Joe24
More serious cyclist than Bonj
- Location
- Nottingham
shauncollier said:
Dynamo lights
- Thread starter Joe24
- Start date
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MartinC
Über Member
- Location
- Cheltenham
It looks like you've eliminated all the blown bulb and switched off type problems. The circuit's not shorting out 'cos the rear light works. Looks like it's an earthing problem.
Traditionally English bottle dynamo systems used a single wire. A single wire from the dynamo supplied current to the lamps and the earth return was through the frame. The rear light was often on the dynamo bracket so earthing wasn't a problem. The front lamp earthed through the steel forks, headset and steel frame. This usually worked but could be unreliable because of bad metal to metal connections between lamp bracket, headset or dynamo bracket. Corrosion or paint often causing problems. The dynamo bracket often had a pointed screw to penetrate (and damage) the seat stay to ensure a good connection.
A better system is to use a two wire system. Supply current with one wire and earth the lamp with another. Continental lamps and dynamo have two terminals and you use two strand wire (loudspeaker wire or you can get specific bicycle dynamo wire - B&M and Schmidt make this).
It sounds like your carbon forks and/or carbon frame are the cause of the earthing problem and you need to use a twin wire system. At the front lamp connect one wire, as now, to the bulb centre connection. Connect this wire to the dynamo supply (as now). Connect the other wire to the bulb casing or a metal part of the lamp body. At the dynamo end connect this wire to the dynamo bracket or casing. The rear lamp appears to be earthed properly so you can leave this as is or wire it in the same way.
Old bottle dynamos weren't voltage regulated so you could blow bulbs if you went too fast. Often blowing the front bulb would cause the rear to blow 'cos of the reduction in load. You can get voltage regulators to wire into the circuit. Shimano and others make them - google for them.
Traditionally English bottle dynamo systems used a single wire. A single wire from the dynamo supplied current to the lamps and the earth return was through the frame. The rear light was often on the dynamo bracket so earthing wasn't a problem. The front lamp earthed through the steel forks, headset and steel frame. This usually worked but could be unreliable because of bad metal to metal connections between lamp bracket, headset or dynamo bracket. Corrosion or paint often causing problems. The dynamo bracket often had a pointed screw to penetrate (and damage) the seat stay to ensure a good connection.
A better system is to use a two wire system. Supply current with one wire and earth the lamp with another. Continental lamps and dynamo have two terminals and you use two strand wire (loudspeaker wire or you can get specific bicycle dynamo wire - B&M and Schmidt make this).
It sounds like your carbon forks and/or carbon frame are the cause of the earthing problem and you need to use a twin wire system. At the front lamp connect one wire, as now, to the bulb centre connection. Connect this wire to the dynamo supply (as now). Connect the other wire to the bulb casing or a metal part of the lamp body. At the dynamo end connect this wire to the dynamo bracket or casing. The rear lamp appears to be earthed properly so you can leave this as is or wire it in the same way.
Old bottle dynamos weren't voltage regulated so you could blow bulbs if you went too fast. Often blowing the front bulb would cause the rear to blow 'cos of the reduction in load. You can get voltage regulators to wire into the circuit. Shimano and others make them - google for them.
Davidc
Guru
- Location
- Somerset UK
Get hold of a resistance/continuity tester (eg a cheap multimeter). if you don't have one then one of your mates will.
Make sure the two connections to the bulb exist (one to the case of the lamp then to the frame connection, the other to the wire connection).
Put the bulb in and check that, using the meter, you can measure its resistance between the lamp casing and the live wire.
If that's OK, you know the dynamo works, the only thing left is the connections to the lamp from the dynamo.
Check them with the multimeter again.
You may find that there is either no connection between the frame and forks, or a poor one, and I'm not sure about the electrical properties of carbon forks.
If there's any doubt run a wire from the case of the front lamp (find a suitable screw, or if none drill it and put a nut, bolt, and shakeproof washer in!) to either the case of the dynamo or a convenient point on the frame (bottle mount for example).
Recheck with the multimeter.
If there's electrical continuity, the bulb's OK, and the dynamo's working it should light up dimly even at very slow wheel speeds.
Make sure the two connections to the bulb exist (one to the case of the lamp then to the frame connection, the other to the wire connection).
Put the bulb in and check that, using the meter, you can measure its resistance between the lamp casing and the live wire.
If that's OK, you know the dynamo works, the only thing left is the connections to the lamp from the dynamo.
Check them with the multimeter again.
You may find that there is either no connection between the frame and forks, or a poor one, and I'm not sure about the electrical properties of carbon forks.
If there's any doubt run a wire from the case of the front lamp (find a suitable screw, or if none drill it and put a nut, bolt, and shakeproof washer in!) to either the case of the dynamo or a convenient point on the frame (bottle mount for example).
Recheck with the multimeter.
If there's electrical continuity, the bulb's OK, and the dynamo's working it should light up dimly even at very slow wheel speeds.
Davidc
Guru
- Location
- Somerset UK
Bulb blowing isn't usually a problem if the dynamo's got the right load.
Nearly all work roughly as a constant current source, so once going fast enough the output doesn't change (much)
BUT if one bulb blows they try to put the whole current through the other one and it goes as well.
To stop that buy 2 of 3 or 5 watt 7.5v zener diiodes. Wire them in series opposite ways round, solder them together and then the pair across the supply inside one of the lamp cases.
Every time the voltage from the dynamo (its AC) exceeds 8.2v in either direction the diodes will stop it going higher. This value seems to protect most bulbs.
(Modern lights already have this system or something close to it, but as yours is an old ex MacB one it won't)
Nearly all work roughly as a constant current source, so once going fast enough the output doesn't change (much)
BUT if one bulb blows they try to put the whole current through the other one and it goes as well.
To stop that buy 2 of 3 or 5 watt 7.5v zener diiodes. Wire them in series opposite ways round, solder them together and then the pair across the supply inside one of the lamp cases.
Every time the voltage from the dynamo (its AC) exceeds 8.2v in either direction the diodes will stop it going higher. This value seems to protect most bulbs.
(Modern lights already have this system or something close to it, but as yours is an old ex MacB one it won't)
Davidc
Guru
- Location
- Somerset UK
jimboalee said:
Certainly true of older ones, new ones are much better and a hub dynamo's best.
The old lucas ones had so much drag that some people couldn't turn the tops with their fingers, and they used to cut into tyres as well even when set up correctly.
Joe24
More serious cyclist than Bonj
- Location
- Nottingham
jimboalee said:
Im wanting the drag, and if there's noise then i just turn the ipod up
Thanks for the help, ill give it a go tomorrow, or Friday, or Saturday(probably Saturday, busy tomorrow and Friday)
Cheers
It may be a faff to get it right, but i have got spare time normally, and i do enjoy it.
Thanks again
