condenser on coil positive is important.

[240260280z]

I thought it was just to supress radio emissions but it is actually part of a tuned circuit to improve spark and to minimize current damage to coil, breaker points or transistors.

[SteveJ]

The next issue is then how much capacitance to go with the inductance of the coil? I'm betting that a 40,000V coil has more inductance than a 20,000V coil.

 

That makes me want to go back through my Amazon wish list and get that inductance meter...http://www.amazon.com/dp/B00GAV6ZRU/ref=wl_it_dp_o_pd_S_ttl?_encoding=UTF8&colid=1D5A9IK8SOSHO&coliid=I2DITKM6A47PQ5&psc=1

[Stanley]

Interesting topic, especially since the condenser on the coil of my '73 looks about 100 years old and I'm wondering about it, and how to find the correct new one. Yesterday, while searching the electrical forum for answers to a different coil related question, I saw a post by Arne, that removing the coil condenser resulted in a performance problem (for his car). Others said it was for radio only.

Since I don't know enough to ask a useful question, I spent some time online this morning looking it up. Don't think I know much more than I did before, but I found this, which at least seems to show that capacitance does make a difference.

[240260280z]

~0.22uF and 500V-ish I think. If the capacitance goes too high, its discharge (too many electrons thus too high a current) can burn up the coil's primary.

 

It seems that when the coils' primary magnetic field discharges, electrons flow backwards to the cap and to ground.  The capacitor absorbs the transient  (back EMF) caused by the collapsing primary field much quicker than without thus it induces a more concentrated impulse into the secondary (causing a better/stronger spark).

 

Apparently without the capacitor, the spark produced is weaker and the back EMF can arc across the points burning them out.

 

 

Here is my fav HV experimenter on youtube:

 

 

 

 

FYI early spark gap coils were rated by how far the spark would jump. I believe one of the largest would produce 1 million Volts (42 inch arc) and had 280 miles of copper wire.

 

Edited April 22, 2015 by Blue

[Zed Head]

Is that Buddy Holly's garage?

[240260280z]

FYI 72 FSM shows coil currents <= 1.5A DC with resulting spark gaps ~ 14mm and condensor of 0.22uF

[Zed Head]

I thought it was just to supress radio emissions but it is actually part of a tuned circuit to improve spark and to minimize current damage to coil, breaker points or transistors.

I thought the tuned circuit part was for points, along with suppressing the arcing to make the points last longer.  Not sure that the same applies to transistor ignition systems.  Arne's problem was with a Pertronix electronic setup and it was never really clear why his had problems but numerous other people with the same setup didn't.

[Jarvo2]

Are we talking condenser or ballast resister?  

[Captain Obvious]

What the condenser does is provide a TEMPORARY low impedance path for the primary current until the points gap is large enough that the "kick back" from the primary is not enough to arc across the gap.

 

That's the nutshell. Here's some detail. Start with some arc theory:

 

1) It's easier to establish an arc across a smaller gap than a large one.

2) It's easier to MAINTAIN an arc than to establish one in the first place.

 

So if you can prevent an arc from forming when the points are first opening and the gap is very small, you might be able to prevent an arc from ever forming at all because the points gap gets wider as the cam rotates and opens it further. in other words, if you can delay the voltage presented across the points gap until the gap is wide enough to withstand that voltage, then you will not get an arc at all. The condenser provides that delay with it's temporary path.

 

The condenser has to be large enough to suppress the gap voltage until the points are wide enough to resist the arc, but as small as possible since any energy that you use to fill the condenser doesn't go into making a spark at the spark plug. The bottom line is you should use the smallest value you can get away with before you start arcing your points.

 

But here's the tricky part... Remember the part about it being easier to maintain an arc than to establish one? That's where the ignition spark performance stuff comes in. You might think to yourself "Heck, I want the hottest spark possible at my plugs and I don't care if I have to replace my points more often to get it. I'm not going to use any condenser at all and that way, I'll get more energy at my plugs."

 

Well, not true...

If you don't use any condenser at all and simply let the arc form across the points gap, you're bleeding energy out of your ignition coil through that arc the whole time that arc exists. So by the time the primary energy is no longer high enough to maintain the arc, you've lost a bunch of energy that COULD have been put into a secondary side spark but is no longer available because it went through the points arc instead.

 

So if you can time it correctly such that the condenser is full at the exact same time when the primary side voltage is not high enough to jump the points gap, then you will get a faster change in the primary current and hence a hotter spark on the secondary side. In other words, the resistance to flow through an already established arc is lower than the resistance to flow though a full condenser.

 

Use the smallest condenser you can without arcing the points. Not only will you not burn up your points, but you'll also get the most rapid change in primary current which creates the hottest spark at the secondary.

 

Quiz on Monday.

[siteunseen]

So with the ZX distributor this is all "pointless"? No need to study for Monday's test?

[Captain Obvious]

Haha! You funny. No quiz for you!

 

Yes, you are correct. The electronic ignition transistor switched coils don't need the condenser like the points versions do. They just have to make sure that their switching transistors are robust enough to handle the primary coil flyback (kickback) voltage without being damaged.

 

Interesting though, the later Z's with the electronic ignition module still DO have a condenser at the coil, but the later model condenser is on the battery side of the coil instead of being on the low side (switched side) like they do in points applications. I'm assuming it's for noise reduction or for a local low impedance source of stored energy.

 

But anyway, that begs the question... The original topic of this thread suggested the importance of a condenser on the coil POSITIVE, not the coil negative. But in points applications the condenser goes on the coil NEGATIVE. So what gives?

 

Blue, was that an oversight on your part, or did I go off and explain the whole points thing in a "normal" application, but miss the mark on the original intent of the thread? I mean, it wouldn't be the first time I answered a bunch of questions that were never asked, but I usually know ahead of time that I'm doing that!

[rossiz]

great info cap'n - truth be told, i've been looking at/wondering about that mysterious little silver barrel ever since i was a kid tuning up the 3.5hp briggs & stratton on my miinibike and never quite understood what it did. just knew it was some kind of magic part that was somehow necessary and prayed i never had to be able to explain it to another kid. it was just something that came in the box of points and you replaced it with your tune-up.

 

fast-forward to a few months ago when i replaced my rotor/cap and i was surprised to see a condensor mounted near/wired up to my coil with no points in the distributor... the good folks at o'reilly couldn't locate a new one, so i just cleaned the contacts, said some magic words and put it back where it was.

 

brought me right back to grade-school days...