About Capacitors

[Ralph Leonard]

Hi, 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 seperated by insulating material such as paper. Engine capacitors are usually two long strips of aluminum foil about an inch wide sandwitched 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, neg electrons on one plate and pos 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. They will remain in this charged condition even when disconnected from the battery. A capacitor stores elect 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 oposite polarity. How can you tell if its good? If you have a ohmmeter connect the test leads together and zero the meter. If the meter wont 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 took the charge. And the prompt movement back to full left means there are no leaks. A leak means there is a small amount of current flowing thru a high resistance bridge across the plates. Did we drive into a patch of fog here? Ohmmeter a problem? Ohmmeter is nothing more than a meter and a battery and the appropriate resistance connected in series. You send current from the battery thru the meter, some resistance and your project. The resistance controls how much and the meter measures it. It should be noted here that even your body circuit will interfere with this test. Dont touch the conductors. Repeat the test while reversing the test leads each time. Test several known good capacitors to establish a reference. Your ohms range dont go that high? Needle movement to small? In the lower ranges this happens to fast for the needle to follow. Add resistance to your test lead until the needle gets on scale. You have a volt meter only? Connect one of your test leads in series with a battery and enough resistance to get on scale reading. After you set your reference readings rember to use the same componants in the future. Capacitors have a important characteristic. When charging, the current builds up on the plate fast but the voltage builds up a little slower. voltage lags the current. This is a important feature when used---guess where!! We'll save that for another day. Not a scientist or engineer, just a BellSouth retiree. I apologize for the long post and I hope this info is helpful to someone.

Ralph in NC

 

[Mark]

Thanks Ralph. A couple of times, I've been doing stuff like trying to fix our microwave where I had to discharge a capacitor. Now I sort of know a little bit about what I was doing.

 

[shermwolf]

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 intermitant 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 mosture migrate into the can (mostly due to temprature 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 surges cause 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 intermittat connection symptom described above is about to cause an an intermitant 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 ingnition 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 you points are clean and gapped correctly replace the capacitor next.If the magneto does'nt work the suspect the coil.

Keep in mind that you will occasionally find a defective (new) capacitor. In my shop I have known good capacito 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 sustitution method is the best way to identify a faulty capacitor.

 

[Simon Thomson]

Hi Ralph great explanation. As you will know its safe enough to carry out tests on automotive capacitors (condensers) at low voltage but as you know capacitors can be downright lethal at higher voltages. I've added a link to the Tesla coil web ring which has hundreds of pages dedicated to literally making your own lightening. Each site has details on capacitors and how they work (some in great detail and a lot of them go over my head). Theres also detailed explanations on coils and sparks etc. I'll add some more links to a couple of the best pages. Be carefull and read the warnings on each page. Regards Simon.


Tesla coil web ring

 

[Simon Thomson]

Hi Mark. For Gods sake and your own do not touch the insides of a microwave oven unless you are absolutely certain you know what your doing. A microwave oven transformer (mot) has no internal current limit and the ht coil is connected to the core of the mot. The ht voltage is about 2kv with no current limit which will kill you outright in a microsecond. Click on the following link and read the warnings. This page has explanations on mots, obits (oil burner ignition transformers), capacitors and links to other sites which have far better pages and explanations. Be carefull. regards Simon.

ps IM in no way criticising you i just dont want you to get hurt. Before i got an interest in Tesla coils i would not have thought twice about playing with a microwave oven but after reading the pages i wouldn't touch one to repair it i'd just get a new one.




High voltage page.

 

[Jessee Zilafro]

Ralph what is the differnce between a capacitor and a condenser? Jesse Z.

 

[Simon Thomson]

The name. Thats it. they are both names for the same component. Simon.

 

[Russ Hughes]

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 condensor (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 metalic 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 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 millionith, or micro-micro Farads, one millionith 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 magnetisim.

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 aproximately 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 interchangably with the term "capacitor" being the prefered one to use today by those who are in the know.

Incidently, 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 supplys 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 embarassing shocking experience.

 

[Jessee Zilafro]

Thanks for the education Russ. The reason I ask the question is because I thought that the radiator looking thing in front of my radiator on my car was the condenser for the air conditioner. Have a good day. Jesse Z.

 

[Ralph leonard]

Thanks everyone for the great response, I was beginning to think no one cared about this subject.--- My wife told me to be careful what I put on here, I would be pegged as a know-it-all. I thought to myself, that can't be true, every time I open my mouth I prove it.--- I agonized over the lengh of the post, so I decided to leave out the warning about discharging large capacitors (which was a big mistake), and also Mike Faraday and the unit of measurememt.--- I use the name capacitor because in formulas it is represented by the letter c whitch stands for capacitance and its opposition to current flow is called capacitive reactance.--- But enough of that, my intent here is to help the beginner have a better understanding of his engine and be able to make some simple electrical test.--- Thanks a million for the microwave oven warning.---I wish I knew how to keep my post from bunching up.---Love this site Harry, Ralph in NC

 

[Harry]

>> I wish I knew how to keep my post from bunching up.

I presume that you mean, "keeping your post from being all in one paragraph?"

Easy, just hit the "Enter" key an extra time between paragraphs. There are also a whole bunch of page formatting commands at the tail end of the Smiley page.

While we are discussing capacitors, I'll place a link to the page below...

http://www.old-engine.com/magneto.htm

 

[Ralph leonard]

Your presumption was right, Harry

I'm trying it now

Thanks again, Ralph in NC

 

[Russ Hughes]

Jessie, it is, you were correct. It condenses a gas to a liquid. Fortunately it doesn't capacatate it into anything

...

 

[Jessee Zilafro]

Russ you just made my day!! Thanks Jesse Z.

 

[Eugene]

Okay so now back to the workbench. If the condensor in a Wick 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? Thanks in advance.

 

[Simon Thomson]

Hi Eugene. Capacitors from your local electronics supplier (I think Radio Shack is the most common Stateside) will work just as well but just remember they do not have the mechanical durability of automotive condensers.

Axial electrolytic capacitors are what you need. The most common automotive value that I've come across is 0.2 microfarad so aim for one at around that value.

They come in a range of working voltages but you need to go for as high a voltage rating as you can get without spending silly money. Generally they cost a few pennies each to buy singly or maybe a dollar for a pack of 10 or 20 or 30 or more.

The main drawback is durability. They only have thin single strand leads which can not handle mechanical vibration as automotive capacitors can. So carry a good supply with you and be prepared to change them on a regular basis as the leads will break with vibration.

Also have you considered using suppressors that are fitted to alternators and car radio and CB power cables to eliminate noise. They are capacitors as well and would be robust enough for what you need.

0.2 microfarad is only a suggested value as your engine can actually run slightly better or worse depending on the value so experiment, It will not harm your engine. Aim for anything from about 0.05 to about 0.6 microfarad.

Dont be afraid to try a non electrolytic capacitor as well and dont be afraid if a capacitor pops. just try a different value.

Keep us posted of your results.

Also if you reset points in say a distributer you might find a slight capacitor value change will improve engine performance. The emphasis here is on experimenting to find the optimum value.

Good luck. Simon

 

[Russ Hughes]

Electrolytic capacitors are polarized devices. The insulation quality depends on the polarity being correct. Reverse polarity can damage an electrolytic capacitor.

A capacitor for an ignition application should be a non-polarized type. A capacitor for an automotive, or ignition service is constructed a lot more rugged than one designed for electronic service. I doubt that Radio Shack would normally carry capacitors that would be ideal for ignition service.

That is not to say that a capacitor designed for electronic service wouldn't work, only that as was said before, it probably wouldn't last very long before it suffered mechanical failure.

Something to think about. Most ignition systems are very much alike in their requirements, and in a pinch, any capacitor designed for ignition service, be it magneto, or spark coil, will probably do the job electrically. The problem mainly is being sure that the capacitor mechanically fits the application. Many capacitors were designed to fit into a specific space and finding one to fit in tight quarters could be a problem. If you can install the capacitor on the outside of the distributor, or magneto, then size id not such a problem. Only the looks, and durability becomes of concern.

Just some thoughts.

...

 

[shermwolf]

Russ, you are right about electrolytic capacitors being useless for ignition point use.

My experience is that that capacity of both magneto and spark coil capacitors range from 0.2 microfarard to 0.33 microfarads. Almost all automotive distributor coils use a 0.25-0.29 microfarard capacitor. The operating volatge 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.

 

[Eugene]

Ralph, Sherm, Russ, et'al. Thanks for all the info. A friend of mine was storing some NOS 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.

 

[Russ Hughes]

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 ordanarly occure, which would eventually burn out the points among other reasons.

I have even used a disc ceramic capacitor in a pinch in place of a regular automotive capacitor. I think I used one with a 1600 volt rating for this for a while until I was able to obtain the correct physical replacement.