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Technical Condenser tech

Discussion in 'The Hokey Ass Message Board' started by 57 Fargo, Oct 20, 2018.

  1. I see all these threads about failed condensers causing drivability issues. My question is, how do they fail in such a way to cause problems? Now, to the best of my understanding the condenser, which is a capacitor, is there to absorb voltage induced back into the primary windings when the points open and the magnetic field collapses , this is to prevent arcing at the points causing premature wear of the points. The condenser is in parallel with the points so I guess my question is, are they developing continuity between the “ plates” in the capacitor not allowing the coil to saturate? I am curious if anyone knows what goes wrong inside one when they fail. These are the things that keep me up at night!!


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  2. Not sure what happens but a quick check on a multimeter would confirm.

    Only two possible options either a short ( plates touching)
    Or
    An open ( plates some how move and the gap between the two is to wide for the voltage to jump)

    I have been collecting old ones I find at swap meets etc the last little bit.
     
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  3. An open shouldn’t cause drivability problems in my head, should cause the points to wear out quickly. I understand how to test them and how they work, I’m wondering how they actually fail. I have never actually had one fail! If anyone has a failed one I guess it should show continuity through it if it’s shorted. Am I missing another scenario that could cause problems?


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  4. A capacitor (condenser) has 2 plates, (conductor, usually aluminum), separated by an insulator (dielectric), which could be Mylar, polyester, etc. When a capacitor develops a short, it may be a direct low-resistance short (metal-metal, where the voltage has jumped or punched a hole through the dielectric, and the resulting "splatter" has left little bits of metal in the dielectric to conduct between the plates, and will show as a short with a multimeter), or the dielectric may break down (partially conducting) only when a high voltage is applied to the plates. This type of breakdown will not be seen with a normal multimeter, as it only puts low voltage DC across the plates. When the capacitor is in the circuit (across your points), it is subjected to many kilovolts, and this is when the dielectric breaks down.
     
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  5. squirrel
    Joined: Sep 23, 2004
    Posts: 48,710

    squirrel
    Member

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  6. tubman
    Joined: May 16, 2007
    Posts: 5,641

    tubman
    ALLIANCE MEMBER

    Both gentlemen above are correct. The case of cheaply made, off shore units should be solved by being careful when purchasing them. The problems "36roadster" described are where the difficulties really lie. When my partner and I developed our replacement "Trash Can" condensers, we found that the temperature and (even more so) the voltage tolerances of these units need to be much higher than one would ordinarily expect. Anything with a voltage rating less than 600 volts would eventually fail in extended use. We ended up using a capacitor rated to withstand temperatures of 200 C and voltages up to 1000. There are capacitors with a higher voltage rating than this, but as the voltage tolerance increases, the size increases and they will no longer fit in our case. We check periodically to see if any new units have been released, as we would like to build our units to tolerate voltages approaching 2000, but there is nothing out there yet. We have dozens of these units in the field without a single report of failure.
     
  7. Thanks guys! Squirrel, I shit the bed and didn’t even think of doing a search, thanks for the link!


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  8. Truck64
    Joined: Oct 18, 2015
    Posts: 4,631

    Truck64
    Member
    from Ioway

    There are different capacitor construction techniques and materials, is something to be aware of. The voltage rating and capacitance might be the same, but they may not be suitable for severe service. The key metric here is what's called pulse rise time or dissipation factor, known as dV/dt. I dug into this a bit online and found the Model T guys had plowed this ground well already.

    The T used the wooden box coils and had trouble with the tar encased condensers after all this time. There isn't a lot of room so it's tempting to use the more compact modern capacitors that are made with a thin foil that is spray deposited with metal. These don't hold up unfortunately. There's a lot of current flowing through an ignition condenser, so they need to be fairly chunky in size because of the construction. I experimented with an "orange drop" 715P or 716P polypropylene film in a Ford points distributor, seems to work just fine. The primary traces look great on an ignition scope.

    What I did notice messing around is points spark more than I thought they would, when looking at them with the cap off. It's a white spark, maybe if it's blue it is hotter and degrades the points quickly? I thought the whole idea was to completely suppress arcing at the points, but some arcing occurs, even with a good condenser. I guess. The more I learn the more I don't know sometimes.
     
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  9. squirrel
    Joined: Sep 23, 2004
    Posts: 48,710

    squirrel
    Member

    There are a lot of considerations with condensers...the capacitance, voltage rating, the current carrying capability, but also the packaging. The packaging problem seems to be the big thing to worry about with normal replacement units. If you can meet the other specs with a non-automotive capacitor, it should be fine. I have had decent luck with Cornell-Dubilier .22 uF at 1000v axial polypropylene film capacitors.

    (note the subtle change in terminology...they used to be called condensers, when our cars were made, but now are called capacitors, in the electronics trade. both mean the same thing)
     
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  10. pitman
    Joined: May 14, 2006
    Posts: 5,049

    pitman

    Recall reading of point matl. transfer, and matching a condenser size, to a coil.
     
  11. squirrel
    Joined: Sep 23, 2004
    Posts: 48,710

    squirrel
    Member

    yup, it's a tuned circuit...the capacitance of the condenser wants to match the impedance of the coil.
     
  12. This? Edit, never mind Jim I just read your attachment from above.
    [​IMG]
     
  13. tubman
    Joined: May 16, 2007
    Posts: 5,641

    tubman
    ALLIANCE MEMBER

    As "Truck64" stated, the "dV/dt" factor comes into play here. I didn't mention it because I don't really understand it. My partner in this endeavor is an electrical engineer who understands all of this, so I leave that stuff up to him.
     
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  14. Mike VV
    Joined: Sep 28, 2010
    Posts: 2,352

    Mike VV
    Member
    from SoCal

    As anyone opened up a NORMAL (read that, cheap) automotive condenser/capacitor ?
    I have. As previously stated above, they are pretty cheaply manufactured. They are wound in a coil fashion with a VERY thin film of plastic (Mylar) placed as the insulator between the metal coil wrap.

    Much of any over voltage, OR...thickness problem in the mylar, with just a little over voltage OR heat, can cause a short. And just like a battery, when plates touch, dead cells, or the condenser, when winding wraps touch... bingo, an internal short and a dead condenser.

    I've had two ignition/points, condensers fail over the years. It's no fun keeping the engine running.

    Mike
     
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  15. ^ and once there is a short the coil could not charge.


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  16. squirrel
    Joined: Sep 23, 2004
    Posts: 48,710

    squirrel
    Member

    Another problem is that there is not a real spring to keep pressure on the contacts, inside, where the film contacts the case and the wire connector. There is just a rubber plug, and as the parts compress over time, the pressure goes away, allowing the film to lose connection and arc.
     
  17. Truck64
    Joined: Oct 18, 2015
    Posts: 4,631

    Truck64
    Member
    from Ioway

    I think the el-cheapo condensers may well short out as their failure mode, though not for very long. They have maybe 3 amps flowing through them. Then it's probably an open condenser. Or they will go intermittent when they get hot. Both coils and condensers really need to be checked at normal operating temperature.

    Another thing I noticed too about the old school condensers from the 50s and 60s while they usually show excellent leakage check when hit with 500+ volts DC, they don't have any insulation resistance. It won't hold a charge anymore. A good capacitor when charged up will zap you good, even after a day or two. Ask me how I know.
     
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  18. pitman
    Joined: May 14, 2006
    Posts: 5,049

    pitman

    He showed Reluctance, in measuring Impedance! :eek:
     
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  19. G-son
    Joined: Dec 19, 2012
    Posts: 775

    G-son
    Member
    from Sweden

    You may have missed that the capacitor is needed to produce a good spark. When the points open the capacitor allows the current to keep flowing through the coil for a fraction of a second, long enough to let the points open so much so the voltage spike in the primary winding doesn't get a chance to cause an arc at the points. Without the capacitor (or with a dead one) the current keeps flowing past the points in the form of an arc, a normal high voltage spike doesn't occur in the primary winding (the arc bleeds it off), and as a result you don't induce a proper "spark voltage" in the secondary winding.

    The capacitor does indeed prevent the arc to form (a big one anyway), but not only to make the points last longer. It is necessary for the points/coil combination to work, without it you can't produce a strong spark. A weak one, sure, but that doesn't make anyone happy.
     
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  20. Ok I am missing something, the primary winding is grounded by the points, when the points open current flow stops and the magnetic field collapses inducing voltage into the secondary, however it also induces voltage back into the primary arcing across the points, I guess I’m not understanding how the condenser helps with spark....


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  21. Truck64
    Joined: Oct 18, 2015
    Posts: 4,631

    Truck64
    Member
    from Ioway

    Condenser acts like an electronic "shock absorber" and increases the spark energy at the plug than otherwise would be the case.
     
  22. I understand that it stores voltage, I don’t understand how it increases spark at the plug....that is done through the voltage induced into the secondary windings. All dependent on the number of turns etc. in the coil. Maybe I just need to accept it!


    Should have said stores electrons.

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  23. s55mercury66
    Joined: Jul 6, 2009
    Posts: 3,980

    s55mercury66
    Member
    from SW Wyoming

    Someone correct me if I'm wrong, but the voltage increase at the plug, is caused by the condensor stopping the voltage loss that would go back across the points. In actuality, it is not an increase, it is more a case of stopping a decrease.
     
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  24. How would that voltage get to the plug? Two different sides of the coil. The primary side, where the points are doesn’t supply the spark plug. By the time the capacitor sees any it has already been induced in the windings, the fact that voltage is induced back into the primary windings is an unfortunate side effect. The capacitor absorbs some of that to prevent arcing. I cannot see how it improves spark.

    Found this on line, this is what I was looking for, a more technical answer. I’m not sure I was able to get my question across



    At the same time, current exits the coil's primary winding and begins to charge up the capacitor ("condenser") that lies across the now-open breaker points. This capacitor and the coil’s primary windings form an oscillating LC circuit. This LC circuit produces a damped, oscillating current which bounces energy between the capacitor’s electric field and the ignition coil’s magnetic field. The oscillating current in the coil’s primary, which produces an oscillating magnetic field in the coil, extends the high voltage pulse at the output of the secondary windings. This high voltage thus continues beyond the time of the initial field collapse pulse. The oscillation continues until the circuit’s energy is consumed.
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    Last edited: Oct 20, 2018
  25. squirrel
    Joined: Sep 23, 2004
    Posts: 48,710

    squirrel
    Member

    What do you think the primary side maximum voltage is?
     
  26. I think it should be somewhere less than about 250 volts, that would be induced voltage. When the coil is charging so to speak it would be charging system voltage.

    The induced voltage would depend on the number of turns, whether or not the coil was completely saturated etc. the 250v max is a number that comes to mind, I may be way off


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  27. squirrel
    Joined: Sep 23, 2004
    Posts: 48,710

    squirrel
    Member

    pretty close, actually...have you ever looked at the primary side on a scope?
     
  28. I have but it’s been years and I’m not sure at the time I knew enough to really know what I was looking at, I will be hooking one up when I get some time


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  29. Everything we deal with now is solid state and coil on plug, I have points in a few old cars but have never changed a condenser


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  30. Truck64
    Joined: Oct 18, 2015
    Posts: 4,631

    Truck64
    Member
    from Ioway

    Still there isn't any question the spark at the plug is a lot better than it otherwise would be, without the condenser. That's an improvement.

    There is no connection between the primary and secondary windings, the high voltage or high tension is induced in the secondary windings. It works both ways though to some extent. The condenser prevents the back EMF from inducing some of that high voltage back into the points, if I understand it correctly. An electronic "shock absorber". I agree that it doesn't add anything to the total plug voltage, but it improves the spark at the plugs.
     
    Last edited: Oct 21, 2018

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