Ignition circuit issue.

Just possibly the SCR is staying on, shorts the inverter, which thermal limits, and recovers later? It's awfully complex.

Back when it wasn't so lethal to ride motorcycles on the street (and I wasn't too bright) I use to make my own CD ignitions.

I liked this topology

because it was easy to drive the SCR gate with a simple RC circuit from the points. A low value resistor from gate to ground ensures turnoff. You could elect to kill the boost converter during the gate pulse, too.

To get a fat spark, you need a pretty wide SCR gate pulse, to let the whole mess ring a few cycles. I usually ran around 80 to 100 mJ per spark, which will fire a plug no matter how fouled. Barely needs gasoline.

Actually, my HV supplies were open-loop 2-transistor DC/DC converters with full bridge rectifiers. It sort of relaxed for a while after being shorted, then started oscillating again, which was nice.

I don't see any sign of that - the voltage across the coil is zero at this point, whereas the capacitor C1 has about 25V across it, dropping to zero each time the circuit is triggered, and then climbing back toe

Looks like I'm using a bit under 50mJ, which seems to be working, albeit with a plug in reasonable condition.

My application is the engine of a generator. It doesn't have points, but has some magnet/coil arrangement that produces a pulse at the correct point in the cycle.

Certainly charging through the coil simplifies things, but I had/have no idea what the inductance of the coil might be. Indeed, it turned out that my coil had insulation failure in the HT side, and I had to source a new one, so even if I'd tried to measure the old one, the new one could have been different.

Finally got the engine running today, using this ignition circuit, and LPG, after many hours spent cursing a PIC32MX that I was temporarily using to generate the trigger pulse - got caught out by yet another PIC errata - input capture doesn't work when the processor is in the idle state.

Sylvia.

I just sortakinda fell into this thread and I do not have the credentials to be jumping in, but I do have to say that that that "magnet/coil arrangement" is called a "magneto" on the backyard and shade-tree mechanic circuits I used to hand on, and it puts out a shocking pulse tall enough to fire a plug on a good day.

There are such things, but this is not that. As originally supplied it was an ordinary capacitive discharge ignition. There's a separate winding that produces the power used to energise the capacitor, and then this magnet and coil device is used to produce a pulse that triggers the discharge into the coil.

Sylvia.

Was that PIC quirk the problem? Otherwise, I'd ask o Whether C4 might be bad, open, or the wrong value?

Or, that not being the case, maybe the LT1243's upset about having its FB pin pulled all the way to +5v (wind-up?)?

Is the LT1243's Vref getting glitched from your reset circuit, upsetting the LT?

Just some wild ideas. From your description it really sounds like it's RT/CT pin-related, and you've already put your finger on it (so to speak :).

Cheers, James Arthur

or are there invisible glitches on the feedback pin, try removing C5?

Using a PIC when you should have gone analog >:-}

...Jim Thompson

Den tirsdag den 28. marts 2017 kl. 01.25.23 UTC+2 skrev Jim Thompson:

why even bother with CDI? most car do fine with regular coils, many coils even have the driver igbt buildin all you need is a 5V driver for a few ms of dwell time

But can it saw Plexiglas ?>:-} ...Jim Thompson

No, and I even replaced it. Its value is far from critical anyway.

It's within spec. Another possibility is that the LT1243 is interpreting the sudden drop in FB as an indicator of an output short, but if so, I'd expect it to object to every such occurrence, not just the first.

The IC is not really designed with this application in mind. Perhaps it just plain doesn't like being used this way.

Sylvia.

Nah! Used a boost converter chip when I should have used a PIC. A 28 pin PIC32MX seems a bit over the top for this, but what the hell? It's not even more expensive.

Don't quite see how that works - does it rely on the reverse breakdown of D3?

Sylvia.

My application is a backup "portable" generator. One useful feature of CDI in this application is that it doesn't require a large input current, meaning that I can start the generator using cheap dry cells, and then let it power itself. No need for limited life failure prone rechargeable batteries that need to be kept topped up lest they die.

Sylvia.

D3 acts normally, the output is a large negative spike.

piglet

Yes, I see it now.

Sylvia.

Yep. I devised schemes to do just that when I was at Philco-Ford in

...Jim Thompson