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capacitive discharge ignition system for motorcycle

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rick1216

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this system's power source is a permanent magnet single phase dynamo with about 20v ac
at cranking rpm and about 60v ac running.the primary side of the output coil is about 1ohm.this triggering can be by virtue of the mechanical position of the dynamo coil or a separate trigger coil. Thanks
 

tcmtech

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AND ? :confused:
 

Chippie

Member
well you need to boost the 20-60v to something more usable....

More detail is required for us to help...
 

Nigel Goodwin

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This sort of small engine normally has an extra coil in the alternator (not dynamo), which is the ignition coil, and directly generates the required high voltage. There's also either a set of points to trigger it, or a magnetic pickup that feeds a CDI unit that triggers it.

I've never seen (or heard of) one that uses the low voltage alternator coil for the ignition - apart from using that coil to charge a battery, and runnign a normal ignition coil off the battery.
 

rick1216

New Member
The ignition systems I have checked on CDI motorcycles have a 20v ac or so voltage at about 200 rpm to around 60+vac running then is stepped up in the CDI module somehow then discharged into a low ohms coil usually .5-1 ohm on the primary and about 5-6k on the secondary with usually 15-25+k volts at the spark plug.I'm thinking I have read or heard of cascading or paralleling capacitors in some fashion in the final stage of the CDI unit to multiply the source voltage to the primary of the high tension coil.This is a 2 cyl engine but want to treat it like two singles. Thanks, Rick:confused:
 

bountyhunter

Well-Known Member
The first part of my post is missing. I would like a circuit that would work with these specs.Thanks, Rick
Rick, I designed and built the CD ignition currently running on my motorcycle. It, and every other CD ignition I have ever seen, is powered by the bike's 12V system. Mine uses the stock points to trigger it, but some are available that use a mag pickup.

I am not sure what you are asking, but if it is how to build a CDI from scratch, I don't think you are going to want to try to do that.
 
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bountyhunter

Well-Known Member
The ignition systems I have checked on CDI motorcycles have a 20v ac or so voltage at about 200 rpm to around 60+vac running then is stepped up in the CDI module somehow then discharged into a low ohms coil usually .5-1 ohm on the primary and about 5-6k on the secondary with usually 15-25+k volts at the spark plug.I'm thinking I have read or heard of cascading or paralleling capacitors in some fashion in the final stage of the CDI unit to multiply the source voltage to the primary of the high tension coil.This is a 2 cyl engine but want to treat it like two singles. Thanks, Rick:confused:
That's not how a CDI works: they take the bikes 12V and run it into a low frequency DC-DC converter to boost it to about 400V. The storage cap (about 2.2uF) is in series from the 400V source to the + side of the coil. The cap charges up to 400V, then discharges into the coil primary when the SCR across the 400V source fires and shorts it to ground.
 

bountyhunter

Well-Known Member
View attachment 33638 I would have to say you are wrong...this system is called AC-CDI which has been used on dirt bikes for decades. I hope this attachment will explane this site gives component values and schematic. Rick:D
I'm not wrong as to how a CDI works. I have built them and serviced them and any decent CDI does just what I listed. There may be some cheap types available for low cost applications (even lawn mower engines generate a spark), but what I listed above is how most auto/MC CDIs operate. You get what you pay for.

Attached is a schematic for the typical auto/MC CDI ignition.
 

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rick1216

New Member
No...I didn't mean to offend...but the AC-CDI system has been used by all the Japanese motorcycle Mfrs since the early 70's This is a rugged simple design... it's best virtue is that it does not require a battery. check it out...it has nothing to do with inferior quality of design or performance or price. Thanks,Rick
 

bountyhunter

Well-Known Member
the AC-CDI system has been used by all the Japanese motorcycle Mfrs since the early 70's This is a rugged simple design... it's best virtue is that it does not require a battery. check it out...it has nothing to do with inferior quality of design or performance or price. Thanks,Rick
I've never seen it on any Kawasaki vintage 1975 or newer unless it is confined to small displacement engines only, I only work on the bigger ones. I realize it's a simple design, but I won't be using one because I am too old to have to push my bike home these days.

The best virtue of the one whose schematic I attached in the last post is that since it uses stock points and coil, the stock ignition can be restored any time the elctronics blow..... and they are notoriuos for blowing.
 
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shed_dweller

New Member
Rick's bang on the money. AC powered cdi was very popular on 70/80/90's bikes.

The Dc CDI powerd stuff wasn't very popular as it cost more and wasn't as relaible.

Transistor igniton was more popular on 4 cylinder bikes, but cdi was favoured on 2 strokes and big 4 stroke twins and singles.
 
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Nigel Goodwin

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Rick's bang on the money. AC powered cdi was very popular on 70/80/90's bikes.
Yes, common place on smaller bikes and off-road ones - my 1970's Yamaha DT400 had that type of ignition. No points, just a magnetic sensor, coils under the flywheel, a CDI unit, and an ignition coil.

Essentially it was just a transistorised and point-less version of the far older battery-less ignition systems used in the 50's and 60's.
 

bountyhunter

Well-Known Member
Maybe these were used on small dirt bikes, but I have never seen that topology on any road bike whose engine was more than a few hundred CCs. I know the schematics of virtually every aftermarket CDI system made back in the 80's when CDI's were replacing standard point ignitions, and every one uses a DC-DC converter front end followed by an SCR as in the schematic I posted. Some of the early factory "electronic ignitions" were not CDI at all, just magnetic sensors driving solid state devices acting as switches in place of points. They offered no real performance improvement, but increased service life since they had no points to wear out or burn over time. The best were the high performance CDI's which used magnetic sensors and had adjustable timing curves. I believe Dyna still makes them.

I don't doubt some makers put cheap ignitions on cheap bikes, I would not run one. I'm too old to have to push my 500 pound motorcycle home.
 
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bountyhunter

Well-Known Member
Rick's bang on the money. AC powered cdi was very popular on 70/80/90's bikes.

The Dc CDI powerd stuff wasn't very popular as it cost more and wasn't as relaible.
That is simply not true. A well made one was more reliable than the stock ignition and would actually start a bike under some conditions where a stock ignition would not: ie, a fouled plug or a battery dropping to low voltage on starter crank.
 

zorbzz

Member
I had a CR250 motorcross bike that had the Ac type CDI ignition. If water got in the flywheel it would not work.
It didnt have points or magnetic pickup it worked like a lawnmower magneto system. As the stator spun past the magnets it would create a voltage that went through the electronics and caused a spark.
I had the CDI fail and yes I had to push it home :) and the replacement CDI cost me $700 so I dont think there cheap.
No wonder you want to make your own.
 
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Nigel Goodwin

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I don't doubt some makers put cheap ignitions on cheap bikes, I would not run one. I'm too old to have to push my 500 pound motorcycle home.
A battery powered CDI is no more reliable than an engine powered one, and is likely to be LESS reliable because there's much more to go wrong. No inverter in an AC one, the HV is generated directly under the flywheel - otherwise the electronics is pretty similar, but most of a battery CDI is the inverter anyway.

If the electrical system fails on an AC CDI bike, you can still ride it home - you can't on a battery powered one.

zorbzz - all the ones I've used work fine under water, because they use a magnetic sensor under the flywheel feeding a sealed CDI unit under the seat.
 

shed_dweller

New Member
They were fitted to some larger bikes. The battery powered ones weren't favoured as making a reliable DC to DC converter was expensive. This isn't the case nowadys. However its the HV coils in the generator that fail most often, so a modern Dc to Dc converter is no less reliable then the old AC powered unit.

Typical running voltage at cranking is 20 but can be as high as 300 when running. If you don't have voltages that high you won't get enough engery into the capacitor to fire the engine at higher RPMs. I suggest maybe your meter wasn't reading the voltages correctly. Many systems have two sets of HV coils, so maybe the signals from the two coils were confusingthe meter. I have an idea that most meters work at 50 herz ac, the engine could get to 200. You need an oscilorscope (well thats what I use before I get shot down and told there's a much better way)to see what's going on and find the true levels.
 
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