Why does my 2nd ignition circuit breaker keep popping?

[WesRagle]

Hi John,

I saw your post about your fuel valve placement on the tank and not going with the revision from Sonex. After seeing that I did the exact same thing.

My not going with the revision from Sonex was an accident. I have an old kit and didn't pay enough attention to print revisions. By the time I asked Kerry about the valve I had already installed the OOPS fittings and really didn't want to back up. I confirmed that my fuel flow is adequate, but not stellar, with a fuel flow test.

There are multiple advantages to taking fuel from the bottom of a Onex tank. For one, the forward face of the sump is not "slanted" like it is on the Sonex tank I am familiar with. So, in the three point stance/landing attitude the fuel valve is pointing upward making the last bit of fuel unavailable. There is also the opportunity for water to collect in the sump only to be released when the tail is lifted. I have a gascolator down stream of the sump so I convinced myself that overall the risk was manageable as long as I took a fuel sample at the gascolator prior to first flight and after fueling.

I would never advise anyone to go against the factory recommendations on something as critical as the fuel system. If you look at my post (ref. http://sonexbuilders.net/viewtopic.php?f=5&t=4903&start=30#p37662), I edited it last September and put in big red letters that the fuel system prints had been revised.

Wes

[gammaxy]

From two sources I’ve been told the 2nd ignition shouldn’t be turned on for more than 15 seconds to a minute without the engine running.

I think the official Dyna S instructions say no more than 5 minutes to avoid overheating and damaging the coil.

Interestingly, I've noticed that after ~2 seconds of no engine spinning, the triggers on my Aerovee stop providing power to the coils. So, at least as long as the engine isn't running, the secondary ignition isn't drawing any current. Not sure if this is typical of Dyna S ignitions or is a change Aeroconversions made to their similar-looking trigger modules.

[Rynoth]

Interestingly, I've noticed that after ~2 seconds of no engine spinning, the triggers on my Aerovee stop providing power to the coils. So, at least as long as the engine isn't running, the secondary ignition isn't drawing any current..

This has always been my impression as well, that the Aerovee secondary coil draw is RPM dependent, i.e. how often it's actually firing. Come to think of it, I do have an ammeter at my battery to measure charge/discharge, but I've never tried switching the alternator off at high RPM to see just how much the secondary coils draw at 3000+ rpm. It's may be in the 8 amp range (4 per coil as Jeff suggested).

[gammaxy]

This has always been my impression as well, that the Aerovee secondary coil draw is RPM dependent, i.e. how often it's actually firing.

This is not quite what I meant. I was pointing out that on the Aerovee, at least, the secondary ignition almost completely shuts down when it hasn't been triggered in a couple of seconds so you shouldn't have to worry about it overheating when the engine isn't running. This might be different than how the Dyna S works.

Your point has come up in the past and inspired me to attempt to capture exactly what's going on:


I used the waveform to estimate what the average current per coil will be at a range of typical RPMs. It decreases by about 13% across the range I considered. I feel that this is is probably barely noticeable in practice. Last time this came up, I expected a much larger change and jerry09w said when he measured it with VOM it was barely noticeable: viewtopic.php?f=6&t=3831&start=10#p33386

[WesRagle]

Hi Chris,

That's good stuff.

So the wave form is just a series of RL charging curves with the R being the 3 Ohms shown on the side of the coil (Ref. https://www.electronics-tutorials.ws/inductor/lr-circuits.html).

I suspect the Dyna S works the same way the AeroVee ignition works. I haven't studied either closely but the button on top of the Dyna S looks suspiciously like the magnet used on the AeroVee secondary ignition.

I've never really questioned how these ignition systems work but now I'm curious. These questions may sound a little geeky but inquiring minds want to know. So here goes.

1) Would it be correct to say that what we are calling a coil is actually a transformer?

2) Is the high voltage generated by the transformer secondary a result of the near instantaneous change in current (and therefor flux in the transformer core) in the primary when the ignition fires?

3) I guess what I'm trying to ask is, are these "Coils" nothing more that a flyback transformer?

Thanks,

Wes

[gammaxy]

Wes,

Yeah, you're absolutely correct and it's not just our ignition coils, but all/most ignition coils are really transformers designed to deliver energy in impulses. Wikipedia's article on "Induction Coil" says these were the first type of transformer, so the confusing name seems to go way back. And you're correct that the sharp drop of current in the primary raises the voltage across the secondary high enough to fire both spark plugs.

[WesRagle]

Chris,

Back in the day we always used the terms coil, choke, and inductor interchangeably, but we never called a transformer a coil. I suppose if you cut one open you would find two secondarys. I've always just hooked the "coils" up and never really understood how the worked. Now I do. Thanks.

Back to the problem John is having:

I recently finished the electrical system and didn’t have this problem before. I could have the master and 2nd (electronic) ignition on all day with no problems.

Without the engine running, as Jeff calculated and as you measured, we should expect 7 to 8 amps of current draw by a secondary ignition with two 3 Ohm coils. That is unless the crank happens to be positioned such that one of the coils is shut down (firing) in which case the current draw would be half that. That might explain why the problem seemed to be related to the installation of the carb heat plate. Maybe only one coil was energized but sometime during the process of installing the carb heat plate the engine was rotated enough to cause both coils to be "turned on". Maybe a little "post hoc ergo propter hoc" goin' on here :-)

Thanks,

Wes

[sonex1374]

Wes,

That's a very plausible scenario. The coil consumes battery power all the time until the triggering magnet aligns with the triggering module which interrupts the flow of power to the coil (there's a small transistor inside the triggering module that does this switching). This interruption causes the stable magnetic field that has built up inside the coil to collapse all at once, and this collapsing magnetic field generates the high voltage surge that fires the spark plug. If the engine just happens to be rotated such that the trigger magnet and module are aligned, then no power will flow thru the coil. Move the magnet a smidgen, then current flows again.

Kinda improbable that is works out like this, but once or twice in a blue moon the stars will randomly align.

Jeff

[WesRagle]

Jeff,

Kinda improbable that is works out like this, but once or twice in a blue moon the stars will randomly align.

Looking at the duty cycle of the current waveform, and considering that there are two coils that could be turned off, it looks like the probability of one coil being turned off is about 15%. At least for Chris' ignition.

Wes

[gammaxy]

Wes,

I just tested your idea and you are correct that if the magnet is stopped next to the trigger, that coil won't draw any current even when it is first powered on. This might not be all that unlikely, especially if the timing had recently been set by pointing the magnet at a trigger.