#1
IP: 50.29.99.20
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another coil thread
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 |
#2
IP: 24.152.132.185
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I'm outta here (again!) to work on my daughter's condo over the weekend so my reply will be brief. Three comments:
__________________
Neil 1977 Catalina 30 San Pedro, California prior boats 1987 Westsail 32, 1970 Catalina 22 Had my hands in a few others |
#3
IP: 108.49.205.7
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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.
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#4
IP: 128.177.72.199
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Quote:
Bill |
#5
IP: 73.55.66.163
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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. |
#6
IP: 206.205.82.3
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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.
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@(^.^)@ Ed 1977 Pearson P-323 "Dolce Vita" with rebuilt Atomic-4 Last edited by edwardc; 12-06-2014 at 10:35 PM. |
#7
IP: 24.152.132.185
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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?
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Neil 1977 Catalina 30 San Pedro, California prior boats 1987 Westsail 32, 1970 Catalina 22 Had my hands in a few others Last edited by ndutton; 12-07-2014 at 08:46 PM. |
#8
IP: 68.60.179.71
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Quote:
__________________
Mike |
#9
IP: 199.173.224.31
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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. |
#10
IP: 198.11.8.218
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Wow - two, technically three puns in one response! Well Done!!
__________________
Jeff S/V Bunny Planet 1971 Bristol 29 #169 |
#11
IP: 12.216.194.200
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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 |
#12
IP: 63.143.240.109
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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. |
#13
IP: 128.177.81.23
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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 |
#14
IP: 184.1.14.64
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Quote:
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.
__________________
-Jerry 'Lone Ranger' 1978 RANGER 30 |
#15
IP: 24.152.132.185
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Thanks guys, you're a class act.
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Neil 1977 Catalina 30 San Pedro, California prior boats 1987 Westsail 32, 1970 Catalina 22 Had my hands in a few others Last edited by ndutton; 12-11-2014 at 08:40 PM. |
#16
IP: 67.232.87.32
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Quote:
+1 Dan S/V Marian Claire |
#17
IP: 50.29.99.78
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This thread proves once again how lucky we are to have this forum.
I'm not sure what I stepped in, but it is clear to me that this topic was handled with honesty, class, and rationality. So much of discourse these days seems to degenerate into cliches and name calling. This is a discussion of ideas, research, supported by outcomes. Very cool. Skywalker |
#18
IP: 72.45.14.161
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It was really a fantastic effort. TWO product lines had random failure issue because of a lack of understanding of slow speed* engines that could spend all day at what would be 20 MPH for a car. Both are now reliable additions to the A4.
* EI systems increase dwell, which means a hotter spark and much better output at high RPMs. At low RPMs the "on time" increases enough to overheat the coils absent the ballast resistor. Last edited by joe_db; 12-12-2014 at 09:19 AM. |
#19
IP: 24.152.132.185
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Quote:
Case in point: MMI's coil is designed and manufactured to meet our resistance requirements WITHOUT the need for a supplemental resistor. It is not an off the shelf coil. It is manufactured to Don's specification. Pretty incredible if you ask me.
__________________
Neil 1977 Catalina 30 San Pedro, California prior boats 1987 Westsail 32, 1970 Catalina 22 Had my hands in a few others |
#20
IP: 161.213.49.150
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Another +1
Quote:
TRUE GRIT |
#21
IP: 24.152.132.185
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[off topic]
Thanks again guys. I'm only one of many on the MMI forum. I've said it before, I wasn't alone in the EI/coil work. Had PLENTY of help. Funny how forums are. I'm hated on another, particularly by the moderation. They demonize dissenting opinions over there. I've been treated rudely, called names and had posts arbitrarily deleted (I promise they were not offensive but merely disagreed with the mod's opinion). I even had a San Francisco law firm contact me as a result of my unwelcome posts in a thread regarding asbestos in boatbuilding. They were considering using me as an expert witness in a case. Yep, this forum is GREAT! If you want to toss about compliments, consider our host and administrator/moderator. OK, back to your regularly scheduled programming. Something about inductance, impedance and reluctance wasn't it?
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Neil 1977 Catalina 30 San Pedro, California prior boats 1987 Westsail 32, 1970 Catalina 22 Had my hands in a few others Last edited by ndutton; 12-12-2014 at 12:11 PM. |
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