25-07-2020, 03:27 PM
I have a 1959 BSA Sunbeam Scooter. There are a number of types of this scooter including a 250cc twin cylinder 4-stroke and a 175cc 2-stroke versions. There are also versions badged as Triumph Tigress. Mine is the 175cc 2-stroke version. It has not been run since 1972. As it has been kept both outside and in a garage for this time it is not in what you might call perfect condition. I started to work on it just short of a year ago. So far I've done brakes, clutch, carburetor, seat etc. I've also built a new exhaust from scratch, done a bit of welding and have the engine running. It is hard to get parts for this scooter. Because of this I would like to build a simple transistor assisted ignition to save burning on the points. There are many ideas on the web but mostly for full electronic ignition for 12V -ve earth machines. Mine is a 6V +ve earth. I have drawn up a circuit for this which I have attached. Please don't laugh at it. My background is in chemistry not electronics but I know there are many people on the forum with a great deal of expertise in electronics who I'm hoping will be good enough to give me some advice. I have quite a number of questions so here goes.
1) Can you suggest a suitable transistor?
2) From what I've read there should probably be a diode across the transistor to protect it. Can you suggest a suitable diode?
3) I've also read that transistors already containing a suitable diode have been developed for the automotive industry. Would one of these be better? If so can you suggest a suitable one of these transistors?
4) Can you suggest a suitable value for R1 and R2?
5) I've included S2 so as I could switch from the transistor, doing the switching, back to the points. Is this good enough or do I also have to break the connection between the collector and earth?
6) I'd like to retain the original ignition capacitor in place. Is there any reason why I can't do this.
Finally I should say that there are at least two different versions of the ignition system for this scooter. One has a separate ignition coil as you might have seen in a car before modern electronic systems. However, in the version I have, the ignition coil is part of the set of coils mounted on the same backplate as the the lighting coil. The flywheel (with magnets in the rim) spins around these coils. I presume that this induces a current in both the lighting coil and in the primary (and the secondary) of the ignition coil. I thought that this was to aid the starting of the the engine with a flat or indeed no battery. I have attached part of a very poor image of the circuit diagram showing this.
The points and primary and secondary ignition coils can be seen in the center. Would this affect my proposed circuit?
Regards,
Ken.
1) Can you suggest a suitable transistor?
2) From what I've read there should probably be a diode across the transistor to protect it. Can you suggest a suitable diode?
3) I've also read that transistors already containing a suitable diode have been developed for the automotive industry. Would one of these be better? If so can you suggest a suitable one of these transistors?
4) Can you suggest a suitable value for R1 and R2?
5) I've included S2 so as I could switch from the transistor, doing the switching, back to the points. Is this good enough or do I also have to break the connection between the collector and earth?
6) I'd like to retain the original ignition capacitor in place. Is there any reason why I can't do this.
Finally I should say that there are at least two different versions of the ignition system for this scooter. One has a separate ignition coil as you might have seen in a car before modern electronic systems. However, in the version I have, the ignition coil is part of the set of coils mounted on the same backplate as the the lighting coil. The flywheel (with magnets in the rim) spins around these coils. I presume that this induces a current in both the lighting coil and in the primary (and the secondary) of the ignition coil. I thought that this was to aid the starting of the the engine with a flat or indeed no battery. I have attached part of a very poor image of the circuit diagram showing this.
The points and primary and secondary ignition coils can be seen in the center. Would this affect my proposed circuit?
Regards,
Ken.