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Need single pulse from ignition coil

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Pinky's Brain

Sun Jan 22 2012, 02:02AM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
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BTW, since this is for initiating a reaction you should realise that the pulse from an interrupted traditional ignition coil does not have an easily controlled waveform ... it will have quite a long tail. Or in other words, you will dump thermal energy into your load.

Patrick

Sun Jan 22 2012, 07:33AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Pinky's Brain wrote ...

BTW, since this is for initiating a reaction you should realise that the pulse from an interrupted traditional ignition coil does not have an easily controlled waveform ... it will have quite a long tail. Or in other words, you will dump thermal energy into your load.

yes, i agree. Does any one even know what a Iggy HV pulse looks like? Are their any oscillograms on the net? if it isnt to critical than maybe it doesnt matter.

EDIT:

and

Sulaiman

Sun Jan 22 2012, 08:29AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

To add to the confusion;

Method 1
Conventional flyback mode, low voltage (12v to 24V) high current primary supply
Store energy in the primary inductance and release via the secondary
Output voltage determined by load and switch rating. (Vout<=Vdsmax x(Ns/Np)

Method 2
Capacitor Discharge mode, high voltage (150V to300V) low current primary supply
Store energy in a primary capacitor and dump it into the load via transformer action
Output voltage determined by transformer action (Vout=Vpri x (Ns/Np)

Method 3
Pulse Transformer mode, high voltage (150V to 300V) supply
(e.g. rectified ac mains + capacitor OR just mains rectified, fired when Vac=peak)
Briefly (<1ms) switch the primary across the supply, avoiding core saturation
Output voltage determined by transformer action.

Due to the relatively poor pri:sec coupling of the cylindrical can type of ignition transformers there will be 'ringing' in all of the above methods.
If you can use the ignition coils that look like normal E I transformers the coupling is better hence less 'ringing'

Patrick

Sun Jan 22 2012, 09:23AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Sulaiman wrote ...

If you can use the ignition coils that look like normal E I transformers the coupling is better hence less 'ringing'

Yes, i agree. This is the "HEI" type, they put out more power, at higher V, with a higher repitition rate than the older oil-can type. supposedly they can out put 40 kV sparks. ive been able to get sparks from cap discharges across 2.7 inches, with sharp points.

I think i used 33uF @ 300V, but i cant remember 14 years ago so well.

Fulmen

Sun Jan 22 2012, 09:58AM

Registered Member #3883 Joined: Fri May 13 2011, 06:30PM
Location: Norway
Posts: 87

Ron: We're not talking seconds but a few milliseconds, it's the time needed for the current to build up in the primary (determined by L/R). Your monostable trigger will work just fine for this, the trigger pulse will switch the FET on, allowing current to flow through the primary and then interrupt it when it switches the FET off again. You can even regulate the output simply by adjusting the on-time.

Proud Mary

Sun Jan 22 2012, 10:29AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

To add to the diversity mentioned by Sulaiman above, old-fashioned ignition coils also came in two basic varieties - the 'ballasted' and the 'unballasted.'

The ballasted type is designed to be used in conjunction with a switchable external resistor in series with the primary. At engine start up, the resistor is switched out of the primary circuit, so applying the full 12V battery voltage to what is in effect a 6V coil. The greater voltage then available at the sparking plugs assisted in starting on cold, damp mornings, and flattish batteries - a typical north European scenario. Once the engine was running, the ballast resistor would be switched into the primary circuit, reducing thermal stress in the coil, and slowing contact and sparking plug attrition.

The unballasted ignition coil was designed to operate at 12V, and does not enjoy the start-up boost of the ballasted species.

And as Fulmen has pointed out above, there is no getting away from the primary L/R time constant of ignition coils, which was designed to optimise performance at the commutation rates derived from engine RPM.

I have on my desk, as a paperweight, a shiny ballasted ignition coil whose primary has an inductance of 684Î¼H and a resistance of 1.4Î© which by T = L/R has a time constant of ~500Î¼S. If we say that 4T is the optimum time to charge up the coil, we can see that the coil will perform best with a primary input of 500Hz.

As RPM rises, the time available in each cycle for charging the ignition coil falls, a problem that is partly overcome by having the secondary resonant (in the example I've given) at 1000Hz i.e. as primary pulse energy falls with increasing commutation rate, the secondary becomes more efficient by way of compensation.

And here is the very coil I used in my example:

Sulaiman

Sun Jan 22 2012, 01:51PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

Wow, I just realised that it's almost 40 years since I worked for Lucas designing electronic ignition systems! (analog and uP/8048)
How time flies.
Anyway, I blame all my mis-rememberings on the passing of time causing decrepitation of grey matter, and pharmaceuticals.

Proud Mary

Sun Jan 22 2012, 02:10PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Sulaiman wrote ...

Wow, I just realised that it's almost 40 years since I worked for Lucas designing electronic ignition systems! (analog and uP/8048)
How time flies.
Anyway, I blame all my mis-rememberings on the passing of time causing decrepitation of grey matter, and pharmaceuticals.

I'm sure you've forgotten more than I ever knew about the subject!

I might have added to my post above that I measured secondary resonance of the coil illustrated and found it lay between 950 - 1050 Hz on account of the high ohmic resistance - 7.5kÎ© - and low Q of the winding. Low Q is an advantage here, so that secondary voltage doesn't suddenly peak at a particular engine RPM.

Patrick

Mon Jan 23 2012, 05:22AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Proud Mary wrote ...

It isn't possible for an ignition coil to output a single pulse - what you will get from a single input pulse will be a train of damped waves from the output, as we are talking L, C and R here.

You could sharpen this up a bit by placing a spark gap in series with the load if you wanted to.

Im going to quote my own work, as i think its relevant. (cursor A1 indicates 5.08kV)

Patrick wrote ...

Turkey9 wrote ...

For the values that he has, it will be pretty linear up to 10 Mhz according to an LTspice sim I just ran.

But that brings up the question, why do you want the capacitors in there? With a purely resistive divider, there are no frequency concerns. Is it because there will be capacitance there anyway since it is a real life circuit and you want to be able to control it?

EDIT: i think ive found the resonant freq of my best HEI iggy coil.... I think its 3-5kHz...

(EDIT: from tough to trough is 178uS)
I only tapped a 9V battery to the HEI iggy and captured this waveform...
5.08 Kv peak, 800 volt ring below ground. 254 uS for the main pulse (i think) 1/0.000254 = 3937 Hz, i think thats close to its resonant freq.

EDIT: i wonder if those first six concave downs could be integrated to find the arrea under the main 5kV pulse?

If there are 6 cylinders, at 3000 rpm thats (3000/60) x 6 = 300 sparks per second.
1/300 sec divided by 254uS would yield about 13. So the dead space would be 12 time periods, for each 254uS spark.

Proud Mary

Mon Jan 23 2012, 02:09PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Patrick wrote ...

Im going to quote my own work, as i think its relevant. (cursor A1 indicates 5.08kV)

You don't specify the secondary load, which, being in series with the secondary, is likely to change things a bit.

Were you to connect a sparking plug in a great big engine block to your coil with the usual resistive suppressor cable, don't you think new complexities would appear in your osciilogram?

Consider what is likely to happen at the spark gap: (1) a streamer phase (2) a spark phase (3) an arc phase (4) an extinction phase

Electrons will be surgingg one way down the cable, and then rushing back the other way, until you get motion sickness just by trying to imagine it all.

"Thine circuit is not of this world," as Jesus used to say.

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