Condenser Capacitiance.

koates said:
You know it would benefit most ordinary people who are trying to understand all of these posts, if the university degree electrical engineers and wanna be NASA scientists could keep their explanations to basic facts. We know you are clever but we don't need to be reminded of it with your every answer. Regards, Basic Kevin.
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Some wisdom there. I think the issue that was attempting to be addressed is the idea that just any old part can be used in any location. Maybe it was being addressed poorly, but think that was the idea. The capacitor is an important part of the ignition system and some thought needs to be given to what is used would have been simpler I guess!
 
MGG said:
Interesting thread. I have read through it twice but I am still unclear about something. When in the ignition cycle does the charge stored in the condenser go to ground, or does it? They are all hooked up with a ground are they not?
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The capacitor has a ground potential but it is isolated from the building & discharge of the coil circuit by the insulating medium the metal foil is wrapped with inside. The idea of a cap is to store its rated capacity for energy until it naturally bleeds off and not just let it go to ground. The magnetic field in the primary coil needs to collapse and build back up as rapidly as the breaker points dwell time allows it to. Without the condenser, the coil would likely only have a couple of weak sparks when the starter is turned since it could no longer function as it should.

Induction or transformer coils as well as capacitors are generally used in alternating current circuits. When direct current is involved, they have to have a half wave pulsating direct current (PDC) in order to function. Old radio sets have the ******** to get PDC and auto ignitions have the breaker points timed to the cylinder firing to get the PDC. When the ignition circuit is turned off or the circuit is opened, then the condenser will rapidly bleed to ground if the points are closed. If the points are open it bleeds off slowly as energy used up in the primary coil.
 
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My initial query was on the lines of my 39 distributor requires a .33 to .36 mf condenser. Some condensers are sold by part No without any reference to capacitance of condenser.

Therefore the importance of getting close to the correct operating value is important due to the effect of too low or too high will have on points condition and life.
Yes?
Phil NZ
 
It's my understanding a little bit on the high side is better, too low is no good. Capacitor tolerance or precision wasn't necessarily very good years ago anyway. So if it actually measures 0.37 for a 0.33 you're OK.

But a 0.22uF installed in lieu of a 0.35uF is way, way off the beam and would likely arc?
 
rotorwrench said:
The capacitor has a ground potential but it is isolated from the building & discharge of the coil circuit by the insulating medium the metal foil is wrapped with inside. The idea of a cap is to store its rated capacity for energy until it naturally bleeds off and not just let it go to ground. The magnetic field in the primary coil needs to collapse and build back up as rapidly as the breaker points dwell time allows it to. Without the condenser, the coil would likely only have a couple of weak sparks when the starter is turned since it could no longer function as it should.

So if the capacitor has only a ground potential would the system work if you did not ground the capacitor by its mounting? How does the energy naturally bleed off, through the points or through its' mounting ground? Or, does the mounting ground only work if there is an internal short in the capacitor?
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A capacitor is like a leaky balloon. It can only hold the energy for a matter of minutes after the power is off and it is connected to the primary coil even if the points are open so it's not completely isolated. In operation it only needs to hold a charge for a split second so the fact that it leaks down slowly is not a problem but hopefully at a much slower rate than required. A leak check is part of the test for a finding a decent capacitor. The capacitor needs the ground potential to do its job storing the charge so yes, the outer case has to be grounded even though the little can is isolated from half of the the metal foil inside.
 
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Crankster said:
It's my understanding a little bit on the high side is better, too low is no good. Capacitor tolerance or precision wasn't necessarily very good years ago anyway. So if it actually measures 0.37 for a 0.33 you're OK.

But a 0.22uF installed in lieu of a 0.35uF is way, way off the beam and would likely arc?
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From my experience, neither of these statements are correct. Whether the capacitance is too high or too low just determines which side of the points erode. The further it is from the ideal capacitance, the more erosion there is. I have been running my .22 mfd condensers on several vehicles with dual points and have not experienced any problems. There is enough tolerance in the system that this difference is not significant.

Because of public perception, I will be producing a version in the mid-thirties when I find a capacitor with adequate temperature and voltage ratings that is small enough to fit my package.
 
Hm.. just thinking outloud, would differences be obvious looking at the spark traces on an ignition scope maybe?

Remember one thing too about auto manufacturers though, they have that "mad man muntz" philosophy - if a part isn't strictly necessary, it won't be there - and, if they could have got away with a .22 instead of using a .33 in a given situation they would have done just that, because they cost 2c less. Many parts selections are just "good enough". Another possible factor, modern poly film capacitors have a lot better electrical characteristics, e.g. virtually no leakage so maybe they work better in real world applications too. Those condensers have a tough job!
 
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I put a pertronix in my 49 back in 2014 when a new condenser failed. Yes it was an expensive one from Echlin. I also bought their coil. I'm running 6 volt positive ground and I have probably put 15k miles on it. Yes I have a spare distributor in the trunk to quickly change it if need be. I met a gut in Oregon driving a healy 3000. He just carried a spare module. He was from Wisconsin so we both had traveled a few miles.
 
I don't know if any company manufactures a top quality condenser for the Model A type distributor. The Echlin FA5 used to be decent but since Standard took over Echlin, it's really hard to say now days. The model A distributor was subject to a lot of heat so carrying a spare was always a good idea. I would assume that all of them use Mylar film or equivalent to roll up the foil in. A person would have to dissect them to find out for sure. The mfd with a can type coil is usually .21 to .25 like all the later ones were.
 
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Paul Bennett said:
What is this "GROUND POTENTIAL" I keep hearing about which does a job by storing a charge?

Do all good capacitors come with a ground potential? Do coils have one also?
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There are as many different capacitors as there are different types of circuits that use them and not all are purposed the same way in function either so this is limited to automotive ignition systems.

It's all about polarity. The charge that is built by an automotive ignition condenser is sensitive to polarity in operation so a grounded case or can is essential to insure polarity for each charge built. A coil has two input terminals so it's polarity is set by the way it's wired and the condenser polarity is also set by the way the coil is wired. The polarity of the electrical system is set by whether the battery is positive or negative ground so that in turn affects the ignition system.

This has always been a good link. http://mgaguru.com/mgtech/ignition/ig108.htm
It shows an oscilloscope pic of a cycle.
 
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rotorwrench said:
The capacitor needs the ground potential to do its job storing the charge so yes, the outer case has to be grounded even though the little can is isolated from the metal foil inside.
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I'd still like to know what you mean by a capacitor needing 'ground potential' to do it's job. Please...Thanks.
 
See if this helps. A capacitor does not allow electrons to flow through it like a resistor does. A charge builds up on one of the two plates, but the other plate must be referenced to ground for this to occur.
 
Potential is another word for voltage. Current is amps and volts is the pressure that pushes them. If you have an open circuit, meaning no vurrent flow, and you measure 6 volts on your volt meter you have the potential to push a certain number of amps in accordance to the resistance of the circuit when it is closed. 12 volts will push twice as many amps. Hope that clears that up for you.
 
Paul Bennett said:
I'd still like to know what you mean by a capacitor needing 'ground potential' to do it's job. Please...Thanks.
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Three tests are used on a condensor.

Capacity (in mfds) is the level of storage the condensor can hold or store . Think of it a a .36 mfds could hold 36 electrons before being full.....

Series resistance is the level the condensor offers allowing some energy to travel to ground. Looks like a path for energy to go , however it isnt a direct path. When the points open ( or begin to open) it appears as a easy path to ground versus jumping the point gap

Leakage is a measurment of this path to ground. Got to have a calibrated level path for ground. (ground potential ? ):eek:
 
Frank Miller said:
Potential is another word for voltage. Current is amps and volts is the pressure that pushes them. If you have an open circuit, meaning no vurrent flow, and you measure 6 volts on your volt meter you have the potential to push a certain number of amps in accordance to the resistance of the circuit when it is closed. 12 volts will push twice as many amps. Hope that clears that up for you.
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I guess I don't understand what you are trying to say here. I always thought it was the opposite. To take the water flow analogy further, The total amount of water flowed is equivalent to watts. Watts is volts times amps. Thus, it takes half as many amps at 12 volts to produce the same wattage as 6 volts. Thats why 6 volt wiring harnesses use larger wire than 12 volt harnesses; the 6 volt harness has to carry twice the amperage.
 
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