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About Capacitors in Ignition Systems

I would like to offer my help to those folks that may not understand capacitors also called condensors. What it is; two metal plates placed near each other and separated by insulating material such as paper. Engine capacitors are usually two long strips of aluminum foil about an inch wide sandwiched between layers of waxed paper, rolled up and placed in a metal case. One plate is connected to the case and the other to a lead out wire. What does it do? When connected to a voltage source such as a battery it will momentarily act as a closed circuit while current flows to charge the plates, negative electrons on one plate and positive on the other. When the voltage across the plates builds up to the battery voltage, current flow will stop and it will act as an open circuit. The plates will remain in this charged condition even when disconnected from the battery. A capacitor stores electric power just like a battery. If a circuit path is connected to the plates it will discharge thru that path. If the voltage polarity is reversed it will recharge to the opposite polarity. How can you tell if it’s good? If you have an ohmmeter, connect the test leads together and zero the meter. If the meter won’t zero replace the battery. Select the highest R times range you have, R times 10,000 works good. Connect the test leads to the capacitor while watching the meter. The movement on scale means it’s taking the charge. And the prompt movement back to full left means there are no leaks. A leak means there is a small amount of continuous current flowing thru a high resistance bridge across the plates. Don’t touch the conductors because you will read leakage through your hands. Repeat the test while reversing the test leads each time. In the lower ranges this happens too fast for the needle to follow. Test several known good capacitors to establish a reference. Not a scientist or engineer, just a BellSouth retiree and I hope this info is helpful to someone. -- Ralph

Your post is correct and of interest. Magneto condensers (capacitors) have several failure modes that a simple ohmmeter test will not identify. The first is the intermittent failure of the leads that connect to the foil capacitor plates within the capacitor. If you have ever taken an older automotive or magneto capacitor apart you will find that the connections are made by pressure on the two ends of the capacitor by a flat spring of some type at the bottom of the capacitor can. Over time, minute amounts of moisture migrate into the can (mostly due to temperature cycling) and corrode the connections. The result is an intermittent connection that breaks down under the charge and discharge cycling of the capacitor. What seems to happen is when the capacitor is operating in an ignition system the current surge causes a temporary weld of the connection, and when the capacitor sits unused the connection breaks down again. This is a very difficult symptom to identify with any simple tests. The second mode is leakage. I have found that leakage of less than a megohm indicates that there is moisture in the capacitor and sometime soon the intermittent connection symptom described above is about to cause a failure of the capacitor. Assuming the capacitor shows capacity, leakage is the most telling indication of the condition of the capacitor. Any leakage at all tells you to replace the capacitor. The old ignition capacitor checkers tested the leakage at 500 to 600 volts and would tend to break down the capacitor if there was any potential for failure. They also checked the capacitance by applying 50 to 100 volts alternating current to the capacitor and measuring the AC current flow through the capacitor. These two tests more closely stress the capacitor in the same way actual operation does. What this all means is if your points are clean and gapped correctly replace the capacitor next. If the magneto still doesn’t work then suspect the coil. Keep in mind that you will occasionally find a defective (new) capacitor. In my shop I have known good capacitor set up with two clip leads that I hook up to the points after removing the connection of the original capacitor one lead to ground and the other to the contact point. This substitution method is the best way to identify a faulty capacitor. – Sherm

I read someplace, that way back when, the early scientist originally thought that the electrons sort of "condensed" much the way steam does, inside the Leyden jar that was originally used as a capacitor. A Leyden jar was a glass jar with a layer of foil on both the inside and the outside surfaces of the jar. The glass jar served as the insulating medium between the two metallic surfaces. The connection to the inside surface was usually made by a small length of chain that contacted the surface of the inside conductor. The chain was attached to a rod, usually with a small ball on the top of it that stuck out of the top of the glass jar. You could charge the Leyden jar from some source of electricity, usually a static electricity generator of some sort. The Leyden jar could be discharged across a spark gap to show that a charge of electrons was built up. In any case, they called these things and their later developments of the same nature "condensors". It wasn't until long after the real operation of the device became clear that the name was changed sometime in the 1930's or 1940's to "Capacitor". This was because the device had the ability to store electrons and thus it had a "capacity" of one value or another expressed in Farads. The device condensed nothing of an electrical nature in its operation.
Most common capacitors are of less value than a "Farad" and are thus rated in micro Farads, one millionth, or micro-micro Farads, one millionth of one million. The term "micro-micro" in the metric system is known as pico, as in pico Farad.
The term Farad came from the name of the scientist Michael Farady (1791-1867) who discovered the capacity effect along with a host of other early developments in the new field of electricity and magnetism.
The value of a "Farad" is equal to the amount of one coulomb of charge for each volt of applied potential. A car battery, while it is not a capacitor being an electro chemical process, never the less, may have an effective capacitance of approximately one Farad for comparison purposes.
In any case, a condensor (old term) and a capacitor (present term) mean exactly the same thing and can be used interchangeably with the term "capacitor" being the preferred one to use today by those who are in the know.
Incidentally, a capacitor in good condition can self charge from the static electricity in the air. A large capacitor, such as found in commercial xenon flash lamp power supplies can self store a charge sufficient to kill someone under the right conditions. Always store capacitors, particularly the larger sizes with the two terminals shorted together to prevent self charging. Always discharge a capacitor before working with it to protect yourself from an embarrassing shocking experience. – Russ

Okay so now back to the workbench. If the condensor in a Wico EK magneto no longer works I can replace it with a capacitor. What should the rating of the capacitor be, to match the old condensor? The electronics supplier has hundreds of capacitors. Can anyone provide a list of what capacitors would be correct replacements for the condensors in the most common magnetos? – Eugene

My experience is that that capacity of both magneto and spark coil capacitors range from 0.2 microfarad to 0.33 microfarads. Almost all automotive distributor coils use a 0.25-0.29 microfarad capacitor. The operating voltage rating should be no less than 600 volts DC. There are now capacitors available in the 0.25 microfarad range rated at 630 volts AC. Since the AC peak voltage is 1.4 times the nominal value these capacitors are actually 880 volt DC capacitors making them ideal for ignition use. I have used these capacitors inside of Bosch magnetos to replace the mica capacitors inside the armature. They can be soldered in and anchored with RTV sealant. – Sherm

Thanks for all the info. A friend of mine was storing some N.O.S. condensors so when the need arose he would have the correct replacement. Wrong! When he started to check them most were dead. So replacing old magneto and buzz coil condensors with a new 0.25 microfarad capacitor rated at 630 volts AC makes sense to me. Not to mention cost, availability, and less storage space required. Thanks again. – Eugene

When you think in terms of a low tension ignitor system, when the ignitor opens, and the spark occurs, there has to be quite a bit of voltage to continue jumping the gap. The collapse of the magnetic field in the low tension coil is similar to a high tension coil's primary. The capacitor across the points is used to suppress the arc that would ordinarily occur, which would eventually burn out the points among other reasons. -- Russ

I would like to expand on Russ's mention of high tension ignition. When the points open the capacitor is connected in series with the coil. The voltage/current generated by the collapsing magnetic field charges the capacitor. This is the voltage/current source that is induced into the stepped up secondary winding. A coil opposes current flow while a capacitor enhances current flow. The enhancement of the capacitor balances out or cancels the opposition of the coil. The result is a faster collapse of the magnetic field and the highest possible voltage generated in the primary circuit. It's true that the capacitor reduces or prevents arcing at the points but its main function is to provide a circuit path for the coil after the points open and to speed up the collapse of the magnetic field. – Ralph

For additional internet reference on battery and coil, buzz coil and magneto ignitions, see:


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