Placement of Ballast Resistor

The ballast resistor should always be connected in the primary circuit of the coil.

To put the ballast resistor in the secondary circuit would affect the time constant charging of the condenser (capacitor), which is essentially in parallel with the breaker points, and is the only path for current from the coil to ground when the points open. Changing the time constant will change the engineered relationship between the coil and the condenser, probably resulting in an under-performing spark.

The ignition coil IS an autotransformer, meaning the primary and secondary share some of the same windings and no part of the internal windings should be grounded to the coil shell or the coil itself would be grounded and thus useless. The high voltage spark comes from a condition in the autotransformer called inductive kickback when the points open.

My H farmall is converted to 12v and has a 6v coil(original IH) and has a ceramic wire wond resister that cuts the voltage to 8!
It also has the IH 6v system condenser.

I read all the posts and am still confused! Sorry

I'm not familiar with the so-called "infamous" Ford *N ballast resistor, but I guess it could have a high positive temperature-variable resistance, which has low resistance when cold, then a higher resistance when heated by coil current, thus applying higher voltage to the coil when starting cold, then lowering coil voltage and current back to normal for the coil when warmed up. I don't know that this is accurate, but such a resistance would work.

The condenser's voltage rating is likely at least 200V or higher to withstand the much higher voltages across the coil's primary winding during magnetic field collapse when the plug fires, so there would be no need to change it for different voltages (6V to 12V).

My take... Hope it helps.

It might be useful to consider the current in the two joined circuits which form the Kettering ignition system. The primary coil winding could have currents approaching 8-10 Amps with the points closed which will, through inductive field collapse in the coil primary, maintain that same current through the points at the instant they start to break open. Without a condenser in place, some of the stored energy will dissipate in a hot arc across the points, saping total field energy available for the high voltage ciruit which triggers at the same time. The arc will also erode the point contacts excesively. However, with the capacitor properly in place and initially at zero volts charge (the points had it shorted prior to this time), the high initial primary current can continue at point opening as the coil's field collapses and the condenser charges. The current then falls at a rate set by the capacity of the condenser. At point opening, all the coil voltages reverse in polarity and spike, then will oscillate (ring) back & forth (AC), falling to zero in a fraction of a second.

If you were to look at the oscilloscope tracings of a system with the resistor in the correct location, and with the resistor incorrectly placed between the points and the coil, they would differ.

The tracing of the system with the resistor between the coil and the points would look like the tracing of a system with a weak condensor.

I have a distributor machine, and an ignition 'scope, so I will try this. Getting a good picture of the trace may be another matter!

Guess I need a DSO!

(12v system)
Power is provided to the coil windings. A resistor is put before the coil and the coil is made to work on this reduced voltage (6 to 8 volts) so we can bypass the resistor during start up for a hotter spark.
If we put the resistor after the coil some will argue that the same amount of power is running through the coil but this does not allow us the opportunity to bypass it during start up.
The power then runs to the points which provide a ground when closed. When the points open this sudden loss of power running through the coil makes the electrical field collapse causing the coil to fire.
The condenser is in the system to provide a false ground for a split second so power will not jump the half open points causing them to pit prematurely.

Rod

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1 - The coil primary (12 volts DC) and coil secondary (spark voltage) are connected at only one point: Both are grounded to the coil shell. Otherwise they electrically independent.

Consequently primary current can flow ONLY in the primary circuit and spark current can flow ONLY in the secondary. Ie. NO spark current flows through the primary ballast resistor.

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2 - That said, since the primary is a simple series circuit the ballast resistor can located at any point in the circuit.

Between the ignition switch and the coil is the usual resistor location. (As you note, placing the resistor here makes it simple to bypass for starting.) However it can also go between the coil and the distributor, or (if there are no other loads fed off the ignition switch...) between the ammeter and the hot side of the ignition switch.

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3 - Ignition park occurs the instant the ignition points open - ie. when they break the primary ciruit. (The high voltage is induced in the secondary by the collapse of the magnetic field that exists when current is flowing in the primary winding). Also the condensor does not act as an open circuit - rather it absorbs the voltage spike as the points separate to reduce arcing at the points.

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Hope this makes sense!

Best wishes n God Bless yall

John T

I wrote my response before my morning coffee - not a good thing for me to do. Thanks for the catch!! ...Bob

John T

First of all, I respectfully disagree with your conrentions about the condensor being an "open circuit" during firing. Although the points open which initiates the collapse of the magnetic field and the subsequent induction of low primary voltage over into the high voltage secondary (mutual inductance transformer action), the condensor (which is indeed an open circuit to DC EVENTUALLY) is not IMMEDIATELY an open circuit when voltage is first appied. A condensor is initially and briefly a "closed circuit" during the time it conducts current unti such time the opposite electrical charges have accumulated on its plate surfaces, ONLY AFTER WHICH it appears as an 'Open Circuit" and DC current flow stops, i.e. it stops conducting.

Sooooo oo, although the points are open as you say at firing, and its true a condensor eventually appears as an "open circuit" to DC, right after the points open SUCH IS NOT THE CASE. I dont have them in front of me, but have seen oscilloscope traces of what occurs in the Kettering Coil Ignition immediately after the points open, and what happens is theres some oscillations and ringig going on in that LC circuit back n forth from the condensor to the coils primary.

Its indeed true, the coil will still fire EVEN IF THERES NO CONDENSOR WHATSOEVER in the circuit HOWEVER the spark will not be quite as strong since theres none of the ringing and oscillations of current and energy transfer that takes place between the coils primary and the condensor when its in the circuit. If the condesnor were too large, there would be NO SPARK as the current flow through the coils primary would be over damped and slowwwww www to change (no necessary sudden field collapse) while if its too small the points will burn up quickly BUT STILL A SPARK. Its the right sized condensor with the right time constant to make the proper LC circuit which yields the maximum spark energy.

It has been my "understanding" that if an inductive wirewound ballast resistor is placed in between the coil and condensor, that it may hamper the ringing (back n forth energy transfer) described above plus may also waste some of that enerfy as it gets converted into pure heat versus potential magnetic/ inductive energy. Thats how I understand the ballast should be ahead of the coil NOT between the coil and the condensor.

NOTE: If I get time later (gotta go help my 82 yo mom) I will dig up some Kettering Ignition theory and try n join back in the discussion and your GREAT question. I cant wait to read other explanations so we may all learn something here.

Thanks agian Thomas, Best Wishes n God Bless, I will check in again later.

John T (long retired electrical engineer, i.e. a lil rusty lol)

Primary and secondary are not "connected".

Allan

First, the high voltage pulse is not DC. The inductance of the coil and the capacitance of the condenser form a resonant circuit which "rings" when it is stimulated by the rapid collapse of the magnetic field in the coil when the points open. A mechanical analog of this is a guitar string which "rings" when it is stimulated by a pick. The wave form is actually a damped sinusoid, modified a little by the non-linear characteristics of the spark gap. The impedance through a capacitor (condensor) is a function of the rate of change of the applied voltage. A fast transient such as the high voltage output of the coil has a very fast rise and fall time and does in fact pass right through the condensor (with some phase shift, which is immaterial to this discussion).

If the pulse did not pass through the condensor, it would arc across the points and destroy them in short order. In fact, one indication of a failing condnesor is metal transfer from one point contact to the other as a result of the arc. If you ever encounter a set of points with a pit on one side and a corresponding mound on the other, the first place to look is the condensor.

Allan, the primary and secondary windings are in fact connected. Check a coil with an ohmmeter. The coil is a autotransformer, with the primary and secondary windings connected in series. There are only three connections, the "+" terminal which is the common connection between primary and secondary windings, the "-" terminal which is the "bottom" of the series windings, and the high voltage terminal which is the "top" of the series windings.

Hopefully, I"ll be out of here in a few hours today. If I get a chance in the next couple days, I"ll post a schematic (or a link to one) showing the current paths.

Thank you for your responses, but I am still confused. The question is more than academic as I am presenting a workshop on point type ignition at the Gathering of the Green conference in March. Perhaps I can be more specific in my questions.
1. How does the secondary circuit return to the coil? Does it arc through condensor while the plug is firing? The voltage in the secondary is certainly dc as the spark line or I should say firing line is relatively flat on the scope and doesn't start to oscillate and become ac until the coil can no longer jump the plug gap. The circuit can't be completed through the points as they are open when the plug is firing. Does the secondary complete its path from the block, through the ground wire, through the battery,the key switch, and back to the junction inside the coil? Either way I don't see how the placement of an additional two ohms could possibly affect the secondary when you are dealing with up to 20,000 volts.

The low voltage primary circuit is from the battery,,,,, to and through an ammeter,,,,, ,,to the switches BAT input terminal,,,,, ,,out the switches IGN terminal,,,,, ,,to and through the ballast resistor,,,,, ,,to and through the coils primary between its lil + and - terminals,,,,, ,to and through the closed points which has the condensor in paralell with them.

Once the points break open the condensor is sitting there empty and discharged and is initially a conductive device and it conducts curent until its plates are charged up then afterwards, its an open circuit to further DC. Immediately after the points open, theres some ringing oscillations in that LC circuit with energy transfer taking place between the condensor and the coils primary. An oscilloscope trace shows that and it eventually dampens out.

NOWWWWW WW if theres no condensor theres still a spark (but weaker) but the points would burn up quickly. If the condensor were to large, there would be NO SPARK cuz theres no sudden interruption of the current flow and at DC theres no transformer action. THEREFORE if the ballast which is inductive (wirewound resistor) is between the coil and condensor, the sparks weaker cuz some of the energy which properly passes between the coil and condensor gets used up as heat due the the impedance of the extra inductance or pure resistance, and if there, the normal ringing taking place between the condensor and coil would be changed. Its the proper size (NOT too large or small) that yields the strongest spark and if you place additional inductance or resistance in that ringing LC circuit the parameters get all changed. PLACE THE BALLAST AHEAD OF THE COIL !!!!! !!!

On every make n model n brand of the Kettering coil ignition Ive ever seen, therefore, the ballast is ahead of the coil NOT between the coil and condensor.

I gave that Ignition Seminar at GOG every year plus at the Two Cylinder Expo and the New York Expo, IM GLAD TO GET SOME HELP WELCOME ABOARD They have me giving four other electrical seminars this year whewwwww wwww

Hope to meet your there

Good Luck n God Bless

John T Nordhoff in Indiana

Some coils may have the bottom end of the high voltage winding grounded to the coil case. Others do not. They are wired with the primary and secondary in series so they look like an auto transformer. They both function in the same manor however. It is only a difference of how the secondary winding gets to ground. In the case of the series wound coil the secondary gets its ground through the battery terminal on the coil. Some of the secondary current does pass through the condenser as well.

The condenser is not there to provide any type of tuned circuit or to cause the circuit to ring for its performance. As soon as the points open the field of the charged core starts to discharge which charges the secondary coil and also the primary coil. The coils are charged to opposite polarity as the magnetic field is discharging.
The condenser is there only to keep the voltage on the primary from arcing over at the points. The primary voltage charges to several hundred volts as the magnetic field collapses.

Yes there will be some ringing due to the capacitor across the coil as it discharges but that is not its purpose. Its purpose is to limit the voltage rise so as not to damage the points. It does this by slowing down the voltage rise at the points while allowing the primary voltage to decay. Its size has to be chosen so as not to discharge too slowly so it allows the magnetic field in the coil to collapse fast enough to charge the secondary. Too large and it would dampen the discharge. Too small and it would allow the voltage across the points to rise too fast before the coil had enough time to diminish its voltage and the points would arc excessively.

As far as the ballast resistor on either side of the coil I don’t see where it would make much difference as the impedance of the primary circuit is so low compared to the secondary of the coil.

On cars with radio interference problems it was common to place a condenser at the positive terminal (battery terminal) on the coil to ground. This kept the return pulse current out of the wiring to the battery and other areas of the car so it didn’t radiate. A shorter path to ground.

As far as polarity of the coil goes I believe that a negative going spark is wanted at the spark plug. This makes the hot spark plug easier to discharge than a positive going pulse.

Regards
Gary

This is why the ignition system will still operate without a condenser but at reduced efficiency. The engine may run poorly in this condition. The points would also burn in a short time.

Regards

See my post somewhere down below as I claim resistor placement DOES INDEED MAKE A DIFFERENCE

John T

Have you also seen internal wiring diagrams showing it on either side or always the same???

John T

John T

I actually took one apart one time to make sure. At least the one I took apart was in fact an autotransformer, with the common point between the two windings at the "+" termonal. There"s a lot of "assumptions" posted to the internet that aren"t always necessarily correct.

I"ll post a schematic when I get a chance to get one drawn up.

Keith

John T