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Old 09-18-2020, 08:35 AM   #1
KGBnut
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Default Dual Condensers?

My grandfather was mechanic from the dawn of the automotive age. He worked mostly for Buick, but could fix anything. He also had a reputation as being an absolute wiz with ignition systems.

My father is also a great mechanic, but never really did it professionally. Recently, Pop was telling me about the first Doodlebug he built when he was about 13 years old. The engine and chassis were out of a 1919 Overland (which I think later became part of Willys). This was during the war and parts for anything were hard to find, especially a 20+ year old obscure car. It needed ignition points and none were to be found. So, my grandfather build a set of point by modifying ones from something else (probably a Buick). He also modified it to have two condensers, one at the distributor and one at the coil.

Does anyone have any experience with this kind of set up? Can you explain why two condensers, and why one would be at the coil? I have heard that if the capacitance is too low, one side of the points will burn, and if it is too high that the other side will burn, so, was he just trying to get that balanced right? Why one at the coil?

I sure wish Grandpop was still around to ask.

Ken
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Old 09-18-2020, 09:24 AM   #2
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Default Re: Dual Condensers?

My A came with a condenser on the coil and one in the dizzy, probably the one in the dizzy wasn't working and one added to the coil, that was almost 50 years ago. A new burn prove condenser in the dizzy and everything is good!
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Old 09-18-2020, 09:38 AM   #3
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Default Re: Dual Condensers?

While we're on the subject of capacitors, what's the proper value of a capacitor for a Model A?
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Old 09-18-2020, 10:14 AM   #4
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Default Re: Dual Condensers?

.22-.27 microfarads is capacity for auto use.


The points really don't care where the condenser is as long as its installed correctly. As quick as those little electrons are the distance between the coil or distributor doesn't mean much.
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Old 09-18-2020, 10:58 AM   #5
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Default Re: Dual Condensers?

I've been running a 60s style condenser connected to the red switch side of the coil for years on my September 29 . I don't even use a condenser in the distributor on that model A . The condenser should run cooler and last longer and is much easier to remove and replace when necessary . I use the condenser that was last used on V8 Fords in the 60s-70s . This is the modern style condenser that is offered by most model A parts vendors . This condenser will work with both original and so called modern points .
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Old 09-18-2020, 11:20 AM   #6
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Default Re: Dual Condensers?

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Interesting thread on condensers.

I have never seen two condensers in a distributor so can’t help with OP’s question.
Just wondering, does the condenser care whether it sees 6 volts or 12 volts?

Last edited by AzBob; 09-18-2020 at 11:38 AM.
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Old 09-18-2020, 12:23 PM   #7
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Default Re: Dual Condensers?

At GMC back in the 50's we installed a radio suppressor (looked like a condenser) on the coil and generator when installing a radio, for noise suppression. I think it went on the positive side. A condenser for the ignition points should be of a certain capacity, adding a second condenser would change that and likely cause the points to have a shorter life.
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Old 09-18-2020, 01:14 PM   #8
Kevin in NJ
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Default Re: Dual Condensers?

As an electrical engineer lets talk about capacitors.

Putting 2 capacitors in parallel will add the values of the capacitance. So you would go from .22 to .44mfd. That means it will absorb more energy. Potentially an energy loss for the coil.

Capacitors are just 2 parallel conductive plates with a separator. They only conduct electricity when the voltage is different between the sides (under ideal conditions).
BUT as they fail they allow for current to flow but a tester will show that the capacitance is still proper.

Usually the voltage for a capacitor is much higher then needed. For the ignition circuit it is up around 450volts. You can actually see some pretty high voltage for very brief periods of time but with very little current. So 6 or 12 volts is not a factor.

The purpose of the capacitor is to limit the arcing on the points. So it absorbs some of what is called back EMF from the coil. If too much current is allowed to flow then you lose some of the energy going to the spark plugs. There is a balance. You need the capacitor at the points or as close as possible because that short bit of wire is a resister. At the quick current levels seen the small resistance will increase the voltage at the points. More voltage means the electricity can jump a larger gap. The major fail point for the type of capacitor used on a car is heat. So clearly getting the capacitor out of the heat would be great. But then it could not do its job as it MUST be as CLOSE as possible to the points.

Modern points are not the answer. The problem is today the points you buy for the modern cars are all done low bid in China. You get luck of the draw. Go read up on Mustang forums and you will find people complaining about failures out of the box and after short use for even the name brand like Motorcraft. At work I buy some expensive computer motherboards because they use high reliablity capacitors. The reason why all my old motherboards have failed, the capacitors go bad. Now I can not complain as these were high end computers and we got 8 to 12 years of 24/7 use.

For all Model A cases I will say you are best to use the burnout proof condensers from a known company. A&L I know makes good ones, or at least used to.

For the people that put he condenser at the coil and the dist. Well they likely are just cutting down on the spark at the plugs. But the wire between is a resistance and it creates what is called a tuned circuit low pass filter of some sort. It could cause the spark to vary depending on rpm or make the dist condenser need to absorb more energy then it was intended.

The radio suppression capacitors would be of a different value. Likely a factor of ten lower in the .02 range as you are trying to clear out a much higher frequency noise. But I could be wrong here, I did not find anything referencing what was used in the later cars. You also have to consider later cars were much different. There would be a resistor or resistor wire in the coil circuit.

What to take away from this:

You can not tell if a condenser is bad without a coil condenser tester or a special condenser tester. A meter just tells you what the capacitance of the device is and a failed unit can still show the correct capacitance (my experience is most will).

Modern built V8 condensers can and will still fail even out of the box. A burnout proof unit is often made from better quality capacitors so they will just work for a long time.

The condenser MUST be as close to the points as possible.

One correctly sized condenser is all you want.

I have intentionally kept the text simple above so hopefully you can understand what I have said.
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Old 09-18-2020, 02:09 PM   #9
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Default Re: Dual Condensers?

The second condenser could have been on the other side of the coil for radio noise suppression
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Old 09-18-2020, 03:46 PM   #10
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Default Re: Dual Condensers?

Kevin in NJ-

Thanks so much for your information. Very helpful.

I was aware of the need to place the condenser close to the points, but I was hoping that an ignition system as used with the Model A (6 v, not 'high performance') would be a bit more forgiving in mounting the condenser at the coil.

For a few months I've been running a NOS USA industrial capacitor rated at .25 mF, 100 v. Its mounted at the coil. No condenser at the distributor. I did this for the same basic reason as most people...get out of the heat, and make it easy to replace. It appears to be running very well with this configuration...perhaps an oscilloscope would tell me differently?

Kevin- Do you think that having the condenser mounted at the coil is a make-or-break issue for a Model A?

Alex
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Old 09-18-2020, 04:17 PM   #11
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Default Re: Dual Condensers?

I'll just say that the coil on the model A is close enough to the points that no problem will happen if the condenser is connected to the switch side of the coil . Lots of people have known that this setup works for many years . There was a time that original type model A replacement condensers didn't hold up well . Back in the day , somebody that knew what they were doing showed others how well this setup really works . I use the same condenser on 12 volts as I do with 6 volts . Same goes if I am using the original or modern style condenser .
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Old 09-18-2020, 04:30 PM   #12
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Default Re: Dual Condensers?

Kevin pretty well covered it but I will add a couple things.

The PRIMARY purpose of the capacitor in a coil /points/capacitor circuit is to tune the circuit to RESONANCE. When this is achieved, the impedance of the circuit is at maximum and current flow is minimum. Current flow being minimum is where the points will have the longest life.
Any deviation from resonance by adding capacitors will shorten points life.

The efficiency or “useful quality” of resonant circuits is measured in “Q’s”. An iron core coil has a very low Q therefore can tolerate a fairly wide variation in capacitor value before circuit efficiency falls off enough to shorten points life so you can really notice it.
Actually the ideal place, electrically, to mount the capacitor is directly across the coil terminals.
Mechanically, this is not a good place for several reasons. Mechanically, the best place has been inside the ignition case.

The distance the capacitor, coil and points are separated will have little impact on efficiency of the circuit due to the very low Q.
In radio circuits, the exact opposite would be true.
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Old 09-18-2020, 04:55 PM   #13
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Default Re: Dual Condensers?

I read about this elsewhere on The Barn so when I ordered a Pertronix 40111 coil I also got a modern style condenser.

Thought it would be handy should the original style condenser fail. All I'd need to do is connect it to the coil post.

I may never use it as I carry original spares and a complete distributor.

But, what the heck. I was bored and I like to tinker on the Model A....
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Old 09-18-2020, 05:53 PM   #14
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Default Re: Dual Condensers?

You should be able to use the same condenser on 6v or 12v. The points shouldn't get more than 8v regardless of battery voltage.
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Old 09-18-2020, 05:55 PM   #15
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Default Re: Dual Condensers?

The model A ignition system pretty well shows that Ford Motors was still on the low side of the technological ladder at the time. Their use of the breaker side of the circuit for the ignition switch was definitely different but when you compare the system to the Model T then it is still highly advanced by comparison. The other major gaff was putting the condenser in an area that can get pretty hot. Maybe that's why they made it so easy to replace.

There is an article in the Early Ford V8 Club's bimonthly magazine where a fellow experimented with condensers of values from 0.15 to 0.3 micro farads. The condensers were tested for resistance, leakage, and capacity prior to his testing so there would be some accuracy for results. All of them were automotive type capacitors of various part numbers and should have had a high end rating around 600-volts. His results were that he noticed no great difference in operation with any of them using the one can type coil for the test.

Pete is right about the resonance being the purpose. For each different design coil that Ford used over the years from 1928 through say 1956 for the 6-volt era, the condenser values were chosen by mathematical calculation to match the inductive reactance of each different coil design. For peak efficiency the resonance is important even though a small variation won't affect that efficiency all that much. It may affect the longevity of the points though.

Condensers can be placed on the outside of a distributor but it should be as close to the breaker points as is practicable. An original model A application has the breaker circuit that extends into the dash and then back out to the distributor through the armored cable so I don't know if I would call that a short run. If a person rewires it to put the ignition switch on the power in side of the coil then that might shorten the wire run from the coil to the distributor a bit. It just depends on whether a person wants to use the armored cable or not.

Back flow of energy released during the breaker opening can have a pretty high voltage. This information is pretty easy to find. Condensers need to be able to absorb that back flow during the magnetic field collapse. A capacitor doesn't care what the voltage is but the coil is affected by current draw depending on system voltage. This is why ballast resistors are used to control current in some applications.

Last edited by rotorwrench; 09-18-2020 at 06:05 PM.
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Old 09-18-2020, 06:05 PM   #16
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Old 09-18-2020, 06:19 PM   #17
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Default Re: Dual Condensers?

My understanding is that the voltage rating is the voltage at which the insulation becomes ineffective (or breaks down). Is this correct?

Why the very high numbers in voltage ratings for these condensers? Its fairly easy to find voltage ratings of 600v, or 1000v, or more.

What is the purpose of such high voltage? You could say its 'safety factor', but that high? If an airplane wing could be rated at 3 times the force it would ever see in service, engineers would be happy.

Is it that very high voltage values are so easy to achieve that we set the bar so high...way past what is really required? Or is there a true engineering requirement why a 6v ignition system should have 600v rated components?

Inquiring minds want to know.
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Old 09-18-2020, 06:34 PM   #18
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Default Re: Dual Condensers?

Quote:
Originally Posted by 1930-Pickup View Post
My understanding is that the voltage rating is the voltage at which the insulation becomes ineffective (or breaks down). Is this correct?

Why the very high numbers in voltage ratings for these condensers? Its fairly easy to find voltage ratings of 600v, or 1000v, or more.

What is the purpose of such high voltage? You could say its 'safety factor', but that high? If an airplane wing could be rated at 3 times the force it would ever see in service, engineers would be happy.

Is it that very high voltage values are so easy to achieve that we set the bar so high...way past what is really required? Or is there a true engineering requirement why a 6v ignition system should have 600v rated components?

Inquiring minds want to know.
.
A voltage spike occurs in the circuit when the points open and the magnetic field in the coil primary collapses. This spike can be 400 volts or more.
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Old 09-18-2020, 07:30 PM   #19
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Quote:
Originally Posted by Pete View Post
A voltage spike occurs in the circuit when the points open and the magnetic field in the coil primary collapses. This spike can be 400 volts or more.
Thanks Pete. I'm still not totally clear (my own fault).

When you say "This spike can be 400 volts or more", you're saying it can be 'a number up to 400, or more'. Effectively, 'any number' would fall into this category.

So to keep this Model A related, is 400v a value that has been measured on a Model A (or similar) system with an oscilloscope or other appropriate equipment?

Future buyers of condensers will be happy to know they are buying condensers properly rated for their Model A.
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Old 09-18-2020, 07:50 PM   #20
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Quote:
Originally Posted by 1930-Pickup View Post
Thanks Pete. I'm still not totally clear (my own fault).

When you say "This spike can be 400 volts or more", you're saying it can be 'a number up to 400, or more'. Effectively, 'any number' would fall into this category.

So to keep this Model A related, is 400v a value that has been measured on a Model A (or similar) system with an oscilloscope or other appropriate equipment?

Future buyers of condensers will be happy to know they are buying condensers properly rated for their Model A.
.
It depends on the primary supply voltage, the individual coil, capacitor and the rpm (interrupt frequency) what the voltage spike will actually be. I have measured 300+ on a scope.

If you are worried about capacitor quality, try one of those brass things Tubman makes. They seem to match most coils found on model A's.
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