The Blue,the Black and the Ugly (White)

[duivendyk]

It does not look like a genuine CDI unit to me.Basically a standard ignition coil with some kind of solid state trigger switch.No sign of a storage cap as far as I can tell.What you showed in pic. 4 was all there was, is that correct?

 

[BoltsMissing]

In Module 4 I tried to get to that square looking piece where the wires lead to.
When I was carfully grinding away that section, the odour was resin, just on that area, unlike the rest of the entire coating. Must wear a mask if anyone is doing this. I think the way they make these is pour this stuff in a mould and allow it to set.
Anyway, it was in there in that square section I was trying to get to, but it's so small, compact and resin just covers every bit of it, every nook and cranny.
That is why I thought burning the stuff off might do it, and time was running out by then.

I was mainly interested in WHAT Copper wire is used.
We get what we pay for and as long we know, that is ok, there is no reason then to frown upon the HT, it is what it is.

 

[duivendyk]

That very thin wire is the secondary of the coil,many turns to get the large voltage buildup to get a high enough voltage to fire the plug. There should also be a primary that gets interrupted, probably when the input voltage ac pulse goes through zero in the negative direction again,There must be an SCR that is triggered on by a pulse derived from the input waveform, this then completes the circuit for charging the coil's primary and build up the magnetic field.Once triggered the SCR stays on until the input goes through zero and goes negative,then it will turn off resulting in the rapid change in current which induces a large voltage in the secondary to fire the plug,like a standard coil ignition points do .There is probably a capacitor from the anode of the SCR to ground to keep the voltage from going up too quickly and retrigger the SCR.It's reasonably clear to me now how it all works.If we used a rectifier that rectifies the negative going portion of the generator coil output,it should not have any effect on the ignition circuit I would imagine.

 

[duivendyk]

Did the ignition coil wire come out of that # 4 part also?, was there still anything else electrical looking in addition to that blob?.I suppose not, but I'd like to make sure.

 

[BoltsMissing]

Nothing special, the tip where the plug lead screws into is simply a screw soldered at the top.
And yes, there was 2 layers of wire, one thicker than the other.
After spliting the coils, I was then able to get to the rod in the middle.
But the part where the blue and black wires go into, very difficult to get to.
Next CDI I splice, I'll simply work on that part, that sqaure piece and trace the blue and black wires endings.
Exactly what we need to know next, what makes the "switch", what are the actual components under all that resin between the plate and entry into the coil of wires.
But I don't have any spare CDI to try, I guess it's up to another HT forum member to have a go.

 

[duivendyk]

Yes, this pretty much confirms what I said before.The socalled CDI is essentially a conventional ignition coil with a primary and a secondary, but with a solid state trigger circuit.The most likely circuit being that SCR setup that I described ,or something fairly close.I had my doubts about that CDI business anyway.Why would a bunch of terminal cheapskates build a CDI ignition? esp if there is no actual trigger input.At least we are now getting the lowdown on the thing.Thanks

 

[arceeguy]

From image 12, looks like nothing more than a stepup transformer inside.

There really isn't more to a CDI than that. Add a rectifier, capacitor, a SCR and a few other components, and Voila! Ignition system with no mechanical "points" to wear out or get out of adjustment. Copper is a very soft metal, so it isn't surprising that the wires in the coil are like hair. Cut open a flyback transformer from a TV and you'll see the same.

It shouldn't make a difference whether you disconnect the CDI from the blue wire, or short the blue wire to ground. Either way, the CDI unit will lose signal and not fire. Each method is no more harmful than the other. You could also wind a separate coil on the magneto, and short out that winding to shut the engine off.

 

[arceeguy]

Yes, this pretty much confirms what I said before.The socalled CDI is essentially a conventional ignition coil with a primary and a secondary, but with a solid state trigger circuit.The most likely circuit being that SCR setup that I described ,or something fairly close.I had my doubts about that CDI business anyway.Why would a bunch of terminal cheapskates build a CDI ignition? esp if there is no actual trigger input.At least we are now getting the lowdown on the thing.Thanks

A "CDI" is a "Capacitive Discharge Ignition". To qualify as a "CDI" all you need is a capacitor and an electronic means of discharging that capacitor into an ignition coil to produce the spark. (the SCR and associated components) Briggs & Stratton, Tecumseh, and many other small engine manufacturers have been using this type of CDI since the early eighties. These CDI ignitions integrate the magneto coil, and trigger circuitry in one unit, where the HT engines have a separate magneto coil and CDI/Ignition coil. Many motorcycles have a separate magneto, CDI Module and Ignition coil. A typical small engine CDI does not have a separate trigger sensor and they work exactly like the HT engine, triggering spark on the falling edge of the AC waveform produced by the magneto.

 

[hill climber]

so what does all this mean are we going to be able to charge a battery?

 

[duivendyk]

Pretty sure about being able to do that , But it would certainly be nice to find out what the peak amplitude of the voltage at the white wire is under no load conditions.The ac averages, say 6-9 volt are not all that meaningful because the output will not be anywhere near a continous sine wave,more like a single ac cycle followed by a period of zero output and then another cycle, and so on.So the peak value,which is what rectifiers actually respond to could be considerably higher than the average that the meter reads and a incandescent bulb responds too.That does not mean that you can't design a simple battery charging circuit,but it is certainly worth knowing about when designing it.I have provided instructions as to how to go about making the measurement (See yesterdays post)