A home-made Capacitor Discharged Ignition unit for an old saw

[Al Smith]

In theory the firing of the SCR should be about at the max transition of the positive rise of the sinusoidal wave form of the low voltage side of the circuit .The problem with that would be the rising voltage because of more rotational speed .Faster it spins the more pulsating DC voltage it generates until it reaches coil saturation .This may or may not affect when the SCR gates .The SCR obviously is the electronic "switch ".
Someplace I have the drawing of a self advancing unit used on a Stihl 048 but it doesn't specify the component values only explains how it works .So with that I can't add any help in this matter .

 

[drf256]

Chy,

You are simply amazing my friend. No other way to put it.

Keep up the great work.

 

[Stihl working Hard]

The outlook of this ignition unit originally installed for her is this;
View attachment 106498
From above view we know there are two coils,

one for the primary-secondary coil which generates high voltage for the spark,

and

another for the exciter and pulser coil, the former of which works for giving induction current to the primary coil, the latter gives timing signal to the thyristor.

The original simple CDI unit had have been installed in between (a) coil and (b) coil painted in green in the above view, which was found broken when she came to me.

Very good thread Chy I admire your tenacity my friend keep it up

 

[chy_farm]

Good evening Al, thanks. How the SCR do its job? to be frank I am not so good at giving best explanation to these things Al, lol. Most always I give it a try and get the better fruit out of it, without digging far into the theory. So this means my poor brain does not love to make more calculations for them, instead I sometimes have some Spice software:s do calculations, there are actually nice Spice;s provided from semiconductor makers like Liner Technologies and Analog Devices.
Chy

 

[chy_farm]

@Al Smith , for instance one of them I love to use is this;

This is a trial circuit I did on the LT-Spice which has been provided from Liner Technologies.
Note that the resistor R2 is '1000000 kilo ohms' which means the spark gap.
Hope this will help you enjoy learning.
Chy

 

[chy_farm]

Chy,
You are simply amazing my friend. No other way to put it. Keep up the great work.

Very good thread Chy I admire your tenacity my friend keep it up

Thank you friends for your nicest comments, glad you like it! All these words from you guys who have long been posting a lot of fantastic articles to this OPE give me encouragement to go more!
Chy

 

[Al Smith]

After all these years it gets confusing to me at times .I was navy technician on submarines .I was good at repairing circuits but not proficient at designing them .That wasn't part of my job nor has it been in the last 40 plus years as I do my job as an electrician in the automotive industry .Any more it's just changing a complete assembly rather than change internal components of same .However I'm certain if you keep at it you'll figure it out .

 

[chy_farm]

Fun to read this @Al Smith, one of my nieces is doing service on a mother ship to minesweepers at our navy, she has been this job for 3 years now. Funny she is an electric engineer on her boat too!!
And a grandson of a guy who visited me yesterday has been on one of the biggest submarines of our navy for two years, he quit police office three years ago and applied to navy. And one more, wife of my first son has father who had been doing service at navy as a pilot for their fighters, he retired five years ago and now doing some farming at his little garden with some bee, but he does not know I am an chainsaw enthusiast, lol.

 

[Al Smith]

Other than the internet information all I have is the drawing in a Stihl tech manual of a coil assembley of a Stihl 042 .When I get time I'll see if I can scan it .
I fine it interesting because most times Stihl is very quite about how things work .That design uses the third magnet in the flywheel to set the advance I think .
If a person could find any info on a Briggs and Stratton "magetron" ignition I believe that also has an advance circuit .
I give you a lot of credit on this .Were it myself I'd have just used a trigger device on a coil and been done with it .

 

[mgr1]

Other than the internet information all I have is the drawing in a Stihl tech manual of a coil assembley of a Stihl 042 .When I get time I'll see if I can scan it .
I fine it interesting because most times Stihl is very quite about how things work .That design uses the third magnet in the flywheel to set the advance I think .
If a person could find any info on a Briggs and Stratton "magetron" ignition I believe that also has an advance circuit .
I give you a lot of credit on this .Were it myself I'd have just used a trigger device on a coil and been done with it .

No timing advance. The magnetron is just a full electronic coil that uses multiple darlingtons instead of breaker points.

 

[Al Smith]

Maybe it's my mistake but I had a coil on a 18 HP twin seem like it had a full advance .It would heat up and likened to burn the starter motor up .Once it cooled it fired right up .That coil btw is some place in one of my junk piles now .
I can't imagine what would cause that coil to fire early .

 

[Al Smith]

So much for Briggs and Stratton .I can't find the Stihl 048 manual which comes as no surprise .However it's on the net if you do a Google .I did but it wanted a log in so I didn't bother with it .

 

[Al Smith]

I had to go find it ,this explains pretty much how or why the timing can self advance http://www.foxvalleykart.com/timing2.html

 

[ray benson]

Other than the internet information all I have is the drawing in a Stihl tech manual of a coil assembley of a Stihl 042 .When I get time I'll see if I can scan it .
I fine it interesting because most times Stihl is very quite about how things work .That design uses the third magnet in the flywheel to set the advance I think .
If a person could find any info on a Briggs and Stratton "magetron" ignition I believe that also has an advance circuit .
I give you a lot of credit on this .Were it myself I'd have just used a trigger device on a coil and been done with it .

I think I have seen it.

 

[Al Smith]

Yep that's the one,thanks .That system uses an SCR (aka thyrister )instead of a Darlington set up but essentially works the same in shifting the trigger as RPM increase .That by the way is why in a previous post I mentioned generated voltage applies to rotating speed .
As a note when you annalize it it becomes relatively simple .The difficult thing to me would be determining things like bias voltages etc if I were to design one .I'd be reading my old theory books for a month if I could even find them any more .You never really forget but my land do you get "rusty" at it .

 

[chy_farm]

Good morning Al, glad to have a nice reading! That's nearly what I have wanting to know about 'automatic' advancing of this kind. But I was not very sure if this works in reality....but anyways this writer says it does, lol. To be frank I yet do not have much understanding on this theory, lol, shame!!
Will try reading this some more times for better understanding though. Many thanks for the nice information.
Chy

 

[chy_farm]

[Off the Topic]
- How does a three phase motor runs? a beginner's thought and experiment to understand its phenomenon. -

It's not a kind of thing that belongs to Ignition, but may be classified to the bigger category as 'Induction', to where this ignition thing goes.

I tried this simple thing this morning, in an attempt to convince myself of the theory.

 

[mgr1]

I had to go find it ,this explains pretty much how or why the timing can self advance http://www.foxvalleykart.com/timing2.html

You are right sir!
When the flywheel spins at a higher rpm, the voltage increase faster. The darlington only need 1 volts to trigger, how faster the flywheel spins how faster there is generated that 1 volt for the darlington to trigger. How faster it triggers how more advancing there is.

 

[Al Smith]

Unless I'm mistaken in looking at the schematic of the Magnetron the resister value would determine what voltage it would take to trigger the first transistor in the Darlington circuit .In this case it was indicated it was one volt .More resistance ,more voltage needed and so forth . By the same token it would depend on the transistor used as well because they don't all have the same values .
One thing to keep in mind these drawings are just a representation of the circuitry because in reality it could be just a tiny little thing but of course much larger than the micro circuitry in say a computer CPU .For example a nova II unit some people use to convert from points to solid state . They are like 15 dollars or less .You could spend a lot of time designing and building one .On the other hand that would not be a bad thing if a person had an interest in that kind of stuff .

 

[mgr1]

The atom chip is like the nova module. From the atom are schematics and buid pics on the net.