About Capacitors


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A recent topic on the SmokStak bulletin board at brings up an item that is responsible for spark plug ignition failure -the capacitor.

I would like to offer my help to those folks who may not understand capacitors, also commonly called condensers.

First off, what is a capacitor? A capacitor consists of two metal plates separated by an 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, then rolled up and placed in a metal case. One plate is connected to the case and the other to a lead out wire. Fine, but what does it do?

When a capacitor is connected to a voltage source, such as a battery, it acts momentarily as a closed circuit as 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 stops and the capacitor acts as an open circuit. The plates remain in a charged condition even when disconnected from the battery. A capacitor, therefore, stores electric power just as a battery does.

But, if a circuit path is connected to the plates the capacitor will discharge its stored energy through that path. And, if the voltage polarity is reversed it will recharge to the opposite polarity.

So, how do you test a capacitor to see 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 its battery. Select the highest R times range you have; R times 10,000 works well. Connect the test leads to the capacitor while watching the meter. Movement on the scale means the meter is taking the charge, and prompt movement back to full left on the scale means there are no leaks in the capacitor. A leak is a small amount of continuous current flowing through 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, and it's a good idea to test several known good capacitors to establish a reference. - Ralph

Good post. Magneto condensers (capacitors) have several failure modes that a simple ohm-meter 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 there is moisture in the capacitor and sometime soon the intermittent connection symptom described above is about to cause capacitor failure. Assuming the capacitor shows capacity, leakage is the most telling indication of its condition. 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 AC to the capacitor and measuring the AC current flow through the capacitor. These two tests stress the capacitor in much 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 you will occasionally find a defective (new) capacitor. I have a known good capacitor set up with two clip leads. I hook it 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 scientists originally thought 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 'condensers.'

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.

Most common capacitors are of less value than a 'farad' and are thus rated in microfarads, one millionth, or micro-microfarads, one millionth of one million. The term 'micro-micro' in the metric system is known as pico, as in picofarad. 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, may, for comparison purposes, have an effective capacitance of approximately one farad.

In any case, a condenser (old term) and a capacitor (present term) mean exactly the same thing and can be used interchangeably, with the term 'capacitor' being preferred.

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, under the right conditions, self store a charge sufficient to kill someone. Always store capacitors, particularly large ones, with the terminals shorted together to prevent self charging. Always discharge a capacitor before working with it. -Russ

If the condenser 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 condenser? The electronics supplier has hundreds of capacitors. Can anyone provide a list of what

capacitors would be correct replacements for the condensers in the most common magnetos? -Eugene

My experience is the capacity of both magneto and spark coil capacitors range from 0.2 microfarads 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

Thinking in terms of a low-tension igniter system, when the igniter opens and 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. - Russ

I would like to expand on Russ' 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 SmokStak is an engine conversation bulletin board with over 15,000 messages on file and is part of the Old Engine series of web sites that started in 1995 as 'Harry's Old Engine.' Harry Matthews is a retired electronic engineer and gas engine collector from Oswego, N.Y., now residing in Sarasota, Fla.

'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.'