Sport Bikes banner

1 - 13 of 13 Posts

·
Registered
Joined
·
2,299 Posts
Discussion Starter · #1 ·
helping friend build a old(1970's) CB 125 as an "intro to motorcycles " bike.
he got the bike for 10$'s and "you get it out of that shed today so i never have to see it again".
it had been parked in the shed since the late 80's.

it was in predictable shape for a bike that had been abandon since around the time i was born.

the original wiring had been eaten by a pack rat at some point. the seat had rusted away.The tank was full of rust. The carb had, well lets not talk about the carb.

anyway i replaced the 6v rectifier with a 12 volt one.

i'm getting 23-38 volts at the alternator, and the rectifier splits our 12.6-13.3v at idle but as the rpm's pick up the rectifier voltage drops down to 9.2v.

any ideas what could be going on.
 

·
old member
Joined
·
13,079 Posts
If that bike has a 6V system, why did you put in a 12V rectifier?
 

·
Registered
Joined
·
2,299 Posts
Discussion Starter · #4 ·
If that bike has a 6V system, why did you put in a 12V rectifier?
it doesn't have a 6 volt system any more, what it has is
an alternator
a 6v regulator with the wires eaten off.
a 6v blinker box, that is in good shape.
a set of points and coil that are all intact.
bulbs that haven't worked since Reagan.

now as for why i am switching it to a 12 volt system,
1.) 12v parts are everwhere and cheep.
2.) with 12vs i need half the amps to do the same work=i can use smaller wires.
3.) resistance in the wires is not as important with 12v as 6v
4.) i am familiar with 12v systems
5.) I assumed that lower current in the dc system= lower current in the AC charging system=the coils in the alternator having it easier with the new voltage.
6.) the 6v rectifier that the bike came with had cooling fins 4X the size of the 12v unit i replaced it with, i assumed that this meant the 6v rectifier was inefficient and that the 12v rectifier would load the system less, wast less power, and make or power available for the rest of the bike.

now i will admit, i do not understand how regulator/rectifiers work. well i kinda understand how a rectifier works, at least for how current flows threw them. what i do not understand is dc voltage regulators, unless they are a zener diode.

even then zener diodes i just see as a one way valve that has a set voltage where they fail at and short out. this means that the yshort any voltage over X amount to ground and limit the voltage in the system.

my experience with things that short out is that they get hot, and waist allot of power. i assume that a zener diode regulator is what the old bike had, and that is why it had such a huge cooling fins. i further assume that we have developed some way to regulate current besides a zener diode and that this new way is more efficient.
 

·
Registered
Joined
·
2,299 Posts
Discussion Starter · #5 ·
Firing the spark plug at higher rpm and giving out 12 v is more than the old alternator can handle?

i am hoping that this is not the answer. and that there is some magic button i can push that will make my attempt to redo this electrical system feasible.

here is what i am telling myself to try and convince myself that i am not pursuing a fools errand.

It makes 12v at idle, and i have always thought that alternator power, and with motorcycles alternator efficiency increases with rpm's. how much power it takes to fire the ignition per revolution is a constant. so if the alternator where maintaining efficiency then the coil load from firing would not cause a decrease in voltage.

that the alternator is loosing voltage them must mean that with increased rpm's the alternator is getting less efficient, since most motorcycle alternators get more efficient with rpm's, this drop in efficiency is because i did something stupid. if i fix the stupid thing then the bike will work.
 

·
old member
Joined
·
13,079 Posts
I'm no expert on coverting a 6V system to 12V, but I did have a 6V VW back in the day (first car) and am an electrical engineer by education. The latter means that usually I know enough to be dangerous.

Agree that 12V parts are much easier to get and cheaper. The issue is to identify ALL of the parts that must be replaced. I'm not sure of everything that should be on that list, but all the light bulbs would be included.

Also agree that doubling the voltage should nominally halve the current, but old connectors might have corrosion causing high resistance and higher current. Don't try to skimp on wire size. The rating of many components might be rated in watts which is volts times amps. Double the voltage, halve the current and the wattage stays the same requiring the same amount of component cooling (cooling fins).

Obviously if a component has a DC voltage rating, it should be changed from 6V to 12V.
 

·
Registered
Joined
·
228 Posts
I'm also not familier with conversion. But I saw you said this bike had points?? If it really has points 12v should not be flowing through. I know the last vehicle I had with points had a resistor in line that lowered the voltage to 9 volts. As this was a 6 volt system I'm going to guess it needs to be even lower than 9.

Apmerage used is not constant. Keep in mind when revving the bike you are going from 600 sparks a minute to several thousand a minute. This may be the load capacity of your stator. I know most 12 volt system stators produce upwards of 50 Vac at idle from any given leg and can spike to 80 at rpm. The rectifier usually also doubles as the voltage regulator. But usually when these go out they "open up" dumping 20 Vdc on your battery.

How did you check the stator? It may be the stator is simply inadequate. Or coupled with the points issue may be overtasked.
 

·
Registered
Joined
·
300 Posts
Do you have a picture? Are you sure it is an "alternator?" Alternators are rather big and not often used on bikes. Most bikes have much smaller and less efficient shunt type regulation (SCR's short excess voltage before it is rectified for the battery.)

The stator assembly should be delivering a lot of current on each phase. Or unplug it and check the voltage at idle...which will be rather high across each phse...50 volts at idle to many hundreds of volts revved up! Under load, the maximum current will drop slightly. This will be very apparent if one phase is shorted out in the coil or regulator/rectifier.
 

·
Registered
Joined
·
2,299 Posts
Discussion Starter · #11 ·
well, got the bike mostly done, now it is just waiting on a new seat.

the problem with the regulator rectifier was that my friend had one of the connections loose, at higher rpms it was breaking continuity.
any way, some answers to questions
What's the story of the Rectifier/Regulator you used? Could it be bad?
it is from a chinese pos that was lying around.

I'm no expert on coverting a 6V system to 12V, but I did have a 6V VW back in the day (first car) and am an electrical engineer by education. The latter means that usually I know enough to be dangerous.

Agree that 12V parts are much easier to get and cheaper. The issue is to identify ALL of the parts that must be replaced. I'm not sure of everything that should be on that list, but all the light bulbs would be included.

Also agree that doubling the voltage should nominally halve the current, but old connectors might have corrosion causing high resistance and higher current. Don't try to skimp on wire size. The rating of many components might be rated in watts which is volts times amps. Double the voltage, halve the current and the wattage stays the same requiring the same amount of component cooling (cooling fins).

Obviously if a component has a DC voltage rating, it should be changed from 6V to 12V.
mostly i used 14 and 16 gauge wires, cause that's what i already had a bunch of. the original components had mostly died, or been eaten. so i replaced every thing that wasn't the points.

Do you have a picture? Are you sure it is an "alternator?" Alternators are rather big and not often used on bikes. Most bikes have much smaller and less efficient shunt type regulation (SCR's short excess voltage before it is rectified for the battery.)

The stator assembly should be delivering a lot of current on each phase. Or unplug it and check the voltage at idle...which will be rather high across each phse...50 volts at idle to many hundreds of volts revved up! Under load, the maximum current will drop slightly. This will be very apparent if one phase is shorted out in the coil or regulator/rectifier.
i used the term alternator because it makes alternating current, that has to rectified, and regulated.
 
1 - 13 of 13 Posts
Top