Thoughts on 8N distributor condensers

The condenser that was supplied with my aftermarket replacement front-mount distributor was only about 75% as long as the condensers I had been using previously, so I decided to measure both kinds of condensers to see what their capacitance was. Not even close! The longer type had a capacitance of about 31 microfarads, while the shorter kind was only about 25 mfd.
I think the value of the capacitance is a fairly critical compromise, because the condenser has to have enough capacitance to efficiently absorb the spark at the points when they open, but not so much capacitance that it would take the condenser too long to discharge, because it needs to discharge quickly in order to "buck" or oppose the current flow in the coil primary, which creates a much larger voltage spike in the primary than would otherwise occur; which steps the primary's instantaneous voltage 'way up above battery voltage, which is necessary in order to induce enough voltage in the secondary for a good spark. I had been using the smaller condenser and did not like the way my 8N was running. I changed it out for the larger version, and it is running well now; however, I fixed a couple of other things at the same time, so I can't prove it was the condenser that made it run better.
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on all of my engines,tractors,equipment I have two condensers wired in parallel to assist the spark
sequence, functions just fine .

leon

I'm curious. Did you compare the two with the same meter?

Would it not make the points start to pit from excessive arching if the capacitance was not enough.

Yes, that is the same meter in two separate pictures.

"Yes, that is the same meter in two separate pictures."

Sorry Bill, I didn't notice that the first time around.
Obvious to me on second review.

Well, Bill, you need more study. 25 &amp; 31 microfarads are off by a hundred to one. 0.25 &amp; 0.31 microfarads is about right. Not really that critical. Second, the condenser does not buck the current, it actually carries the current that was flowing thru the points prior to opening.

A year or so ago, a fella wrote an article for The N-News on condensers with his vintage HeathKit Condenser Checker. I'll see if I can dig it out and pass on any more information. Now, I only had one issue with a 'bad' condenser, but never 100% proved it was the root cause. Back before we got everything from Cheena, US companies, and even some foreign ones we helped get established, i.e. India, Mexico, and Brazil, had some sort of Quality Control in place. I never use to replace the condenser when doing a tune-up, and never had problems until the one time I did install the new condenser that was included in the tune-up kit. Now I take my chances and usually replace it only because I have no way to verify if old one is good or bad and they are included in the new kits. There has been talk in recent months/years of condensers being bad out of the box and I can only surmise it is because of the inferior design and manufacturing. I'm not sure if doing a static capacitance check is sufficient. Maybe the electrical gurus like JUMOR and Bruce(VA) will reply further.

Tim Daley(MI)

if parts are made overseas, any part, and then packaged in a well know American company box why would said American company settle for poor quality control from the supplier ?

(quoted from post at 09:31:36 10/05/17) if parts are made overseas, any part, and then packaged in a well know American company box why would said American company settle for poor quality control from the supplier ?

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ummmm..........uhhhu......maybe profit? Just a WAG.

Yes, JMOR, you are absolutely right in your first statement: .25 and .31 Microfarads they are. Thanks kindly for correcting me.

However, you're wrong in the second part of your post. You seem to be assuming that the only function served by the condenser is to absorb the spark at the points when they open and, as you say, at this point the current is flowing through the points in the same direction as it was flowing when they were closed. But you're completely overlooking the second and, arguably, more important function of the condenser: to discharge after the points open, thereby bucking the current then flowing through the coil primary, which causes the voltage in the primary to fall almost instantaneously (much faster that it would have dropped without the condenser bucking the current flow). Without that instantaneous voltage drop (downward spike in the voltage), only battery voltage would ever appear on the primary, and the turns ratio of the secondary to primary would have to be about 10,000 to 1. That would make the coil too large for automotive use. The downward voltage spike caused by the discharge of the condenser into the primary creates a spike of hundreds of volts on the primary, permitting the turns ratio to be a more reasonable and practical 100 to 1.

Tim Daley said, inter alia: "I'm not sure if doing a static capacitance check is sufficient."

Right, Tim, a simple capacitance tester like mine pretty much just tests if the capacitor is shorted or open, and if its capacitance is in the ballpark or not. It doesn't test how efficiently the condenser charges, or if it has significant leakage. In order to do that, you need a fancier capacitor checker that can measure "ESR", or equivalent series resistance.

connecting condensers/capacitors in parallel adds the capacitance of the two.

(quoted from post at 09:56:30 10/05/17) Yes, JMOR, you are absolutely right in your first statement: .25 and .31 Microfarads they are. Thanks kindly for correcting me.

However, you're wrong in the second part of your post. You seem to be assuming that the only function served by the condenser is to absorb the spark at the points when they open and, as you say, at this point the current is flowing through the points in the same direction as it was flowing when they were closed. But you're completely overlooking the second and, arguably, more important function of the condenser: to discharge after the points open, thereby bucking the current then flowing through the coil primary, which causes the voltage in the primary to fall almost instantaneously (much faster that it would have dropped without the condenser bucking the current flow). Without that instantaneous voltage drop (downward spike in the voltage), only battery voltage would ever appear on the primary, and the turns ratio of the secondary to primary would have to be about 10,000 to 1. That would make the coil too large for automotive use. The downward voltage spike caused by the discharge of the condenser into the primary creates a spike of hundreds of volts on the primary, permitting the turns ratio to be a more reasonable and practical 100 to 1.

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o, Sir, on both counts. Not assuming, not claiming the only function is to "absorb the spark at the points". As I said, the primary function is to provide a path for primary coil current flow as points open. It is a misconception that the energy for the spark comes from the condenser. The stored energy being released is from the coil inductance (energy=0.5 X L X I X I). Were it not for the condenser, much of that inductively stored energy would be converted in to heat at the point contacts as they open, with no alternate path for that current to flow (normally thru the condenser). If we had a "perfect switch" then no condenser would be needed, but there is no "perfect switch". Solid state devices such as transistors, SCR, Triac, etc. are closer to perfect. They switch in micro or nanoseconds without the slowness of a mechanical switch &amp; accompanying arc. Most if not all SS ignition systems do not have the condenser. Aside from the technical/physics of it, another view would be that if the energy did come from the condenser, SS systems wouldn't work! That "bucking current" and "only battery voltage would be available..." stuff is thin air. As is "the discharge of the condenser into...". The fact is that the condenser is only discharged some time well after the spark has ended, not before or during the spark. See oscilloscope trace. There are many books explaining Kettering ignition systems for the layman, and many text books delving into the minutia of inductive/capacitive circuits that will require some background &amp; deep study to fully understand.

Very interesting, JMOR, and while I can tell that you do know what you are talking about, the articles I have read say the opposite: that the capacitor's discharge back into the coil primary is critical to making the Kettering system work. FWIW, even Wikipedia says this. I have been studying electricity and electronics almost my entire life, which makes me realize that some of the points you are making bear further investigation. I'll just have to keep studying the subject. I hope you will do the same, and we can have another discussion about it sometime. Thanks for responding.

JMOR, let me toss this one out for you. If it unnecessary in the Kettering system to have a condenser, as you say, so you would never get more than battery voltage across the coil primary, how do you explain the fact that the coil turns ratio in an automotive-type ignition coil is in the range of 100 to one, rather than 1,000s to one?

OTOH, it seems to me that an engine with a Kettering-type ignition WILL indeed run with the condenser disconnected, so you may well be right!

(quoted from post at 11:23:33 10/05/17) JMOR, let me toss this one out for you. If it unnecessary in the Kettering system to have a condenser, as you say, so you would never get more than battery voltage across the coil primary, how do you explain the fact that the coil turns ratio in an automotive-type ignition coil is in the range of 100 to one, rather than 1,000s to one?

OTOH, it seems to me that an engine with a Kettering-type ignition WILL indeed run with the condenser disconnected, so you may well be right!

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ake a metal bar &amp; wind a lot of wire around it (now you have an inductor). Coil is inductor. Hold one end of inductor wire to neg batt post with left hand, and other inductor wire in right hand and proceed to connect to pos batt post (current flowing &amp; building magnetic field in inductor), then remove from pos post. This sudden release of stored energy in magnetic field will result in that voltage rising to how ever high it needs to be in order to try to maintain the current value that was flowing before the disconnect. Since your body resistance is a lot higher than the winding resistance, that voltage will be high enough to produce body current sufficient to get your attention &amp; probably result in very immediate release of everything you are holding. All this with no condenser!
Back to coil. Since the coil (inductor ) has a second winding, the rising voltage mentioned above from the magnetic field stored energy, can either or both rise high enough in the primary or secondary to try to maintain that current. P.S. Prior to the spark arc on your tractor being established, the secondary voltage at say 20,000 volts results in 200 volts across the condenser (primary voltage), but as arc established, the arc plasma is a low enough resistance that it sustains at around a thousand volts for the duration of the spark (until all the stored energy in the inductor is dissipated). During this arc time, the condenser voltage remains steady as it is set by the voltage across the plasma arc.