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Thread: Condenser keeps burning up

  1. #11
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    Thank you, that was a great. Live and learn. Bob L

  2. #12
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    Thanks for all your very informative posts.I always enjoy reading and learning from them.
    Thanks for taking the time.
    Tom

  3. #13
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    With the contents of my previous post being digested, it's time for desert. Most people, even ones who work with them, only learn one characteristic of capacitors, i.e. their capacitance (say, 0.18 micro-Farads). However, if you look into it in more detail you'll see that for any given capacitance value there is typically a choice of a half-dozen different "types" of capacitors (tantalum, mica, electrolytic, ceramic, etc.). Each of these has its own advantages and disadvantages. For example, with electrolytics you get a lot of capacitance in a relatively small volume for a low price, but they are polarity sensitive and have a lifetime of only a few decades.

    The description of what a capacitor needs to do in an ignition circuit dictates the properties it needs to have. The capacitance value C along with the inductance of the ignition coil L determines the characteristic frequency with which the current flows into the capacitor and back out again as the points open. Since L is set by the coil that the bike's manufacturer used, this determines the C the capacitor needs to have. Although, operation isn't incredibly sensitive to the precise value of C so that if the original capacitor were, say, 0.18 micro-Farads, 0.24 uF would work fine.

    Unfortunately, a little knowledge is a dangerous thing, and knowing what C value their replacement capacitor needs to have has sent many people to Radio Shack to buy replacements of the same value, which then failed. The reason they failed is those capacitors lacked the required "pulse current" rating. Although less than ten amps flows through the circuit in steady state, when the points open a much larger current flows into the capacitor given by current = capacitance x (time rate of change of voltage).

    The voltage across the points, and hence across the capacitor, was 0 V before the points opened, but as soon as they opened the "instantaneous" disruption of current caused a voltage of several hundred V to be developed. Hence, the time rate of change is several hundred V in a microsecond, resulting in a pulsed current of hundreds of Amps into the condenser. Although that large current only flows for a brief time, it damages the internals of most capacitors because they are not constructed to handle it. Circuits that generate large pulsed currents are relatively unusual and most common capacitors will fail.

    I'll only briefly mention just one other essential property, breakdown voltage. In simplest form a capacitor is two plates of metal separated by a thin insulator. Since the capacitance varies inversely with the thickness of that insulator, the thinner it is the smaller the overall capacitor can be and still have the required capacitance. Unfortunately, materials suffer "dielectric breakdown" if too high a voltage is placed across a thin layer (actually, it's the electric field that causes this, which is E = Voltage / thickness) so there are limits on how thin the dielectric can be, and hence how small the overall size a capacitor can have for a given C and voltage rating. Also, different spacer materials are better or worse at this with, of course, cheaper materials being less resistant.

    Anyway, the properties of a given capacitor (capacitance, pulsed current, breakdown voltage, operating temperature range, size, etc.) are the result of various tradeoffs that were made by the manufacturer with its final use in mind. As a result, even if it has the required C, the odds are fairly small that a capacitor selected at random from a catalog will actually work in an ignition circuit.

  4. #14
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    Hi BoschZEV -

    Would you object if I merged your three posts relating to condensers and make it a sticky in the Electrical Forum? I think its a long term good post that others will want to refer to in the future. I would add tags as well if you haven't already included them. Thanks for always taking the time to provide these concise explanations.

    Your thoughts?

    Mike Love
    AMCA Forum
    Moderator

  5. #15
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    Quote Originally Posted by ihrescue View Post
    Would you object if I merged your three posts relating to condensers and make it a sticky in the Electrical Forum?
    No problem. Electricity seems to stump more motorcyclists than other topics do and I'm happy when I can help.

  6. #16
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    Excellent thanks

    Mike Love

  7. #17
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    Okay I have a question on condensers that has been going back and forth for some time between me and a buddy. Will a HD run with out one and will one start without one? I say they won't but he says that the battery and coil are enough. I say it requires that stored charge in the condenser to provide enough spark to fire the plugs. I have told him that if he think s they will to pull his condenser and try it he refuses. He is a electrical engineer and no doubt know more about electricity that me however I have had condensers go bad and know the symptoms and remedy is replace it. I admit unless the condenser has completely failed it may start or run with an intermittent issue (which I have found rare) but when they are done that is it. What say the experts?


    Tom (Rollo) Hardy

    AMCA #12766

  8. #18
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    Quote Originally Posted by Rollo View Post
    he says that the battery and coil are enough. I say it requires that stored charge in the condenser to provide enough spark to fire the plugs.
    The answer will make both of you unhappy. You're both wrong.

    As background, ultimately it is your engine that provides the energy for the spark. Some of the h.p. of the engine is sucked off by the electrical system and used, either via a magneto or a battery, to drive current through the primary of a coil. The amount of energy stored in the primary is 1/2 times the inductance L of that coil times the square of the current flowing (e.g. 2x the current results in 4x the energy). Even if there is enough voltage to create a spark, it takes a certain minimum amount of energy to ignite the fuel, so the problem is a two-fold one of storing enough energy in the primary, and then transferring that energy at the right time and with minimum loss to the secondary where it sparks the plug and ignites the fuel.

    As for your friend, the energy that is needed for the spark is stored in the primary of the coil and the problem is transferring that energy to the secondary and from there to the plug. As I've already discussed, simply opening the points on a DC circuit results in an arc that continues to conduct electricity. In principle, if the points could be pulled apart far enough and fast enough the circuit would be interrupted and the spark would happen at the desired point BTDC. But, the simple cam and rubbing block system used in any of our magnetos or distributors doesn't come anywhere close to accomplishing this. In principle, a solid state device might exist that accomplishes the required instantaneous interruption of the ~10 A in the primary circuit while surviving the several hundred V developed across it, but that's not what you and your friend are arguing about. Anyway, if you remove the condenser the points will arc like crazy every time they open, dissipating energy that is needed to ignite the fuel and firing the plug at random times. Still, the engine might fire, and might even manage to run horribly despite this, but it's wrong to say that the battery and coil alone are enough.

    As for you, although charge flows in and out of the condenser on a time scale of milliseconds when the points open, it's not that the charge in the condenser provides the spark. Think of your bank account as like the condenser, and you as like the coil. Your bank stores money that you earned, but even though you may have the bank transfer some money to a merchant, it is you who has paid the merchant, not the bank.

    As somewhat of an aside, I've already said it requires a certain minimum energy to ignite the fuel, and that the energy comes from the LxI2 stored in the coil. More energy could be stored if the coil were larger, but that would require more turns of copper wire, which costs money. It also could come from a higher current, but ultimately that has to come from a bigger generator, i.e. more copper wire, which costs money. So, it shouldn't be surprising that manufacturers don't give us electrical systems that do much more than the minimum needed. Hence, the electrical system has to be kept in very good condition (e.g. connectors that aren't corroded) or there will be problems with how the engine runs.

  9. #19
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    Thanks for the reply. I think I understand all that you said but to me electricity is a mysterious thing. All I know is like you say the system needs to be kept in excellent condition to work properly and go for a nice ride in the country.

    Tom (Rollo) Hardy
    AMCA #12766

  10. #20
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    Quote Originally Posted by Rollo View Post
    to me electricity is a mysterious thing.
    You're not alone among motorcyclists in having this sentiment. The opening sentence in the chapter on "Elementary Principles of Electricity" in the 1935 edition of Dyke's Autombole and Gasoline Engine Encyclopedia is:

    Electricity: No one can tell you what electricity is.

    Although thanks to James Clerk Maxwell that statement hasn't been true since 1865, word still hasn't leaked out to everyone in the general public.

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