red ignition coil primary resistance as a maintence item ch

[n502pd]

Ok, all, I have been thinking again, so be aware! I am putting together a fly parts kit, and want to include at least one ignition coil. You know the red ones that sit on the fire wall. Investigating on line ( I know, shouldnt believe everything found), but there are several coils available, with primary resistances of 1.5, 2.0, 2.2, 3, and 5 ohms. As haas been said, the lower resistance coils use more current, and get hotter durring use, and maybe will fail prematurely if not cooled correctly. That might be hard to do in an airplane. N502PD has small blast tubes already on coils and regulator.

Here is what I wish to find out from those of you who would measure the primary resistance of one or both of your sonex supplied ignition coils,( I do not denegrate anything on Sonex supplied coils) and either send me a PM, or list the results here, along with the total time on them. Test conditions can be either with one wire removed (preferred), or with no wire removed, and measured between the ternimals on the top of the coil, not to ground. I tested mine with and without wire removed and saw no difference in resistance. Leave master off, pri and sec ignition off.. ships battery does not have to be disconnected, that is up to you.

My reason for this request is, while testing an aerovee install with about 10 mins run time, the coil measured had 5.0 ohms, and my bird (TT 45 hrs) measured 3.8 - insturment error of .7 ohms, yielding net of 3.1 ohms. Since others have said in the past on this forum, the lower resistance coils run hotter due to increased current, and can be prone to failure at inapropriate times, like take off(??) as an example. If this trend is trackable, I am of the opinion that actual failure may well be adverted if the reduction in primary resistance can be tracked over time. The no-go resistance has not been determined yet, but data from 'all youall' should help me decide if I need to add this to the condition inspection list of thing to do.

Thanks for any input that can help with this process!

Fly safe, greasy side down, at least when landing!!

[sonex1374]

Joe,

I see what you're getting at, but I think this is going to be difficult in practice to map out. Getting enough data on the coils is going to be slow going. As an additional consideration, I've had coils go bad that bench tested OK with a meter. There was no doubt is was acting up in flight, but after being removed it gave all the signs that it was fine. My theory was that heat and vibration encountered in flight caused intermittent failure, and that couldn't be detected on the bench.

What would be interesting would be to monitor the temperature of the coils in flight and see just how warm they get under different conditions. I'd like to understand how hot is too hot, but there's no data on that currently.

Jeff

[n502pd]

yes I need the temp data too, and there are temp strips that can be put on heat sinks in the electronic world to see just how hot thet are. Let me look into finding some.
Part of what I should have asked for is weather there are cold air tubes blowing on the upper firewall on any installs. i do have cold air blowing on the mags, coils and regulator and so far have not seen anything alarming. You said you have had a coil checkout on the bench fine. Did you think to check the secondary winding for low resistance?
and depending if the primary is wound as a true pri/sec transformer, the hv side should not be shorted to primary, otherwise its bad if it is. however, if the coil is wound as an auto transformer, both the primary and secondary are connected resistance wise. and from what I have seen, primary to hv terminal could have something like 10k to 15k ohms, which isnt necessairly bad for an auto transformer.

I just need some info on what others are seeing, so as to not just reorder a coil from online suppliers, or from sonex until I can decide if I indeed, may be barking at the moon, if you know what I mean! thanks much for the reply!

[lakespookie]

Interesting topic I never considered the thermal load although I will be going with the 5 ohm coils do to power considerations based on the panel and equipment I want to run on my build. All that being said it would be interesting to investage diffrences in the internal coil design as i would assume some care would be taken to the termal dissipation and the higher ohm coils may have a diffrent design internally although i doubt it due to manufacturing/tooling costs.

[n502pd]

Most of the different resistances for the coils on the interweb are for motorcycles, and as such are pretty much is cool air, and probably can get away with being lower resistance, which gives more and quicker raise of the energy spark to the plugs. In our application, we...I... am interested in not having a failure at unexpected times. If I can conjur up a relationship between resistance, time in service, with or without cooling air, and outright fai;ure, then maybe something could be said about when to replace the coils prior to having to be hauled out of a field somewhere!!

[Onex107]

The left ignition coil and the voltage regulator were replaced within 3 hours of each other at 280 hours. At that time there was no outside air introduced to the firewall area. The only air to that area comes from the cylinders. I attributed the failures to heat. The sockets in the coil had green corrosion in the bottoms. I replaced the coil with the Sonex coil ACV-A01-25. I don't know how to test it but I still have the failed coil and regulator.
About this time I was experiencing surging at idle after flying. I attributed this to a mild case of the burps and added a blast tube to the front of the Aeroinjector. My fuel system and exhaust are fully insulated and wrapped. Problem stopped. That may have increased the coil and regulator life also. Keep tuned.

[Jerry09w]

These electronic ignitions draw a lot of current and get hot quickly if turned on with the engine not running. They should be turned on immediately before hitting the starter button, or as in the Jabiru electronic ingition upgrade instructions just after hitting the starter and the engine is turning. To keep my voltage regulator cool, I moved it to a bracket mounted on the airrcraft skin to the left of the pilots knee which is cooled by the slipstream.

[mike.smith]

480 hrs on the original coils. Mounted 1/3 of the way down from the top of firewall on passenger side. No blast tubes or outside cooling. No corrosion. No failures. I routinely turn off my secondary ignition (coils) during ground operations, and once in cruise climb. So I generally have them "on" only for startup, takeoff and landing. At higher RPMs the engine runs better and charges better without them. During ground ops, at low RPMs, it's the only way to keep the battery from beginning to discharge.

[bvolcko38]

My red secondary coils draw 10 amps. 5 amps each I would imagine. Here's the thing though, when the engine is not turning there is no current draw through the coils.

[Rynoth]

These electronic ignitions draw a lot of current and get hot quickly if turned on with the engine not running...

Here's the thing though, when the engine is not turning there is no current draw through the coils.

While these 2 statements appear contradictory, they both can be true. The culprit is the trigger magnet. If the crankshaft/prop is at the particular angle that the trigger magnet is being picked up, the coils may try to constantly fire if switched on (with the engine not rotating.) Otherwise, with the trigger magnet/crankshaft in any other position, the coils will charge and then not fire, halting the current draw. This is my understanding anyway.

Under "typical" conditions I would expect that when the engine is shut down the prop/crank would come to a rest between compression strokes and away from trigger magnet ignition points (which would be approaching the top of a compression stroke.) But a number of factors (such as hand-turning the prop, shutdown position ending up at the trigger point, tight crankshaft movement) could result in constant activation of the coils.