another coil thread

[Skywalker]

This has been an interesting couple of days. My interest in the prop has lead me to have a brief discussion with Tom at Indigo. Nice guy, willing to share knowledge and experience. I've learned a lot.

Electronics is my weakest area of knowledge, but obviously, many of you have keen interest in the topic, so here's a question that came from my discussion with Tom.

Why is it a common belief on the Forum that just because a coil reads 4.0 Ohms across the brass studs, 3 Amps of current will flow through that coil when 12 VDC is applied across the stud?

The real answer is that Ohm Law, V=IR, only applies to a pure resistor. A
coil does indeed have resistance but it also has inductance, reluctance, and
impedance which makes Ohms Law not applicable. A very simple means of
proving this is to install a ballast resistor in an ignition system and then
measure the voltage drop across the resistor. You can then use Ohms Law to
determine the amperage. If you perform such a test, you will find that the
current through a coil with about 4 Ohms resistance and a 1 Ohm ballast
resistor with 13.3 VDC applied is only about 1.31 amps. Without the
resistor, the current is about 1.55 amps.

Ok, do what you will!

Chris
Skywalker

[ndutton]

I'm outta here (again!) to work on my daughter's condo over the weekend so my reply will be brief. Three comments:

1. Inductance, et al applies to the behavior of the coil secondary. Our studies involved only the primary side.
2. Theories aside, our real world experience applying Ohm's Law to the primary circuit to control coil heat has thus far been 100% successful - and predictable.
3. Next time you talk with Tom, ask what formula he used in the development of his primary resistor product. In other words, how did he come up with the idea of a primary resistor and how did he determine a one size fits all 1.5Ω resistor was the proper value?

[hanleyclifford]

There is such a variety of coils out there it is impossible to have a "one size fits all" solution. Neil and I differ on how to achieve a satisfactory voltage at coil+ but I think we agree that it must be done to protect the coil. The rubber meets the road at coil+: I have a voltmeter dedicated to continuous monitoring at that point. I routinely grasp the coil after long running to make sure it is not overheating.

[Administrator]

Three comments:
Inductance, et al applies to the behavior of the coil secondary. Our studies involved only the primary side.
Theories aside, our real world experience applying Ohm's Law to the primary circuit to control coil heat has thus far been 100% successful - and predictable.
Next time you talk with Tom, ask what formula he used in the development of his primary resistor product. In other words, how did he come up with the idea of a primary resistor and how did he determine a one size fits all 1.5Ω resistor was the proper value?

Yep.

Bill

[romantic comedy]

I agree with Neil and Bill.

The primary side of the coil is just many turns of wire.

The linear relationship of E=IR applies. There maybe some changes in resistance and thus current, due to changes in temperature.

[edwardc]

Wait,wait, you're all right.

The impedance of the coil (sort of tha AC equivilent of resistance) is a function of both the inductance of the coil and the frequency of the non-dc portion of the voltage. When the engine is running, the points (or EI) are putting a square wave voltage onto the coil primary, not a simple DC voltage. Impedance is a complex topic (math pun intended), and we wont go into it here other than to say that the AC current thru the coil is a function of it's impedance. The faster the engine turns, the higher the frequency of the points colsing, and the higher the impedance, resulting in lower current and total energy in the coil. Non-EE mechanics intuitively know this because the shorter dwell time with the points closed at higher RPMs doesn't give the magnetic field as much time to build, so there's less energy stored to make the spark.

When the engine is stopped, however, the points remain closed (or open a smaller percent of the time). If they're open, and you turn on the ignition, no current flows and the coil's in no danger. IF, as is more likely, they're closed, pure DC voltage is applied to the coil, and the only thing limiting the current through the coil is the resistance.

[ndutton]

My position is unless there are some new ignition system failure reports showing a pattern, we've done enough analysis. If anybody else wants to dig into secondary inductance, impedance and reluctance (I'm feeling a twinge of reluctance myself ), to what end or goal I have no idea, go right ahead.

I remembered my very first post on this forum. It was on this same subject. After lurking for a while I saw a pattern of EI conversions followed by engine shut downs, then resuscitated by replacing the old points. I asked if EI is so great, why all the failures and why keep the points plate at the ready?

[marthur]

Impedance is a complex topic (math pun intended)

i imagine that Ed is on to an entirely different phase of humor here : )

[joe_db]

For those who love the engineering details:
The ignition circuit is far more complex than DC current flowing through fixed resistance. It takes a fair skill at pretty complex measurements and formulas to accurately model an ignition system. It is possible that two coils with identical DC resistance will have different impedances and different current flows when used in a running engine. The circuilts invovled are very similar to tesla coils or spark gap radio transmitters if you want to do some research.

For those who are tired of getting stuck with burned out coils:
AC, DC, or RF - in all cases adding a resistor reduces total system current and coil heat. I had an Indigo system that had a huge appetite for coils. Now the ballast resistors have become a known fix the coil issue has been fixed. Thomas Edison style science - just try it and see what happens - has shown us that the resistor is the solution.
If anyone really wants to go for the ultimate solution, there are ballast resistors available that change resistance as they get hotter.

[BunnyPlanet169]

i imagine that Ed is on to an entirely different phase of humor here : )

Wow - two, technically three puns in one response! Well Done!!

[smosher]

Here's my cut .

engine rpm = 3600
dist rpm 1/2 of engine 1800

number of cyclinders 4
dwell angle, 40

freq = 3600/2/4/40 = 11.25

reactance coil = 2pifl = 2*3.14*11.25*.008 = .56

I found some coils listed as 8 mh, so I used that

impedance coil = sq rt R^2 + reactance^2 = 4 ohm coil *4 +.56*.56 = 4.03

Total impedance = ballast resistor 1 ohm + coil impedance 4.03 = 5.03

you can substitute Impedance for resistance in ohms law in calc amps.

Not sure how you came up with your readings, what kind of meter and did you
have it set to ac or dc. As your readings do not compute.

Steve

[Skywalker]

I've been trying to catch up with things, so I'm a little late in putting up this clarification from Tom. Here it goes.

First I must correct a typo I made in the previous comments posted by
SKYWALKER. The ballast resistor specified should be 1.5Ω. Additionally,
I performed some additional bench testing today to update some specific
data points before I shared the information with the world wide web.

The idea of a primary resistor came about as a result of several Indigo
coil failures even though the coil resistance as measured across the
primary studs was on the order of 4.0 Ω. The coils were running very hot
in service, about 200F, and it was realized that this was too hot and lead
to damaging thermal expansion within the coil. Various threads on the MMI
Forum confirmed my suspicions and convinced me that I should look into a
Primary Ballast Resistor as a means of promoting reliability and extending
coil life for the Indigo system.

I basically conducted considerable bench testing with an operational
Indigo EI and coil and various high wattage resistors in an effort to
reduce the current through the coil (thus reducing its operating
temperature) yet keep the voltage level at the coil sufficiently high to
keep the A4 running properly. It has never been my intent to develop or
promote my solution for the Indigo system as a "one size fits all"
solution. I did, however, through my test program, find that a 1.5Ω
ballast resistor will reduce the operating temperature of the Indigo coil
from about 200F down to about 165F while keeping the voltage to the coil
above 10 Volts. At a battery system voltage of about 12V, it was found
that the current through the coil and ballast resistor (voltage drop across
it of 2.2V) was 1.46A and the voltage across the coil was right at 10V. At
the high end of normal battery system voltage, 13.8V, it was found that the
current through the coil and ballast resistor (voltage drop across it of
2.5V) was 1.66A and the voltage across the coil was right at 11.4 V. For
the purpose of determining the current in the above data points, Ohm's Law,
V=IR, can be used relative to the voltage drop across the ballast resistor.
For example, 2.5V = I x 1.5Ω, therefore I = 1.66A.

I was satisfied that the 1.5Ω ballast resistor was and is a proper
solution for most "normal" A4 electrical systems with the Indigo EI and
coil. For those who may choose to run their system voltage at a level
greater than 13.8V or those who want to "dial in" a specific voltage
across their coil, then a 1.5Ω ballast resistor may not be the right
choice. Likewise, any coil other than the coil supplied by Indigo, which
by the way is a product of Andover Coils here is in the USA, may or may
not respond properly to a 1.5Ω ballast resistor.

Probably the easiest approach for an unknown coil would be to install a
ballast resistor of known value, measure the voltage drop across it with
the A4 running, and then determine the voltage across the coil [(System
Voltage) - (Voltage drop across ballast resistor)]. If the Voltage across
the coil comes out in the 10.5V to 11.8V range, you should be good to go
from a coil heating standpoint and yet have a sufficient voltage for
sparkplug firing. Note it is important that the ballast resistor be rated
about 50 Watts continuous to insure that it will hold up under the stress
of the application.

I hope this somewhat clarifies the mystic of coils and ballast resistors a
little.

[Administrator]

Success has many fathers, as they say.

Forum members will remember Neil's original work on the subject, even if not mentioned in the comments quoted in your post.

Bill

[roadnsky]

...Forum members will remember Neil's original work on the subject, even if not mentioned in the comments quoted in your post.

Without giving away too much private info...

I can say that not many on the forum know how much work and organization Neil put into helping us figure out the EI/Coil snafu.
This was an ongoing issue with our A4 community for YEARS until he figured it out.
It is now a NON-issue.
Obviously he has contributed in many, many other areas for our A4's.
Thank you Neil.


'nuff said.

[ndutton]

Thanks guys, you're a class act.