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Checking a Condenser (Capacitor)

cobbadog

Registered
Hi everyone. I have a Lucas RS1 rotary magneto and the spark is not that bright. It actually does run the engine but the colour of the spark is weak. I was thinking that maybe the condensor was weak and I wanted to do a simple test to check it. I have read stories over time here on this forum that people would charge a condensor up and toss at someone and when they caught it they would get a shock :eek:. So the question here is; This magneto has had the condensor replaced before my time of owning it and it has a 'film capacitor as the condensor with the readings of 0.22 mfd and 600 volts.
Is it a simple matter of removing the condensor from the magneto and using a jump starter battery of 12 volts touch each end of the condensor then, ground out both ends of the wires and I should see a spark?
Cheers John.
 

Andrew Mackey

Moderator
Last Subscription Date
05/14/2017
Re: Checking a condensor

Take an Analog Ohm meter (one with a needle, not numbers), set for highest reading. The needle should jump up and then rapidly drop to zero. If the needle does not drop down, condenser is bad. If the needle drops, Let the condenser sit for about 30 seconds, and then retest. The needle should not move under ideal conditions. If it moves very slightly, the condenser may be marginal. If it moves a lot, or does not drop down when recharging, then it is bad. After charging, short the condenser. You should see a spark, as it discharges.

If you have a digital meter, again, set for highest Ohms, charge the condenser and let the condenser sit 30 seconds. Then short to discharge. You should see a spark. If you do not, condenser may be bad.
Andrew
 

Darxen

Registered
Re: Checking a condensor

Well a Capacitor "stores electrons" ! The more it can store,,the higher micro-frad number will be, or uf value. The voltage rating is its normal maximum working voltage..if you exceed that voltage,,the materials used to construct the capacitor can break down,or "arc thru" destroying or discharging the capacitor !

When you connect a DC voltage to a capacitor, with the - or negative lead to negative on the battery, or ground,,and then take the + or positive lead,,and touch it to say +12 volts on the battery, electrons will flow from the positive charge into the capacitor, till the capacitor charge is equal to the source,,or in this case say 12 volts. The "size" or Uf rating,,will determine how "much" or many electrons it can hold !! A 10,000 Uf capacitor will hold far mare than a 10 Uf capacitor !! The voltage rating,,is the maximum voltage it can hold,,before breaking down !

A highly charged capacitor can KILL a person !! A capacitor is what is used in a unit used to start a persons heart, defibrilator,shocks them,,like 15,000 volts or so..but at a low current. If you charge a capacitor up to say 50,000 volts..without a limiting resistor,,someonce can get KILLED....nothing to fool with !

When you use an "ohm meter" to test one,,what you are really doing, is putting a charge from the ohm meter into the capacitor,,and when that charge is going in,,,you will see an ohm reading,,on the meter,,then when the capacitor is FULLY charged,,the reading will go down to zero..how fast depends on the size of the capacitor !! A small one,, say 5 Uf, you will see a really quick blip on the meter,,and it is charged,,but say for a 1,000 Uf one,,it will peg the needle, or show a short,,then slowly come down, as the capacitor gets charged.
Once it is charged,,there is no more current flow thru the meter,,and no more reading,,it will show open !! Now if you WAIT say 5 seconds,,and REVERSE the leads,,as soon as you touch the second lead,,ZAP,you will see a big kick,,which is now discharging the electrons into the meter,,showing a short,,and then when it charges up again,,backward,,it will slowly read from a short down to what appears is an open,,that is when it is fully charged again !

If a capacitor is "leaky", it will self discharge fairly quick..and lose its charge ! Capacitors are made from several different materials..some with oil..and are "self healing" if they were to arc over,,or thru the dialectric !! this is good and protects the capacitor.

Capacitors usually last a long time,,easy to replace,,and if you do, use the same values as on the old cap! Also, observe polarity,,if it is marked !!

Good luck,,and don't play kid games with charging a capacitor and throwing it to someone,,if they have a pace maker,,you can KILL them,,it happens !! Most people dont't walk around bragging about they have a pace maker,,I have 2 nieghbors a Father and His Daughter have one,,heredity heart problems,,I never knew it,,until I was doing some ARC welding,,he backed away real quick,,then told me !! Thank God !

Take Care,,
 

Craig DeShong

Subscriber
Last Subscription Date
12/13/2019
Re: Checking a condensor

Nice reply Darxen and Andrew

All I'll add is that the coil and the condensor form a "tuned circut" and the capacitor rating (in micro-farads) is set by the electrical properties of the coil.

This is why you need to choose a replacement condensor with the same uf (micro-farad) rating as the original for the ignition system to work properly.
 

Elden DuRand

In Memory Of
Age
78
Last Subscription Date
12/22/2017
Re: Checking a condensor

Craig:

I also offer kudos to Darxen and Andrew for their 'splanation.

The 0.22 microfarad 600 volt capacitor, if good, will be a good replacement although, as a rule, just about any magneto or point-type "condenser" will work. From checking some of them, the capacitance ranges from about 0.15 to 0.47 microfarad at around 600 volts.

The oscillation of the coil/condenser circuit when the points open is called a "damped oscillation" or "ringing oscillation". Many years ago, what were called spark transmitters used ringing oscillators tuned to relatively high frequencies to transmit Morse code.

"Ignition noise" is caused by this oscillation as ignition systems act like little transmitters.

Now, do you feel fully informed??:) For myself, I feel much better having gotten that off my chest.

Take care - Elden:wave:
http://www.oldengine.org/members/durand
 

cobbadog

Registered
Thank you all for the very informative information. I will get my meter out and try it. As for installing it negative to negative and positive to positive, the new film capacitor is not marked one way or another.
So does that mean that it does not matter which way I install it?
Also, I do realise the horrible affects that palying with these things and I had no intention of playing silly buggas with it. It is just that when I tried to load the capacitor with a jump start battery I got no response when earthing both terminals (no spark). I certainly would not load it and throw it to someone. But I do thank you very much for the reminder of its ability.
Cheers,
John.

---------- Post added at 04:01 PM ---------- Previous post was at 03:44 PM ----------

I just went out to do the test using my old analogue multi meter (one with a needle). I did the test and with the meter on the highest setting the needle only just jumped and slowly went to rest. I left it 30 seconds as suggested and carried out the same test and the needle did not move. This was good. I tried bridging the two wiires after I remove the meter but I got no spark.
I redid the test and then I swapped the polarity and this did make the needle jump a small amoutn, about the same as when I first connect it.
Then I looked at what was written on my capacitor and I think I may have the wrong part. It reads 224K 630V.
I am not sure if 224K is the same as 0.22mfd. As I live in the country area of NSW we dont have many choices of electronic stores to visit and ask about getting the part I want.
So is 224K the same as 0.22mfd?
Cheers John.
 

JoeCB

Subscriber
Last Subscription Date
02/28/2019
This "capacitor" has a wire from each end and is marked 224k?
It sure sounds like you have a resistor of 224K ohm value and not a capacitor.

Joe B
 

Elden DuRand

In Memory Of
Age
78
Last Subscription Date
12/22/2017
John:

You're okay with that capacitor. When the value of capacitance is written as you describe it, you can translate it into something meaningful by noting that 224 means 22 picofarads times 10 to the fourth power.

In other words 0.000022 X 10000 = 0.22 microfarad.

This method of marking values was started when capacitors got too small to print .22 on the capacitor body. Actually, if you go to a value of 22 picofarad, there are too many digits to fit on the component.

You have a film dielectric capacitor, the same type as is usually used in automotove ignition systems. The only difference is the way it is encased. There may be a black band on one end of the capacitor you have but they are actually non-polarized and, for your use, you can hook it up either way.

film capacitors are made by taking a couple of long sheets of aluminum foil and separating them by an insulator then wrapping them up into a cylinder. The black band refers to the lead that is connected to the outside layer of foil. In some applications (radio, etc), the outside foil is connected in such a way as to eliminate or minimize electromagnetic or static interference with other parts of the device it's used in.

I think that should cover it. :)

Take care - Elden :wave:
http://www.oldengine.org/members/durand
 
Last edited:

Dempster

Registered
They weren't that hard for me to find even in Australia with Google search. Go with the ones that have they have with the 630V rating. .22 mfd should work fine in that mag but if you are getting excessive pitting of the points step it up to .33 or .39 or .47 mfd. The yellow metal film tubular type or the brown polypropylene ones they show should work fine.

http://evatco.com.au/webcat1p12.htm
 

piewagon

Registered
Capacitors (a.k.a. condensers) used for coil point applications can be a trap if you are not aware of the various types of capacitors that are out there. Most folks know that capacitors have a value (usually in microfarads) and a tolerance as to how far away from the marked value that the part can be. A film type capacitor commonly has a +/- 10% tolerance. Then there is the working voltage which for ignition coil operation is often 400V but many people buy a 600V part "just to be safer". The problem is that the "typical" part that they then buy at the local electronic hobby store is a fairly inexpensive common part that is a metalized dielectric type part. At the link furnished elsewhere in this thread - that would be the yellow tubular part. You will notice that it is way cheaper than the Sprague type 716 polypropylene part listed. The type 716 part is NOT a metalized type part and it also has a different dielectric being polypropylene. What does all this have to do with anything ignition related? Well my company makes Model T Ford coils and repro buzz coils for stationary engines too and if we used the yellow parts - they would go about 5 miles in a Model T and then quit while the type 716 part (which we don't use either) would likely never fail. It has nothing to do with the quality of the parts. What you need to understand is that there are other ratings besides value, tolerance, and working voltage. The most important rating of all with regards to capacitors used for ignition points, magnetos, and coils, is the dV/dT rating. Not commonly found in a catalog but published by the manufacturers data sheet, the specification definition refers to the terms dV and dT. The term dV means "change in voltage" and dT means "change in time". There is a rating as to how fast your can safely charge and discharge a capacitor. If you charge it and discharge it at too fast of a rate - it will heat up inside and quickly fail. The rating of dV/dT is a rating of the allowable change in the charge voltage of the capacitor during a give change in time. The rating on a capacitor that will work reliably in a Model T coil is more than 700Volts/microsecond. Written the common way is that the rating needs to be dV/dT=700 or greater. You will find that the type 716 capacitor at .22 and 400Volts is well over that spec while the yellow metalized part likely doesn't have any rating at all for that. Why no dV/dT rating? Because it is not designed for use in applications that have really high charge and discharge currents as happens when the points close and force the charge in the ignition capacitor to ZERO in almost zero time. That is a huge pulse of current with very high dV/dT.

Unfortunately the most common capacitors you can find at .22uF and 400 or 600V are metalized type and their dV/dT ratings if published are probably about 25-30 which is at least a factor of 20 less than necessary hence they don't last long if you try to use them. Usually the value of the capacitor is not too critical nor is even the working voltage if you get at least 400V but ignore the dV/dT rating and you will get burned. Its a lot of work do dig the tar out of an old coil and then install a new but wrong type capacitor in there that only works for a few minutes of running.

I am new to your forum and only wish to help but this is a trap that a lot of coil rebuilders have fallen into. I could post a link to my company's web site so you could read more about this but not sure if it is OK to post web site links so I didn't do it here. We have capacitors with high dV/dT that are reasonably priced but I think in Australia it would make sense to find a source local.

Hope this makes some sense.
 

Dempster

Registered
Capacitor or condenser pretty simple either works or does not. These smaller capacitors are not going to hold a big charge like a large capacitor in a BIG electric motor. Check it for leakage, capacity, series resistance if fails any of those test replace it. If you don't have a way to replace it and got weak orangish yellowish spark instead of a snappy hot blue spark replace it after all it is probably the least expensive thing to replace on any magneto.

Page 31 of the FM magneto fundamentals booklet shows basic condenser construction.

http://www.oldengine.org/members/diesel/Magnetos/MagFmBook.htm
 

brknwng

Registered
I found you guys looking for a solution to my problem. I have an old 1962 Ironhead Sportster 'magneto ignition' that I was having problems with. I want to thank each and every one one of you for your input into this thread because I learn more from you guys in one day than I have from HD in 30 years. Thank you very much! My bike bike runs better than it ever has. Mechanicals I know but, I'm a little shy on the ignition side. thanks again! Glad to be here.
 
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