Coil Tests

[Tim]

Don recently sent me several coils to test on a rebuilt A4 I have in my garage. The test was to measure voltage at the coil + terminal and current through the coil primary at various engine RPM. I first measured the resistance of each coil and then measured the voltage and current at 0, 700, 1500 and 2000 RPM. The 0 RPM measurement is with the ignition switch on and the engine off. I have the Pertronix Ignitor installed. I tested 4 coils total, three that Don sent me and one that I had purchased about ten years ago, an Ametek coil with part number CAS-4012-1. The results are attached in a PDF. The voltage was measured at the coil positive terminal and the current meter was connected between the coil negative terminal and the black wire to the EI module.

Some observations from the tests:

1) The current is highest when the engine is off and the ignition switch is on (0 RPM). In this situation, the current through the coil primary can be calculated by dividing the coil + voltage by the coil internal resistance, I=V/R
2) The current decreased in the primary circuit when the engine was started and continued to decrease as RPM increased. This is due to the expanding and collapsing magnetic field generated in the coil as the EI/distributor switches the current on and off. The faster the current is switched on and off, the more the magnetic field opposes the current flow.

[joe_db]

THANK YOU - VERY COOL
This does illustrate why the A4 is hard on coils - we spend a lot of time at low RPMs and low RPM = high current flow.

[hanleyclifford]

If I operated under any of those 4 scenarios on a trip south my guess is I wouldn't get a coil past NYC.

[Tim]

Hanley,

Your own instrument panel shows that the current through your coil is similar to my test results. Why would you think that the coil would fail? The coils I tested all came from MMI and Don has stated that he has never heard of one of his coils failing.

[hanleyclifford]

Hanley,

Your own instrument panel shows that the current through your coil is similar to my test results. Why would you think that the coil would fail? The coils I tested all came from MMI and Don has stated that he has never heard of one of his coils failing.

Yes, current is similar but voltages and resistance are very different.

[Tim]

From Don:

"OK Tim, I’m finally back from my travels.



Thanks again for your great test! A couple points of interest;



1) Although the Blue Standard coil started out with lower static resistance, by the time you took it to 1500 to 2000 RPM, it had almost caught up to the other coils in terms of decreasing current flow. Doesn’t this mean that it builds inductive reactance at a different rate from the other coils? The implication being that you can’t judge a coil’s dynamic performance based on its static primary resistance alone.


2) Your test (pretty much duplicating in-service conditions), confirms that either of the Standard UC-15 coils or the NAPA IC-14 keep current flow well below the Pertronix 4 amp restriction.


3) I know of no current flow restrictions imposed by the coil manufacturers, so I see no reason to suggest that the current flow values shown in your test should create excessive heat build-up. For folks operating in very hot climates, a more direct way to minimize coil heat would be to increase the ventilation in the engine compartment or perhaps even move the coil away from the engine.



Tim, if you have the time, I really think your test results and your conclusions would be very useful and welcomed on the Forum. I’m not aware of any credible discussion regarding the profound effects of inductive reactance and I think these results would go a long way to reduce all the concern over the need for additional external resistance. I’d consider not reporting on the Amtek coil, simply because I don’t think they’re available anywhere and their unusually high primary resistance might create some misunderstanding. I googled the name and couldn’t find any connection to ignition coils.



Thanks again for your great report. I’ll find lots of use for the information in providing tech support for customers who call in with questions; many times having been confused by Forum information on the subject.



Don"

[ndutton]

Tim, I propose the amperage is the same at different RPMs when the circuit is closed via the EI. I further propose the measurement differences are a result of meter interpolation between on and off and is a function of time within the meter. The time function accounts for the measurement variance.

Great testing. Is anyone suggesting a strategic change in EI application or is this for deeper understanding?

[Tim]

The current is definitely decreasing as the engine RPM increases. The tests show that the highest current in the circuit is when the ignition switch is on and the engine is off. It is this situation that puts the coil at the greatest risk. None of the tested coils went over the Pertronix 4 amp limit.

[hanleyclifford]

Anything under 14 volts, unresisted, at coil+ is not realistic in this time of big alternators and extensive electrical systems.

[hanleyclifford]

The current is definitely decreasing as the engine RPM increases. The tests show that the highest current in the circuit is when the ignition switch is on and the engine is off. It is this situation that puts the coil at the greatest risk. None of the tested coils went over the Pertronix 4 amp limit.

It is precisely the Pertronix "4 amp mantra" that needs to be ditched to clarify thinking.

[ndutton]

Well, I'll disagree about the current changing despite the measurement data. As I said in my previous post, the effect of meter interpolation is not being considered. Because you're reading lower numbers on your meter you're concluding the current is changing. I'm suggesting the meter is displaying an interpolated value trying to deal with on and off inputs.

When the EI is closed the current flows at full value, when the EI is open there is zero current flow. Current is not a function of time (although coil saturation is).

I'll not belabor the point, had my say. You can proceed however you choose.

[Tim]

From: "Don Moyer"
Sent: 9/8/2015 7:22:18 AM
To: "'Tim'"
Subject: RE: AC/DC

Looks like our emails are crossing in cyberspace! I think I now have a complete understanding of AC/DC as it relates to our coil project.



I don’t think I mentioned this to you before, but when I talked to Pertronix last Friday, they reminded me that until an engine is started, the alternator voltage isn’t even present, so our use of 14.2 volts to calculate amp flow through the module are flawed since prior to starting, voltage would only be battery voltage. Moreover, as you are now demonstrating, as soon as the engine starts, inductive reactance enters the mix and dynamic measurements of resistance (impedance) become quite different (higher than original static resistance measurements) so concerns of high current flow through the Ignitor module and coil are greatly reduced.



Don

[joe_db]

Just FYI: The ignitor module burning out wasn't the issue, the coils burning out was the issue. As a data point, my Standard UC-15 coil with no resistor at about 13.9-14.1 volts has been good for years now. I estimate it has at least 500-800 hours on it.

[Don Moyer]

I wrote Tim privately in an effort to better understand the concept of inductive reactance. Prior to his postings, my understanding was that we were having coil failures due to exceeding a current flow of 4 amps through the primary circuit (a restriction originally posed by Pertronix) unless additional ballast resistance was added to bring total static primary resistance to around 4 ohms.

It was for this reason I asked Tim to test several coils that were in common use on the Atomic 4 (without external resistance) to see if his measured current flow through the primary system ever exceeded 4 amps at any point within the normal operating range of the Atomic 4. I asked him to start with the ignition switch "ON" and the engine not running, to the recommended cruising power setting by Universal of 2000 RPM.

Tim's test data is what it is and everyone is free to react to it (or not). That's the nature of public forums. I have no professional credentials to bring to the table, but intuitively, Tim's test makes a good case for me that inductive reactance is real and is a credible component of the actual total current flowing through our primary ignition circuits. Don

[ddomino]

Coil test data clarification

As a Tartan 30 owner who is electrically challenged (the owner, not the boat!) I need a clarification on the test data set provided by Tim. In his note he states that "...the current through the coil primary can be calculated by dividing the coil + voltage by the coil internal resistance, I=V/R". I assumed that the amp values in the table were calculated using this formula, with the resistance value taken from the column "Coil", and the + terminal voltage from the columns related to the various engine RPM values. The amp values in each cell are much lower than a value computed using the formula, e.g for the Black Standard at 1500 rpm, I=V/R> I=13.8/3.7> I=3.73 amps. I am clearly missing something in the discussion, so could someone clarify?

Thanks,

dd

Later edit:

OK, nevermind on this one. RTFQ, I see in Tim's post (finally) that the computed value only applies to the zero RPM case, since no current is being produced yet. The other values come from direct current measurement (not computed) between coil negative and the EI module