Another Coil question

OKAY over the years Ive seen several different internal coil wiring diagrams WITH TWO DIFFERENT CONNECTIONS:

On both, of course, one end of the the high voltage winding terminates out the top high voltage output tower. BUT heres the differences.

A) Some show the other HV winding end connected internally at the coils + terminal..... .

B) Others show it connected internally at the - terminal..... ..... ....

NEXT I hope we alllll lll agree the coil will still produce a spark regardless if wired at the wrong primary polarity buttttt ttttt t the coil is more efficient and can impart more spark gap energy discharge across the plugs gap IF WIRED AT THE CORRECT POLARITY.

NEXT I hope we alllll lll also agree its easier to emit electrons from a hotter (plugs electrode tip) to a relatively colder (plugs ground strap) surface. That the reason why in a vacuum tube theres a heater under the cathode where electrons are emitted up to the cooler plate anode (hope I got that right lol been a while since Purdue)

SOOOOO O QUESTIONS FOR DISCUSSION

1) Hows about those different coil diagrams Ive seen?? That just a misprint n if not are some for Pos others for Neg ground or what alls going on anyway?????

2) Does the coil work best at the correct polarity cuz reversing its primary polarity connection causes a reversal in the current flow direction over in the HV secondary n its easier for current to flow from the hot tip to colder ground strap ORRRRR RRRRR RR does swapping the + and - primary polarity coil connections change the HV secondary current flow at all???

3) If changing the primary polarity dont affect the HV secondary current flow, then perhaps its the old right hand screw rule of electromagnetic field North/South polarity and the coils internal winding geometry n turns direction that makes it more efficient if used at the correct polarity. Any agreement or thoughts????? ????

FOOD FOR THOUGHT Ive never done it but a friend has used the lead pencil test in an open 1/8 inch space made in the spark plug wire circuit where one can observe curent flow direction from the lil carbon sparky flow direction. He informs me the spark plug circuit current flow CHANGES if you swap the coil leads. He also indicated if the coil polarity is correct the plugs firing voltage is a few thousand volts LESS then what it is when wired backwards. That confirms what Id think we alllll ll agree on its best if wired right, Im just trying to figure out all the reasons why.

Sorry if this bores others, if I had all those neat books our friend jd has Id read up on this.

FWIW I still believe after alllll l the other pages fun discussion that typical 12 volt IHC n Deere n others thirties thru fifties coils that ARE NOT the old ones like jd showed and DO NOT have the seperate internal compartment as indicated by that ring on the outside achieve their apporx 3 ohms of primary resistance (one thing that makes em for 12 volt use) because of more wire or more turns instead of any internal discrete ballast resistor. HOWEVER I dont doubt the ones jd showed, Im ONLY saying most that dont have the internal compartment or arent the older ones he showed are still NOT internally ballasted, just use more primary wire i.e. more primary resistance..... Thats my story n Ima stickin to it lol

Thanks yall God Bless n Happy New Year

John T Loves sparky chat well DUH you say lol

My thinking is that the secondary current would have to be reduced with incorrect polarity. As I see it, the resistance at the plug tip is higher when current has to jump across the spark gap from the cold ground eletrode versus jumping the gap from the hot electrode to the cold ground. But the induced voltage is the same in each case since it is a function of the primary current and the turn ratio. So therefore the secondary current would have to be lower in the case of incorrect polarity operation. The spark power (the integral of eidt over the spark interval) would be less for the incorrect polarity than for the correct polarity.
My simple minded approach is to see the spark gap as a series resistance, whose value is dependent on the direction of current flow.
I hope this makes sense to you. It didn"t to the other guy.

I think you have the right idea, but actually there's no close connection between the induced voltage at the coil secondary and the turns ratio. Why? Because the voltage at the spark plug is WHATEVER IT NEEDS TO BE.

How can this be? Well, let me give you a couple of extreme examples: Let's say you short the secondary coil output to ground. What's the induced voltage? Zero! Now let's say you hold the coil wire a quarter inch away from ground and get a nice spark. Now the induced voltage is close to 20 thousand volts. The nominal spark gap of .035 inches is a compromise that gives a nice hot spark reliably under a range of conditions.

The ENERGY discharged by the spark plug is limited to the energy stored in the coil, and it's going to be pretty much constant over a range of conditions. However, the way that energy gets discharged is going to vary. You might short out the electrodes, in which you'll get a high current that's dissipated by the resistance in the coil and plug wires. Or you can a have a wide spark plug gap, which would mean high voltage but weak current: a "cold" spark. Ideally, it's somewhere in between: plenty of current across the plug gap to generate enough heat to ignite the air/fuel mixture.

Sorry, but you can't treat plug gap as a resistance, because its behavior isn't linear. You can MEASURE that resistance during the instant of the spark, because you can capture voltage and current on an oscilloscope. But you'll find that it isn't useful information; resistance is simply a derived value from voltage and current. In this case it varies from infinity to something fairly low back to infinity in a few milliseconds. When the spark jumps the gap, it is because the voltage at the gap exceeds the breakdown voltage of the air/fuel mixture; resistance doesn't come into play at all.

One of the many useful gadgets that J. C. Whitney has given us over the years is the "spark magnifier" (I'm not sure of the name) that went between the spark plug and plug wire. These are just spark gaps that forced the coil voltage to rise higher than normal before discharging. Surprisingly, these might actually work on an engine with fouled spark plugs; you'll sometimes get the same effect when you pull a plug wire loose and all the sudden a missing cylinder starts firing because of the extra spark gap.

My two cents on John's original post:

1. I think that most guys who draw ignition system schematics have no idea how an ignition coil is wired internally. And it really doesn't matter how the coil is tapped; the polarity of the secondary is determined by the physical orientation of the two windings, not by how they're tapped.

2. I've heard that the reason spark polarity matters is because the center electrode is hotter. Makes sense to me. I seem to also recall that spark plug erosion will be greater if the polarity is backwards; that makes sense as well.

The nice think about these sparky discussions is that nobody has ever seen an electron, so you can make up all kinds of stuff!

Yall take care now n again thanks for the fun discussion.

John T

1. A coil point system on an OLD tractor.

The primary winding of the ignition coil is wound with a small number of turns and has a small resistance. Applying the battery to this coil causes a sizable DC current to flow. The secondary coil has a much larger number of turns and therefore acts as a step-up transformer. But instead of operating on AC voltages, this coil is designed to produce a large voltage spike when the current in the primary coil is interrupted. Since the induced secondary voltage is proportional to the rate of change of the magnetic field through it, opening a switch quickly in the primary circuit to drop the current to zero will generate a large voltage in the secondary coil according to Faraday's Law. The large voltage causes a spark across the gap of the sparkplug to ignite the fuel mixture. For many years, this interruption of the primary current was accomplished by mechanically opening a contact called the "points" in a synchronized sequence to send high voltage pulses through a rotary switch called the "distributer" to the sparkplugs. One of the drawbacks of this process was that the interruption of current in the primary coil generated an inductive back-voltage in that coil which tended to cause sparking across the points. The system was improved by placing a sizable capacitor across the contacts so that the voltage surge tended to charge the capacitor rather than cause destructive sparking across the contacts. Using the old name for capacitors, this particular capacitor was called the "condenser".

More modern ignition systems use a transistor switch instead of the points to interrupt the primary current.

2. A modern system where the coil is on the sparkplug.

Some modern engines have multiple ignition coils mounted directly on the sparkplugs. Instead of single voltage pulses, they may under some engine conditions produce three voltage pulses. The coil arrangement shown is on a Dodge engine.

Sometimes I think we try to put modern ignitions system in the same category as the older tractor systems believe me there is no way to compare these two systems. Walt Anyway I'm almost 70 and when I studied the ignition system back in the olden days this is what they taught me. Please its just a simple Faraday Law system of collapsing a magnetic field within a coil of wire to create a voltage. So lets not get all this fancy stuff that none of us would ever understand anyway.

But it doesn't matter. That was a long time ago and the physics of it is the same regardless.

Mark

I was just looking through a 1930s Delco ignition parts and systems catalog. One interesting item is the "three terminal" coil they list. It was used along with a polarity-changing switch. Polarity would be manual changed once in while to change the direction of metal transfer on the ignition points.

Also shown is:
1. Round type with straw-colored shell. This coil does NOT incorporate an internal resistance unit or a condensor.

2.Round type with black-shell. This coil does not incorporate the condenser but does have a resistance unit on the outside of the coil-shell.

3. D-type with Bakelite shell. Carries the resistance unit carried upon the end of the shell, but does not incorporate the condensor.

4. D-type with black shell. This type incorporates the condensor within the shell and the resistance unit towards the end of the shell.

Spent a while with s similar discussion on another board several years ago. I don't think the issue ever actually got resolved.

I took an automotive coil apart one time to see how they were actually constructured. It's actaully an autotransformer, with the input tapped part way up. Of course, the part of the autotransformer that the primary current flows through is wound with much larger wire than the part that only the secondary current flows through.

What this does is increase the spark voltage by having the back EMF of the primary add to the voltage produced by the secondary. When the points open, the bottom end of the autotransformer is connected to ground via the condenser (for purposes of this dicussion we can ignore whatever part of the secondary pulse flows back through the battery).

Note also that the induced voltage polarity is the reverse of the DC input polarity, so the negative voltage is applied to the center conductor of the spark plug and the positive voltage is applied to the engine block. For the pulse voltage, the engine block would actually be positive with respect to the battery by several hundred volts due to the back EMF from the primary winding. This results in the total voltage across the plug being the back EMF of the primary plus the induced voltage in the secondary, resulting in a hotter spark.

If you leave the "+" terminal connected to the battery with a positive ground connection, the resulting high voltage will be negative at the center conductor of the plug, resulting in a weaker spark.

I can think of two ways to fix this. I'm not sure how positive ground coils are actually wired, as I've never had a chance to take one apart & find out. The first way would be to simply wind the coil so that the primary is wound in the opposite direction of the secondary, so that the magnetic field relationship between the primary and secondary remains the same with a positive ground as a negative ground coil would have.

The second way to do it would be to connect the common point of the two windings to the points/condenser and the lower end of the primary to the battery, such that current flows through the primary in the opposite direction to compensate for the reversed polarity (this is what happens when you connect the "+" terminal to the points and the "-" terminal to the battery in a positive ground system). This connection can be thought of as being a conventional transformer with the bottom ends of the windings tied together rather than an autotransformer. In this case, the spark polarity would be correct (center electrode negative), but one would not gain the advantage of having the back EMF in the primary adding to the spark voltage.

I suspect this has a lot to do with the differing ways the connections are shown in various references. I have never been able to find a definitive answer, nor determine unequivably whether both connections are actually in use.

Keith

Lotta info there so I"ll just shotgun this.

Probably where the secondary low voltage winding is connected is a matter of convenience for the manufacturer. As stated, it makes little difference when considering the secondary.

Service manuals that I have visited indicate that the points are designed for 5 amperes of interrupt current. That sets the DC resistance of the primary circuit, however you get it, primary coil resistance, and or a current limiting resistor that may be located anywhere, including inside the coil case, which makes the coil case very hot by the way.

From what I know about the subject, the direction of current flow in the plug helps to determine the terminal life and I don"t remember the preferred direction. Hence OEM"s polarize the applied voltage accordingly.

When I was a technician serving in the military, I was instructed on the electron theory. Made great sense for troubleshooting. When I got the different education, where hole theory (what happens when an electron moves on to another location and leaves behind this hole) was the norm, I found it very helpful in "designing" circuits whereas electron theory complicated the process. Prior to that it (hole theory) was extremely confusing and I couldn"t understand why it was taught.

The capacitor/condenser in a conventional ignition system does generate an LC circuit (with the inductance of the primary of the ignition coil, including what"s reflected from the secondary) which has a natural resonance, meaning it can and will oscillate if un-iterrupted (like if someone sticks a diode in the circuit) and the field (oscillations) decays at the rate of the circuit resistance, as said, unless interrupted.

The purpose of the capacitor is not necessarily to resonate into ringing oscillations, but to control the release of the L di/dt energy [measured as 1/2 L(i squared)] from the coil, developing across the contacts as they break. The coil attempts to "keep the current flowing regardless of the circuit resistance". Without the cap. the coil will (theoretically) generate whatever voltage it takes to keep that (instantaneous) current flowing, so when the points open, circuit resistance goes to infinity, and hence per Ohm"s law, voltage across that infinite resistance (the point gap) would "theoretically" go to infinity resulting in a horrible arc and terrible contact degradation.

Well, you can"t do that and have your points last for any length of time, so you install a device that operates exactly opposite to an inductor (the cap). As the points open, with the cap wired across them, the cap starts sucking up the current (starting from zero volts since the points had it shorted out) and the voltage across it starts rising in a sinusiodal fashion (LC circuit resonance).

The cap is sized for two things: Large enough to keep the voltage height and rate-of-rise below the ignition (arc) potential of the point gap (which makes it need to be large) and secondly, to do this fast enough to be ready for the cycle to repeat for the next firing of the next plug (which makes it need to be small). So you have to select the value accordingly.

Transistor ignition systems operate significantly different. The energy to fire a plug has been determined to be a certain number of "joules" (watt-seconds, that is Voltage x Current x Time). A capacitor of a distinct size is charged to a distinct high voltage (several hundred volts)and holds the charge [energy...1/2 C(Vsquared)] usually because of a "charging diode". This charge is held until the trigger pulse to the firing switch (transistor, FET, SCR or what have you) coming off a magnet on the flywheel/distributor, or whatever moves, that went by a coil to produce the pulse.

The pulse triggers the electronic switch component which essentially shorts out the circuit and the transfer to the ign. coil primary is accomplished, usually in 1/1000th the time of conventional systems and at 2 times the value, because of the circuit parameters and component values. Obviously, the latter is much better in firing your plugs.

Nuf of that.

Mark

I do a lot of reading in older tech manuals written late 1800s up to the mid-1900s. I enjoy them because they are much more heavy on theory and backround/supporting information. They are much more thorough and much less myopic then much of what I see passed today as technical matter.

I've got schematics and diagrams of hundreds of igntion systems. Also got many performance charts. Very generally speaking, the types of coils being discussed here can vary 10-15% when used with one polarity or the other (according to the charts, not my own testing). I won't start citing the explanations given ad nauseum, and there are many.

One persistent problem - as I see it that pops up on these forums - is complaints about discussing "old" information. I take issue with that. Some old information was correct then, and is still correct at present. Not everything new is better and of more truth value. Even such things as Newtonian rules were not abrogated by Quantum Physics and newly observed behaviour of sub-atomic particles. Adding to our knowledge is not the same as erasing all the former and replacing it.

This all is starting to remind me of some past discussions of the Hole Theory versus the Electron Theory, and postive ground versus negative ground.

Oh well, I enjoy it and often learn new things here by paying attention to what other people post. This is still an antique tractor forum, is it not?

In regard to the coils and internal resistors - as I said before, I haven't been in the habit of cutting them apart - can't say I ever cared that much. But, if you notice the photo posted by "John" - it shows the Delco with an internal separate resistor. I have several diagrams showing the same for 12 volt systems used in 1950s - 1960s British and German autos, tractors, and boat engines.

Surely the inductive kick (Ldi/dt) of the primary winding can add or subtract from the voltage created in the high voltage winding from the collapse of the magnetic field of the core. But is it significant? I think that a couple hundred volts from the primary turns compared to 15,000 or so on the high voltage winding isn't significant. So I think it doesn't matter which of the three connetions are. That its more important to match the output polarity so the hot electrode emits electrons better.

The high voltage generated in the coil gets clipped by the break down of the gaps in the secondary circuit, first the one at the rotor and then the one at the plug inside the cylinder. The function of the coil is to make available enough voltage to cause a spark at the plug under all operating conditions which has to include the supply voltage, the time of the dwell (during which the primary current rises relatively slowly), the varying caps at the rotor and each plug and the compression pressure in the cylinder at the time of firing. The pressure in the cylinder will vary a great deal from throttle closed to throttle open and even with throttle open will vary with the engine speed becuase of flow limitations in the entire intake system. So the voltage the gaps clip the voltage to depend on all the engine operating conditions.

The voltage created from the primary is V = Ldi/dt, that's a differential equation that has a time varying solution and the instantaneous solution is dependent on the polarity of the current. The polarity of the magnetic field in the core depends on the direction of current through the primary, so the polarity of the voltage generated in the high voltage winding when that magnetic field collapses is dependent on the direction of the primary current.

This gets more complicated by the addition of the condenser that makes that primary current oscillate giving a series of voltage pulses of both polarities that makes that (sans condenser) wimpy yellow spark turn to a fat blue spark.

I'm thinking that the first gap breakdown limits the voltage so that the opposite polarity pulses don't spark at the plug because of that observed difference in plug firing voltage according to polarity. I'd rather check that with a scope than a pencil lead, I'd see more detail.

Gerald J.

• Ignition systems

The better transistorized systems are capacitor discharge where they charge a capacitor to some 300 volts and hook that to the coil and let that capacitor current ring the coil and create the output voltage.

Gerald J.

The capacitor, charged to the 300 (or whatever volts) is a bang waiting to happen. It stores the energy that goes to the coil to operate the plug.

This stored energy (measured in Joules....Watt-seconds)is defined by the equation 1/2 C x (V squared) where C is the value of the capacitor in farads and V is the value of the stored voltage in volts. Notice V is squared, so increasing the voltage twice gives you 4x the stored energy, hence, with a good dielectric, you can reduce the space required to do the job. That's why you charge the C to 300 volts, rather than ring it out at 12V.

The switch (transistor, FET, MOSFET, SCR) dumps the cap into the coil and the secondary voltage takes off like a rocket (because the coil is unloaded....no current flowing). This is how you get your 40,000 volts in 40 microseconds which 2 cycle outboard mfgrs. just love. Also, note the gap on the plug is now much wider, like .045 -.060 rather than .030-.035 of the old conventional systems.....course you need better insulated coils and spark plug wires now with the higher voltage.

When the voltage hits the firing point of the gap, it arcs over and clamps the secondary of the ignition coil to a very low voltage....say 50 volts and it stays at that voltage till the coil has released most of it's energy (it got from the cap) which becomes insufficient to maintain the arc across the plug gap and the plug shuts off resulting is some residual oscillations in the coil as the core eats what energy is left....not much.

And that's the way it is. Just so happens this is the principle that pulsed RADAR systems for aircraft and ships have operated on for many 10's of years.

HTH

Mark

Walt

• Another way to look at it.