Condensers

Over the years I heard many explanations of what a condenser is doing in the ignition circuit and the usual answer is it keeps the points from arcing by "absorbing excess energy". That is the conventional wisdom because the condenser is really a capacitor and everybody knows that's just a temporary battery right? Can be. What a condenser/capacitor basically does is block DC current (but it will flow DC current until it reaches a predetermined "charge" based on size and time of charge) and pass AC current and that's exactly what it does in the ignition circuit. It can be used as an AC filter or volatile DC battery (won't hold charge long term without refreshing).

The instance the points open the primary winding flux field (magnetic lines of force) collapses and induces a current of reverse polarity in the circuit and the condenser discharges thinking its seeing AC voltage and recharges the pri winding which collapses when the condenser is discharged etc. If you have the right size condenser for the coil you are using you will get three to five oscillations between the coil and condenser.This can be seen on an O'scope. If the condenser is too large, it will rob the primary winding of current needing to build the magnetic field and use it to charge itself - resulting in a weaker spark than the system is capable of. If it is too small it will not be able to handle the discharge of the initial flux field collapse and the points arc; metal is transferred and smoke is made! :shock:

I hope that helps anybody that's always wondered. I can explain in more detail if someone is curious like me. Even though its a simple circuit, it is more than it seems. When you have a coil (inductor) and a condenser (capacitor) in series with one another and switch the polarity back and forth it becomes an AC active circuit where wierd things happen because of ELI the ICE man. The current gets out of phase with the Voltage...Seriously folks. That's no chit mon.

Mike

Right on !

If you open up any computer, you will find its generally packed with those little or micro capacitors (condensors in old-speak). They prevent 'ringing' down the wires, and thus from spurious signals getting things confused. So a slightly different purpose, but the same principle....

Capacitors are also often used to smooth out DC voltage fluctuations, much like an air tank connected to a compressor helps to regulate pressure...FWIW fluid analogies sometimes help to understand electricity (for me at least), voltage is analogous to pressure and current is analogous to flow

Comoz reports the value to seek is .2 to .22 uF for Norton size coils and contact area.

A primary coil winding resists current flowing into it d/t the backwards EMF from expanding magnetic flux, till equilibrium of current flow reached by the pure copper wire resistance. If coil continues to flow current if becomes same as toaster heater coils. When when power cut to coil by open contacts the flux field collapses to induce hi voltage in the secondary coil in opposite polarity flow to what 1st charged it up. Like stretching a spring one way then let go it springs back the other way, or direction of current flow in in coil case. Initial flux collapse voltage is enough to over whelm the tiny battery like condensor for one to 3 cycles. Then the condensor should be sized to absorb the rest of the decreasing voltage/polarity swings-rings, so prevents AC current arcing across points to blow metal off and pit or heat enough they weld together.

A capacitor-Lyden Jar, allows DC current to flow until its saturated-fully charged by the voltage shoving e-'s in or pulling them out, then stops-resists flowing more current until voltage is enough to just arc-burn right through its dielectric insulation separating its surfaces. AC Voltage applied to a capacitor-condensor within its rated capacity will flow right through, but above this a condensor blocks AC back and froth flow. DC current tops off a condensor so DC in one direction can flow as if no condensor present.

Can think of the condensor like a small spring such as in line with Norton front cable brake switch if you pull hard enough the rubber/spring switch is stretched enough to transmit pull to brake shoes, if less force applied then light may come on but not the brakes.

Manufacture of condensors vary enough its worth trying a few to see least amount of point arcing yet still allows a good surge to fire plugs.

That's a good analogy. There's a few things that aren't "quite" correct ( or complete would be a better term), but definately good enough for the layman to understand.

If I wanted to be picky though. The only time a cap will not flow AC is if the frequency is so high that it's faster than the ability of the cap to charge and discharge . The situation Jim mentions will attentuate (clip) the signal voltage but still pass AC. The only other thing is that the flux field built in the primary "pushes" the voltage in reverse when it collapses but that's getting nitpicky. The coil/cap oscillations for a "perfect match" is 3-5 instead of 1-3, in a "normal" battery point ignition. I've never checked a Norton. That's just been the standard in Automotive for many years.

Jim may be like me, sometimes I take liberties in explaining electronics for the sake of understanding the basics without going to far.

By the way, all electronics is nothing but FM. Also for those of you who accidentally "let the smoke out" of thier systems, I do sell cans of replacement smoke, you just have to devise the interface to get it back in. I haven't figured that part out yet! Spending too much time on quantum physics and time travel. :lol:

Mike

Yep if ya size the cap right it can act as an audio freq filter letting say deep base to pass to the sub woofer but not the higher freq sounds it can't reproduce.
Fun stuff any Time Lord knows.

Mike,
I haven't heard the F***ing Magic term in a while. I always though it was related to Frequency Modulation. But who am I to know. Then we have antenna erections and excitations, I could go on, but you know. I used to feed my managers buzz words like flux condensers, just to make the look stupid when they would repeat them. It was always hilarious when people in the know would look at them like idiots. They never caught on either.

Dave
69S

Real motorcycles like Nortons all came with real flux capacitors so who's fooling who?

DogT said:

Mike,
I haven't heard the F***ing Magic term in a while. I always though it was related to Frequency Modulation. But who am I to know. Then we have antenna erections and excitations, I could go on, but you know. I used to feed my managers buzz words like flux condensers, just to make the look stupid when they would repeat them. It was always hilarious when people in the know would look at them like idiots. They never caught on either.

Dave
69S

Click to expand...

I was wondering if anyone was going to pick up on the FM! That's a term like Write Only Memory (WOM chips). Back in the early 80's, Texas Instruments or HP (foggy mind) had a taped training series on the Motorola 8080 series of microprocessors (If you remember that 8-bit dinosaur...You are an Olephart too!). The last tape was a very believable lecture on WOM chips complete with pin-outs etc. It was always funny to see people around the room start going...Hey, wait a minute...what's wrong with this picture? Of course there were always those that were looking around going; "Whut's so funny?"

Mike

hobot said:

Yep if ya size the cap right it can act as an audio freq filter letting say deep base to pass to the sub woofer but not the higher freq sounds it can't reproduce.
Fun stuff any Time Lord knows.

Click to expand...

The absolutely most entertaining thing you can do with a cap:

Get the largest electrolytic cap you can find - charge- toss to friend - run. :mrgreen:

That's a good analogy. There's a few things that aren't "quite" correct ( or complete would be a better term), but definately good enough for the layman to understand.

Click to expand...

Being an EE for 35+ years I can attest that there's a whole lot of electrial "not quite" in this thread.

Olephart is quite right in saying that there are many different thoughts and theorys as to how condensors work and what their main function really is. I researched this very topic extensively this last January and found all kinds of different answers on the internet. Most answers as to what was the condensor's purpose, were that they reduce arcing at the points or that they help provide a hotter spark. Actually they do both, but asking which one is the main function is like asking which came 1st, the chicken or the egg? I found that the answers as to how the condensor actually accomplished these 2 important functions varied widely. I also found that it was hard to find accurate and UNDERSTANDABLE information on condensors when used in the "Kettering" style ignition system that we are talking about. After searching the web on and off for about a week, I found that the best explanation was right under my nose all the time. A 2005 edition of the John Deere Fundamentals of Service manual (Electronic and Electrical Systems) was in the training center library where I work. I will be referencing this manual as I post this reply. I want to address a couple of fhe most common misconceptions on this topic that I saw this last winter. I am not referring to any comments stated on this thread, so please do not take offense.

It appears that most everyone understands that when the points open,the magnetic field that is surrounding the primary winding collapses. This collapse induces a very high voltage into the secondary winding which produces the spark at the plug. What many are not aware of is that this very same collapse also induces a voltage back into the primary winding itself. This process is called "SELF INDUCTION". This self induced voltage is in the opposite polarity of the 12 (or 6) volts that is powering the primary winding and is being switched by the breaker points. This self induced voltage is called "CEMF" (counter electromotive force. Electromotive force means voltage.) This CEMF may reach a voltage level as high as 200-300 volts. (roughly 10% of the secondary voltage depending on the coil design) It is this CEMF that must be controlled or the points will be destroyed in a very short period of time due to arcing. The condensor (ignition capacitor) provides a place for the CEMF current to flow while the points are SLOWLY opening. (Keep in mind that each coil on a stock Norton will be firing 45 times per second @5500 RPM) This condensor charging provides enough time for the points to open far enough apart so that the voltage can't jump the point gap. Not all arcing is eliminated by the condensor. During the 1st millionth of a second as the points begin to separate, a very weak, and harmless spark will be seen. Controlling this point destroying arc is the #1 function of a condensor.

There are many misconceptions as to how the condensor helps create a hotter spark. One very common (and very incorrect) misconception that I read many times is that the condensor discharge is added to the secondary voltage. The condensor is wired into the primary circuit, not the secondary. Furthermore the .22 mfd current is a very tiny amount. (micro means millionth, so we are talking about .22 of a millionth of a farad!) Getting the highest possible voltage induced into the secondary winding is dependant on a FAST collapse of the magnetic field (flux) surrounding the primary winding. Our CEMF complicates this task. If arcing were allowed, current would be flowing through the primary for a longer period of time which would slow the magnetic collapse, resulting in a weak spark. Keep in mind that the condensor is being charged with CEMF immediately after the points break the flow of the primary current (6 or 12 volts). It charges quickly as the points continue to open to a SAFE gap. Once charged, the condensor provides resistance to the CEMF, and rebounds current back into the primary winding, thus resulting in the desired, fast collapse of our primary magnetic field. The 3 to 5 oscillations seen on the scope indicate a rapid breakdown of the magnetic field, and properly functioning condensor, as the current from the CEMF is blocked.

Sorry about the lengthy butt-in, but all of this was still in the back of my head from this winter's research, and, when the topic was brought up, I felt compelled to share it with people that are interested. This is MY VERSION of what is stated in our John Deere training manuals, it is probably not perfect, but I believe it to be quite accurate. I have tried to express it in the simplest and most understandable form that I can. I hope it makes sense to you !

Mike Wolf

maylar said:

That's a good analogy. There's a few things that aren't "quite" correct ( or complete would be a better term), but definately good enough for the layman to understand.

Click to expand...

Being an EE for 35+ years I can attest that there's a whole lot of electrial "not quite" in this thread.

Click to expand...

If you're talking about something I wrote, please correct me. I would not be offended at all. I too have been in electronics for many years and it was along time ago that I studied theory. Especially since everything is on a chip nowadays! Even at 60 I still prefer not to propagate incorrect info!

Mike

mw, I tried to keep it as "layman" as I could (simple) and still get the point across. I was trying to get at the cause of the pendulum effect of the pri coil and condenser as simply as possible. I didn't mean to imply that it was an active circuit with the ELI/ICE comment, I was just joking around. Like the time travel machine I'm working on! I retired 12 years ago and even though the basics never change, my memory does!

As to many things being written about the circuit, that's why I posted what I did. I love it. You will find that the explanations of Automotive - tractor - etc. engineers on how this circuit (2 counting the secondary) works is sometimes completely different that what Electrical Engineers will say. It has made me scratch my head more than once! I was trying to stay away from too much detail. My main point was that size does matter and I was trying to get there as simply as possible. For instance you used the term self-induction/CEMF and I said "it pushes back" . With that said other than a couple of minor points, we are saying exactly the same thing.

I will have to say though, I don't remember ever observing (o'scope)100-300 volts on the primary side during the discharge cycle. Your explanation sounds like an Automotive trainers explanation! (Please take that with humor ) I say that because the condenser is charge to DC potential when the points close. Therefore it is not the "charging of the condenser" that prevents the arc. so when the polarity of the coil reverses due to CEMF so does the condenser, therefore it is discharging.

And the beat goes on! I enjoy the discussion, not proving anybody right or wrong. I hope I can grow and learn until the day I die.

If you're green, you're growing, if you're ripe, you're starting to rot."

Mike

More in site on electrical vs mechanical analogs

http://lpsa.swarthmore.edu/Analogs/Elec ... alogs.html

Now this is a condenser, although typically it's called a capacitor because of the size. I doubt if it would be very good in the ignition circuit.



Dave
69S

OK now ya gone hijacked the post into more pleasant uses of big ole capacitors.
I use a pair or more in 1,000,000 uf rating each to get the TwacK-peaks to subwoofer bass hits no battery or amp can dump current spike loads fast enough to reach on their own. Charge one up and toss to a non-friend, while yelling hey watch this to on lookers.

Oh yeah, 000 gauge leads to tolerate the surge w/o lose.

Mike;

You kept it simple, I got carried away. I probably won't do that again because I spent waaay to much time writing all of that and I really never cared for composition and typing classes anyway.

I have never actually seen the 200-300 volts on an O-scope either. Do the scopes show primary voltage? Its been so long since I used one (1974), I don't remember. Anyway, I found that figure in several web resources that I visited. I think it sounds reasonable. Think about this; How many volts would it take to jump the point gap, and be able to cause the metal transfer that it is capable of? I NEED TO CORRECT MYSELF. I said that this 200-300 volts would be about 10% of the spark voltage. It is 1%. Sorry about the mistake.

Here is a link to what I thought was the best resource on this topic that I found on the internet. It is about different ignition systems . It covers this CEMF quite well and touches on the "pseudo AC voltage" that you talked about, plus a whole lot of other good info. http://www.jetav8r.com/Vision/Ignition/CDI.html

Mike

Principles of Inductive Discharge Ignition - patented by Charles Kettering in 1904:


Any coil of wire posesses an electrical property called Inductance (symbol 'L', unit 'Henry'). Inductance has the ability to "store" electric current in the form of a magnetic field.

When the points close, DC current flows through the ignition coil's primary winding and "charges" its Inductance. A magnetic field is generated around the coil. When the points open the field begins to collapse and the stored current discharges. An induced voltage appears across the primary:

e = L di/dt

where e = electromotive force (voltage)
L = coil inductance (in Henries)
di/dt = rate of change of current (amps per second)

This induced voltage is typically in the range of 200-300 volts peak (it's AC), enough to give you a nasty bite if you're unfortunate enough to be touching the points when they open.

The ignition coil is actually an autotransformer (both windings connected) with a typical turns ratio of 100:1, which yeilds 20-30KV at the plug (Bzzzzt).

This condensor charging provides enough time for the points to open far enough apart so that the voltage can't jump the point gap. Not all arcing is eliminated by the condensor. During the 1st millionth of a second as the points begin to separate, a very weak, and harmless spark will be seen. Controlling this point destroying arc is the #1 function of a condensor.

Click to expand...

Correct. The main purpose of the condenser is to absorb that discharging current long enough for the points to open. Otherwise all the stored coil energy would go into a spark at the contacts and burn them up quickly.