post
Nor an aluminum frame I would imagine

I have a Raleigh beach cruiser - aluminum frame that i'll fit lights on as well, that will probably need a earth wire.

Love these old lights.. I have a box full

I heard about magnetic generators, great idea, but I suppose you'd need steel rims to make them spin
An aluminum frame should be fine as the return conductor.
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Old 03-13-22, 03:06 AM
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Originally Posted by unterhausen View Post
some newer headsets have something non-conductive in the way.
No sure if I have seen a headset that is fully insulated from the frame but there might be. If so Andy's thought about wiring around is a great solution.
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Old 03-13-22, 03:07 AM
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Originally Posted by unterhausen View Post
some newer headsets have something non-conductive in the way.
No sure if I have seen a headset that fully insulated from the frame but there might be. If so Andy's thought about wiring around is a great solution.
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Old 03-13-22, 11:09 AM
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Originally Posted by le mans View Post
Nor an aluminum frame I would imagine

I have a Raleigh beach cruiser - aluminum frame that i'll fit lights on as well, that will probably need a earth wire.
Aluminum conducts electricity better than steel
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Old 03-13-22, 12:46 PM
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Originally Posted by le mans View Post
can someone tell me why?
Just like automotive, the chassis the the ground.

Same with a lot of other small engines, like lamps for snowblowers.

The only time you will want to run your own (negative) ground wire is when you want more isolation for sensitive things, like zero-gauge for car audio amplifiers.

The amount of current coming from a vintage dyno that is needed to power an incandescent is plenty to overcome whatever resistance ohms you think a bike frame might make.
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Old 03-13-22, 05:58 PM
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Originally Posted by Andrew R Stewart View Post
It is relatively simple to attach a wire to jump around the headset, and insure continuity. Andy
Many industrial bearings have (sometimes literal) brushes which short across shaft bearings so that current doesn't flow through them, as arcing from bearings to races can damage both.
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Old 03-15-22, 03:40 PM
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Originally Posted by dsbrantjr View Post
Many industrial bearings have (sometimes literal) brushes which short across shaft bearings so that current doesn't flow through them, as arcing from bearings to races can damage both.
Not likely to be much arcing at 6 volts/3 watts (0.5 amp).
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