[b]8N Capacitor Questions[/b]

Soundguy · Mar 27, 2006

Re: [b]8N Capacitor Questions[/b]

I view 8v batteries in 6v systems as horrible bandaids, if done incorrectly. Usually you get an 8v battery that is undercharged.. or if you can tewak the charging system to run it.. you get reduced life on the lamps, and more primary coil current. In the case of ford frontmount tractors, if you are running an 8v system, my recomendation is to get one of the new 12v coils, and run that coil, with no other resistance.. ont he 8v setup... probably work fine. Those front mount 12v coil generally need 1/2 ohm of resistance inline with them to run on a 124v system.. but on the 9-10v system they are just about right. have to remove the ballast resistor as well.

as for caps and voltage rating.. capacity of the cap will make more of a difference than the working votlage, compaired to the max voltage. Max voltage on those automotive caps will be in the neighborhood of 60-120-200vdc. If a cap had a working voltage of 6v.. it should still be suitable for 12v apps. All my tractors that were 6v that have been converted to 12v are still running the same caps that come in the 6v tune up kits.

For round coil sidemount system that were designed for 6v, and run on 8v.. I think I'd expirement with a little inline resistance.. probably in .1 - .6 ohm range, and see what I could do about points life, vs spark quality. Ventalated points, if available, can help too.

As for time on, with ignition not running.. I wouldn't want any. When the ignition engine is off.. turn off the key. Obviously a few seconds / minutes won't hurt. but remember the contact surface are of the points is small, and they are conducting up to 4a.. and that will start heating the primary of the coil. These are meant for duty cycle.. not steady state...

Soundguy

Keoke · Mar 27, 2006

Re: [b]8N Capacitor Questions[/b]

Fred, most automotive Kettering ignition capacitors are rated to handle up to a 450v spike voltage induced back from the secondary winding into primary winding when the pts open.
Soundguy makes a good point on just what is his primary current flow on this 8v system? Anything more then 4 amps here is bad news. A 12v system usually has 1.5 ohms on the ballast resistor and another 1.5 on the primary winding for a total of roughly 3.0 ohms that will give about a 4 amp flow.
A 6v system does not run a ballast resistor and has about 1.5 ohms in the primary winding only for again 4 amps.
This 8v oddball may be happy with two ballast resistors (1.5 ohms each) wired in PARALLEL for a total resistance of .75 ohms. These two resistors in series with the OEM 6v coil (1.5 ohms) will give a total resistance of 2.25 ohms. So, an 8 to 9.5v primary voltage with 2.25 of primary resistance in series should have 3.55 to 4.22 amps running through it's coil and points. I am guessing that his generator is running around 9.5v to chg the 8v battery.
Bottom line, we need to insert a digital amp meter in the primary circuit and see what the amp flow is while engine is running at mid speed to remove the guessing game on primary current flow.
If that number is somewhere between 3 to 4 amps all is ok. If it is higher then current flow is too high and the capacitor is being subject to much higher spike voltages on primary collaspe then designed for and thus dying an early death.
The failure is due to the wax paper insulation between the plates breaking down (puncture) due to higher then designed for spike voltages.

Almost forgot, but the polarity question somebody raised should also be checked out. Regardless, as to whether you have a pos or neg gnd system the voltage at spark plug should be of negative value. Quick check here is to take a sharp pencil and place point in an artifical gap between plug wire tip and and spark plug with engine running (don't get zapped). Look for a flare coming off the pencil lead, if flare is on the plug side secondary polarity is correct if on the wire side, incorrect.
Voltage required to bridge (ionize) plug gap will be 30 to 40% higher if polarity is incorrect (pos) but plug will still fire in most cases however coil has to push (emf) harder.
Electrons (spark) will flow from a hot object (center electrode) to a cold object (gnd electrode) 30 to 40% easier with a neg to pos push.

lots of luck,

Soundguy · Mar 27, 2006

Re: [b]8N Capacitor Questions[/b]

Yep.. good old edison effect.. or more modernly refered to as thermionic emission..

Soundguy

( engine running (don't get zapped). Look for a flare coming off the pencil lead, if flare is on the plug side secondary polarity is correct if on the wire side, incorrect.
Voltage required to bridge (ionize) plug gap will be 30 to 40% higher if polarity is incorrect (pos) but plug will still fire in most cases but coil has to work harder.
Electrons will flow from a hot object (center electrode) to a cold object (gnd electrode) 30 to 40% easier with a neg to pos push.

)

Fredex · Mar 28, 2006

Re: [b]8N Capacitor Questions[/b]

George: We are generally thinking alike on this; 8v = more current w/o ballast = heavy kick to the cap = possible trouble.
I mentioned the battery polarity. And now that you've discussed it further I have another possible. Coil polarity. It's been years, but I think these things are connected internally like an auto-transformer; that is, one end each of primary and secondary are connected internally to one of the terminals. The other low voltage terminal is connected to the other end of the primary winding. Again check my memory, that is the terminal that goes to ground. If the low voltage coil terminals are reversed, that "common" terminal will be above ground and thus also get some kick-back voltage from the secondary - BAD news for cap and points. I'm going to have to dig our my dad's 1940 Automotive Electrical System book. They didn't teach us a lot about ignition systems in electronics school.
Fred

Dave_Lilly · Mar 28, 2006

Re: [b]8N Capacitor Questions[/b]

This thread has had a lot of replies about burned capacitors, but you may just be having a switch problem. I used to be the tractor guy for my brother-in-law. He started having problems getting his Cub Low-Boy started and keeping it running. He called me the first time he had a problem starting it and after much diagnosis I determined that he had a bad capacitor. We replaced the capacitor and all was OK for a short time. When he had problems again I went back to look at the Cub and once again it looked like a bad capacitor and a replacement fixed the problem. He wound up buying capacitors by the box and when he had trouble with his Cub he would replace the capacitor. Sometimes he would only run for 50 feet and the tractor would quit. A capacitor replacement would fix it for a while. Long story short, I finally found out that the pull on/push off ignition switch was making intermittent contact. A replacement ignition switch fixed most of his problems. The only problem he had left was what to do with his boxes of good capacitors.
Sometimes these old tractors will drive you nuts trying to get them to run. I had another brother-in-law who was having problems getting his 9N started, but that's a whole nother story.

Keoke · Mar 28, 2006

Re: [b]8N Capacitor Questions[/b]

Fred, you are right in that one end of the seconcary winding is connected to one of the internal primary terminals. If the coil is used with a neg gnd system then it's the pos terminal (battery end). If coil is made to be used on a pos gnd system then the neg terminal (still bat end) is tied to one end of the secondary. The other end of the secondary winding is tied to the coil tower tap. An ohmeter check from coil tower terminal to either of the two primary terminals will tell which one is internally tapped to the secondary. As you well know turning off current quick on any coil (even a relay coil) will cause cause a spike voltage. However, in the ignition coil, if this voltage spike is run thru the primary winding first it will dilluted before getting to the cap. So, you are right on the mark when you talk about getting the cap wired to the right side of the coil prirmary winding. Since one side of the coil primary will be dealing a higher voltage spike then the other we had better get the cap tied to the low spike side of the primary winding.
Bottom line, don't get the battery side of the coil (internally wired to secondary) hooked directly to the cap. This means on neg gnd systems plus terminal goes to battery and minus goes to breaker pts (gnd).
If you are using a pos gnd system, and since most coils are made to work with neg gnd systems, you better get your ohm meter out and check to see which primary lead is internally tapped to secondary winding. And then make sure that this is the guy that goes to the battery feed (ignition sw).

PS. on Dave's cap killing tractor, it is possible that every time his ignition switch went into an unintended open circuit (a good shake) he got a primary spike that took out the cap. A new switch that kept the circuit from going open, when not intended, may have fixed the problem for him. This, of course, is only a guess.

cheers,

Dave_Lilly · Mar 29, 2006

Re: [b]8N Capacitor Questions[/b]

The "cap'"killing tractor was not killing the capacitors. The pull on/push off switch had internal corrosion that would sometimes let the tractor start, sometimes not. If it did start the vibration from driving the tractor would shake the switch enough to reach a corroded, non-conductive area that basically turned the tractor off, due to the corrosion not being conductive. It did not "kill" any capacitors, but would just turn the tractor off because the switch vibrated to the non-conductive, corroded part of the switch. I accidentally discovered the problem when I had a volt meter connected to the ignition. When bro-in-law pulled the on/off switch there was no meter reading for voltage. The light in my head lit so I had him pull/push the ignition switch while I watched the meter. Sometimes I got a reading and sometimes I didn't. I told him to replace the switch and he never had a "cap" problem after that.

Keoke · Mar 29, 2006

Re: [b]8N Capacitor Questions[/b]

Hi Dave, I agree with you that the intermittent ignition switch surely could have been killing off the capacitors. Every time that ignition switch went open (unintended) it could cause a spike voltage (more the 450v) to the capacitor. This not good for a cap rated to handle up to 450 volts. An ignition coil will fire every time flow of current through the primary is interupted. It does not matter whether this interuption (open) is on the key side or the points side of the primary. However, the reason an unintended break on the input (ignition sw) side of the primary (collapse of primary winding) causes so high a spike voltage is because the rotor at that point is not aligned with the dist. cap terminal so the high secondary voltage has no way out of coil. Accordingly, this high voltage (up to 30kv) inductes back into the coil primary winding (capacitor) and burns out the insulation (wax paper) between the cap plates.
Of course when the points break the primary circuit (intended) the rotor is aligned with dist. cap terminal allowing the high secondary voltage to continue on to the plug and bleed out.
Dave, please give the last paragraph on my previous post about your brother-in-laws tractor a second read.

take care,

Dave_Lilly · Mar 30, 2006

Re: [b]8N Capacitor Questions[/b]

A very articulate response but you ignore a basic situation. A bad ignition switch that makes intermittent contact that kills the ENGINE is no different than manually turning the tractor off. After I found out what was the real problem with my brother-in-law's tractor, I went back and checked the "killed" capacitors with an ohm meter. If a cap is burned, or "killed", you will get a zero reading on the ohm meter. A good capacitor will show an immediate jump in resistance then fade back to zero after it is charged. He kept all of the "bad" capacitors, so I went through all of them with the ohm meter and they were all good capacitors. A capacitor serves one purpose and that is to absorb the voltage induced when points open and the electromagnetic in the coil field collapses. If you have a bad capacitor you will get a continuous spark across the points, firing cylinders randomly and burning up the points.

Soundguy · Mar 30, 2006

Re: [b]8N Capacitor Questions[/b]

( If a cap is burned, or "killed", you will get a zero reading on the ohm meter )

0 reading on an ohm meter indicates a short.

( A good capacitor will show an immediate jump in resistance then fade back to zero after it is charged. )

Actually a good cap will show an initial 'short'/low resistance.. then gain resistance as it charges up to near infinity.. or.. depending on how good your meter is.. untill the m-ohm scale is reading the leakage of the dielectric material.

There is also another failure mode.. 'open'. That is a plain 'infinity' reading.. with no needle swing.

I think you got your 0 and infinity ideas backwards with respect to meter swing. Ohm meter starts as 'open circuit', on infinity. When you touch the leads to calibrate/'zero' it, that is when you read zero. When there is a 'short'. The initial current flow of the cap charging is seen as the short'low resistance measurement. as the cap charges, it's resitance rises.. etc. This is all for dc based electronics.. when you get to ac and capacitors.. it all changes due to charge/discharge, equating to constant bi-d current flow... etc...

Soundguy