Why does my 2nd ignition circuit breaker keep popping?

[Bacon8tor]

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. Now, when I turn on the master then the 2nd ignition switch the 5 amp breaker pops after about 30 seconds. I re-checked the wiring and everything seems okay. I’m using the Dyna S Ignition. I’m running 16 gauge wire from one of the coils to the switch, then breaker, then bus. I’m assuming the ignition is grounded to the engine block.

I’m wondering if this has anything to do with an electric carb heat plate I installed on my Zenith carburetor. These have been successfully installed on Bing Carburetors to prevent carb ice. It wasn’t until after a prolonged test on that system (15 minutes) that I started having problems with the 2nd ignition. I’m wondering if there’s a connection here. Even after letting the carb cool down the 2nd ignition breaker still pops. I checked the wiring and neither system touches the other. The carb heat plate warms quite nicely. I hope it’s not adversely affecting other components.

[Area 51%]

According to my Aerovee assembly manual, there's supposed to be a 10amp fuse/breaker in the system.

[Rynoth]

Mine has a 10A fuse and 18AWG wiring. If I recall, 18AWG is probably the minimum wire size for the secondary ignition on an Aerovee, 16AWG would be just fine. It draws right around 5 amps continuously.

Remember, the fuse/breaker's main purpose is to protect the WIRE, not the equipment.

[bvolcko38]

My secondary ignition draws 10 amps

[Bacon8tor]

I think I found the answer to the problem. 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. This can cause overheating in the wires. I’m not sure which “wires” those are (coil?) but damage is most likely done to the ignition “module” which I think is the round thing on the top of the engine (I have the Dyna S Ignition on a Hummel). That would be an easy fix but if it were the coils that would most likely require engine removal for access. Ugh!

I should have specified that I have the Dyna S Ignition and not the Magnetrons like on the Aerovee. The 5 amp fuse is the correct size. I don’t know what the Aerovee says about turning on the ignition without the engine running but I wish the Dyna S manual would’ve cautioned against it. Maybe they assumed no normal person would do it in the first place. I flipped it on for the electrical test just to see if I had it wired correctly (no smoke or popped breakers). When that didn’t happen I saw no reason to turn it off immediately. I just didn’t really think about it one way or another. I was too busy sitting in the cockpit making airplane noises with various switches flipped here and there. But, lesson learned! Please don’t make this same mistake.

[WesRagle]

Hi John,

The 5 amp fuse is the correct size.

Just an FYI, I emailed Scott and asked him what size fuse to use for the secondary ignition. He told me to use a 10 amp fuse.

Wes

[Bacon8tor]

Thank you, Wes. I know of other builders who have never popped a 5 amp breaker on this system. Can’t remember where I saw where 5 was the correct size but if 10 is better my order is in!

[sonex1374]

A typical 3 ohm coil supplied with 12 volts will draw approx 4 amps (Ohms Law: V=I*R, so I=12/3 = 4 amps), and in doing so will generate 48 Watts of heat (P=I*V: P=4*12 = 48 watts). This is a fair bit heat being generated (think of grabbing a lit 60 watt incandescent light bulb). With no cooling air flowing over them the coils could easily overheat. I think a best practice with these ignition systems is to turn them off when they are not needed, such as taxiing or when the engine is not running. Some pilots even turn them off for periods in flight to save the current draw and help recharge the battery.

In any case, I think a 5 amp fuse for a single coil or 10 amp for two coils would be appropriate.

Jeff

[WesRagle]

Hi John,

I think a best practice with these ignition systems is to turn them off when they are not needed, such as taxiing or when the engine is not running.

That's exactly what I intend to do. Also keep in mind that with the Hummel the primary ignition is an impulse mag. To be as kind as possible to the starter you can/should start the engine on primary ignition alone. Just do a quick check of the secondary before taxi and then re-engage the secondary in the run-up area while doing the "formal" run-up.

I'm going to go ahead and say this too while I'm thinking about it. Since the Hummel primary ignition *is* an impulse mag, and the Zenith carb has no mixture cut off, we need to be especially mindful while handling the prop. A single point of failure, like a broken P-lead, combined with that distinctive clunk of the mag, could lead to a very bad day. As an added precaution, on the final flight of the day, I intend to kill the engine by turning the fuel valve off and letting the fuel in the float bowl burn before turning the ignition off.

Wes

[Bacon8tor]

Thanks Wes. So many people here have given good advice. This is my first build and it really has been a learning process.

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. I ordered the smaller valve based on the part number you gave and also installed it on the forward side of the tank “drop down.” Let me know how it goes shutting off the valve to kill the engine. I prefer not to put stress on the valve fitting but I’m not comfortable with leaving fuel in the carb either.

Regarding turning off the second ignition during various phases of flight I’m concerned about possibly fouling the plugs?