A new ignition circuit

[Walsheng]

I will be following as soon as you guys start speaking English again!

John

 

[123RWO]

What is the function of C1? Speaking of multi-spark pulsers, this link has some info: http://www.sci-experiments.com/HighVoltagePulser/

RWO

 

[jgedde]

What is the function of C1? Speaking of multi-spark pulsers, this link has some info: http://www.sci-experiments.com/HighVoltagePulser/

RWO

Don't get me started on that one! It's a myth that the condensor across the points does nothing more than protect the points from arcing.

It does do that with points, but it also beefs up the spark by creating a resonant circuit with the coil. The same thing holds true when using a transistor as a switch.

You will get a spark without a condensor (or C1), but for all intents and purposes, the coil's output voltage duration will be shortened. This is of course a practical explanation - keeping it in English!

John

 

[dsage]

Yeah. For sure you're going to waste a lot of battery power at low RPM's but once the engine moves up toward the RPM near to the repitition rate of the 555 it starts trimming back sparks until you have only one just like without it. It takes a bit of balancing point dwell and timing components to get it where you want.
The selling point for commercial multi-spark ignitions is easier starting and better low end torque. The claims come from when a performance engine is running slow the mixture is usually crap because the cam timing is tuned for high RPM's. So theoretically one spark might not take. So the answer is - hit it a few times to be sure. Model engines usually have poor carburetors so everything helps.
A small 4Ah or 7Ah battery will last quite a while running a small hit-miss engine. I'd say long enough to cover a weekend show either way.

It's also cool to say you have a multi-spark ignition.

I'm sure someone will chime in that a 555 is old school but they run on a wide range of voltage and don't change timing vs voltage so you could run the same circuit from say 5v to 25v and still be fine. You could use a processor and have no discrete components but then you'd have to have a regulator and the ability to program a processor etc. etc...

I have a small circuit board layout in Eagle for the whole thing.

It's all trade offs.



Sage

 

[canadianhorsepower]

Don't get me started on that one! It's a myth that the condensor across the points does nothing more than protect the points from arcing.

It does do that with points, but it also beefs up the spark by creating a resonant circuit with the coil. The same thing holds true when using a transistor as a switch.

You will get a spark without a condensor (or C1), but for all intents and purposes, the coil's output voltage duration will be shortened. This is of course a practical explanation - keeping it in English!

John

I'm happy to see that I'm not the only one that knew this.
one recent post create a storm when I mention that:fan:

 

[dsage]

Boy this is a busy place. A whole slew of replies popped in while I was editing my response - which is Waaay back there now.

Yes the EEVIC engine that Dave Bowes designed is exceptional. I'm not sure if you get to the shows but he brings it to Cabin Fever and NAMES. Only NAMES this year since they changed the Cabin Fever date. He is in the the same club as me.
tsme.ca
Unfortunately the technology does not scale up so well for a lot of reasons which he could explain to you. Lots of companies have tried.

We digress.

Sage

 

[dsage]

John:

Do you have a quick calculation for a proper capacitor value?

Sage

 

[jgedde]

John:

Do you have a quick calculation for a proper capacitor value?

Sage

Alas, no. Certainly it'll depend on the the inductance of the coil, turn ratio, etc. But beyond that, I tend to think it'll depend also on the dynamics of the spark gap, engine requirements, etc...

I think the good news is that it isn't all that critical.

John

 

[123RWO]

Re: C1. I have heard the resonant tank circuit theory for a long time, but the transistor ignition circuits I have seen in the literature and including the couple I have designed and built didn't use the parallel cap. I never had a problem with low output even at 6000 RPM ( 4 cylinder). How about running a test for spark peak voltage and duration with and without the cap when you get your new system going? That should put the question to bed one way or another.

RWO

 

[jgedde]

It's no theory, it's basic AC electronics. http://www.allaboutcircuits.com/vol_2/chpt_6/3.html

If you look at elelctronic ignition circuits for cars, you'll see them there too. There are many examples, some good, some bad to be found by Googling "Electronic Ignition Circuits".

Again, you will get a spark without the cap and your engine will likely run fine.

John

 

[tornitore45]

The circuit is clever and will work for sure. Clamping is a good feature 180Vzener x 100 ratio will clamp at around 18kV secondary, plenty for a spark but not enough to fry the transistor or the coil is the gap is too high.
Basically you AC coupled the drive contact/hall sensor the ON state can only be temporary.

I had the same concer about a straight Kettering (inductive) system and opted for a Capacitive discharge. We like to run our model engine as slow as possible to show all the moving parts. At LOW RPM the coil is practically on all the time at highest current. The energy to charge the cap is fixed, no matter how low the RPM, even not running the cap get charged once and the current goes low after that.

I bought a CDI kit mostly to get the impregnated HV coil. The circuit works but is pretty cludgy design. I plan to redesign the boosting charge pump and utilize the coil which is really small.

 

[jgedde]

Thanks for the good words. The clamping in the circuit of post #11 is built into the IGBT and happens at 370-430V. You'd think because of the 100:1 turns ratio, the clamping would occur at 40kV, but because of the parasitic capacitance and core losses in the coil, the output voltage is lower.

What CDI kit did you use?

John

 

[jgedde]

John:

Do you have a quick calculation for a proper capacitor value?

Sage

I've done some more research and experimentation on this subject. As it turns out the cap in parallel with the switching element (transistor or breaker points)does matter. In fact, in my test setup, I get no spark with no cap (using breaker points). .01 uF gives me a small (but likely usable spark), .1uF gives me a robust spark. I couldn't resist the potential for damaging the coil, but .1 uF could give me sparks 1/4" long with a 6V supply. Beyond .1 uF I didn't see further improvement. And, at higher values of capacitance, the output voltage was sufficiently lowered to prevent the spark from occurring at all!

Bear in mind that this is but one case: using a only one coil and breaker points.

Without the cap present, the oscillation frequency on my coil was 1.67 MHz! Certainly much too high to couple well to the secondary with an iron core and not enough time to ionize the spark gap. .1 uF lowered the frequency to 38 kHz.

That said, it turns out there is much more to designing with an ignition coil than I expected. I needed to add intrawinding capacitance and leakage inductance to my PSPICE model to get it to reflect reality. The leakage inductance made the biggest effect.

So, as far as a capacitor value, I haven't yet come up with a reasonable equation to cover most possible cases.

John

 

[varonicaleon]

It's very informative for me thanks for share.

 

[jgedde]

It's very informative for me thanks for share.

Thanks. I've been trying to beg or borrow a high voltage scope probe to make life easier during experimentation so I can use a regular oscilloscope.

I have an old CRT type ignition analyzer, but it's big, bulky and has no waveform storage capability so documentation would be nearly impossible.

John

 

[Lakc]

Thanks. I've been trying to beg or borrow a high voltage scope probe to make life easier during experimentation so I can use a regular oscilloscope.

I have an old CRT type ignition analyzer, but it's big, bulky and has no waveform storage capability so documentation would be nearly impossible.

John

That kind of specialized equipment wont likely be easy to come by. I just inductively couple into the primary for the most part. Couple turns of wire wrapped around the primary lead. Will work the same for the secondary once I get that far.
I can only dream of a scope with storage capability I have an old HP scope with a finicky trigger circuit.

 

[123RWO]

Well, John, let's think about your assertion. Let's look at cars per your suggestion. How about GM cars? The GM HEI ignition is certainly well known and many millions were made over many years. As to performance, the late versions were good to 8000 RPM on a V8. Did it use a C1? No. Of course, the HEI is now obsolete as the car makers have all gone to coil-on- plug or coil-near-plug. Some of the systems put the transistor switch inside the individual coil can. Do the coils have a parallel cap? None that I have seen. My old 86 Nissan put the transistor switch on the coil's mounting bracket. There was no C1.

RWO

 

[jgedde]

Well, John, let's think about your assertion. Let's look at cars per your suggestion. How about GM cars? The GM HEI ignition is certainly well known and many millions were made over many years. As to performance, the late versions were good to 8000 RPM on a V8. Did it use a C1? No. Of course, the HEI is now obsolete as the car makers have all gone to coil-on- plug or coil-near-plug. Some of the systems put the transistor switch inside the individual coil can. Do the coils have a parallel cap? None that I have seen. My old 86 Nissan put the transistor switch on the coil's mounting bracket. There was no C1.

RWO

RWO,

I'm not so sure yet that we can or cannot draw an analogy to a full size car coil wiuth huge inductances as compared to the small 6 or 12V coils used in model engines. The fact remains that without a cap across the breaker points in my test setup, I get no spark.

I've been struggling to get a PSPICE model of my ignition coil that reflects reality. :wall:As of this morning I am much closer with the help of a fellow EE. A phenomenon known as leakage inductance dramatically changes the behavior of the coil vs an ideal model. Also, interwinding capacitance has an effect.

We have been successful in measuring these parameters with my 6V coil. ;D

I have a GM HEI coil from my Camaro (Optispark not distrubutor cap HEI). At some point I'll characterize that as well and can make a model of that. Maybe that will tell why the cap is not used.

There seems to be a lot of conflicting information about whether a cap is or isn't needed and I'm honestly questioning under what circumstances it's needed.

I'll agree HEI doesn't use a cap. On the other hand, my little coil won't sparkat all without a cap. I need to reconcile this for my sake as well as the gang here at HMEM. It would be great to come up with some rules of thumb as to allow guys to make a working ignition system on the first try. Do you know if anyone has ever drawn a schematic of what's inside there? I remember having one apart like 20 years ago and there was a hybrid circuit inside.

In any case, the good news is that I got my parts for the ignition circuit shown in post #11 and I'll be building it up over the weekend.

John

 

[dsage]

John:

A small point but back in post #33 you should edit to correct your statement that the capacitor goes across the COIL (I think that was a typo). As you know the cap goes across the POINTS (and you've said that all along previously) The cap across the points makes the cap and coil a series resonant circuit of L1(coil) and C1(cap) when the points open (or the transistor turns off). From other posts I think that's what you meant to write. Certainly a cap ACROSS the coil would suppress any oscillations not enhance them.

It would be good to clarify this for those reading in the future looking for information.

As Lakc has pointed out a scope connection to the primary side is just as effective at looking at the secondary directly due to the transformer effect. In fact I believe your big ignition system analyzer connects to the primary side. As I remember them you could see all 8 cylinders at once as the same coil supplied sparks to all 8 in turn.
I always wish I had one of those. I once saw plans to convert a TV set to an ignition analyzer.

Sage

 

[123RWO]

Here is the GM HEI circuit or at least one that will work with the original coil and reluctor: http://www.jaycar.com.au/images_uploaded/MC3334R0.PDF
Note the 350V Zener clamp used with nonselected MJ10012 and the wide voltage range.


RWO