Condenser keeps burning up

[ihrescue]

Excellent thanks

Mike Love

[Rollo]

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

[BoschZEV]

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.

[Rollo]

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

[BoschZEV]

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.

[T. Cotten]

... word still hasn't leaked out to everyone in the general public.

As long as the smoke don't leak out,...

I'm good.

....Cotten

[shermanpup]

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.

Can you post the proper Condenser's ratings in Micro farads and DC voltage ?

[BoschZEV]

Can you post the proper Condenser's ratings in Micro farads and DC voltage ?

That's not possible because it depends on the inductance of the coil and, especially in early years, manufacturers used a wide range. Even in later years the range is large. Tables in a Merc-O-Tronic tester manual for "modern" (post-1960) magnetos show condensers ranging from 0.1 microFarad all the way to 0.85 microFarad, although most are in the range 0.18-0.22 microFarad.

As for voltage, ones rated 400 V or higher should be fine. Although, other parameters matter as well.

[shermanpup]

Rob, I think the word your looking for is rectifier. Come on Bosch. Ya got my interest spiked.

In electronics power supplies,after a transformer reduces the AC voltage,a rectifier changes an AC sine wave to DC halves of the sine wave, converting AC to DC according to what I have learned . A full wave rectifier has 4 diodes and a single diode is a half wave rectifier. In electronics applications , the converted sine wave is filtered through polarized capacitors to smooth out the "humps" and make it a smooth DC voltage. This is how line voltage is converted to DC to power most electronics.I know this is little off the topic , but that is what a rectifier does.Not trying to start a battle.

[shermanpup]

That's not possible because it depends on the inductance of the coil and, especially in early years, manufacturers used a wide range. Even in later years the range is large. Tables in a Merc-O-Tronic tester manual for "modern" (post-1960) magnetos show condensers ranging from 0.1 microFarad all the way to 0.85 microFarad, although most are in the range 0.18-0.22 microFarad.

As for voltage, ones rated 400 V or higher should be fine. Although, other parameters matter as well.

Thanks for the info. It gives a capacitance range and a DC voltage rating . Can you recommend a replacement condenser /capacitor? I have a 1934 RL if that matters. Is there a range for the coil inductance?
Mili-henrys I would guess? Thanks Bosch
Jim

[BoschZEV]

It gives a capacitance range and a DC voltage rating . Can you recommend a replacement condenser /capacitor?

I'm not sure what you're asking. What is the 'it' that you are referring to?

No matter what, the number of different coils produced over the years makes it impossible to know what to recommend unless a specific coil/condenser combination happens to be listed in a table.

[shermanpup]

I'm not sure what you're asking. What is the 'it' that you are referring to?

No matter what, the number of different coils produced over the years makes it impossible to know what to recommend unless a specific coil/condenser combination happens to be listed in a table.

The "it " is the capacitance ranges and DC voltage you gave me, sorry for the confusion . The other info I asked for was the inductance range of the coils, if you had that too. Not super necessary, Thanks for the answers

[BoschZEV]

The other info I asked for was the inductance range of the coils

I measure the inductance of coils I work on but, unlike condenser values, I can't remember ever seeing that information tabulated. I think the reason is it's relatively easy to measure capacitance and as a result many general coil testers have a function on them to determine the value, but inductance isn't as easy to measure and requires equipment the average automotive shop doesn't have.

[shermanpup]

Hello Mr. Bosch"
Thank you so much for sharing your knowledge with the rest of us who are trying to increase our antique motorcycle electrical knowledge. "Ignition coil" in Wikipedia has a good explanation.

[BoschZEV]

the rest of us who are trying to increase our antique motorcycle electrical knowledge.

There's no doubt that electronics is a subject most motorcyclists shy away from. Although automotive electronics isn't trivial, it certainly doesn't require an electrical engineer to understand it. Actually, most EEs don't understand it. Before someone attacks me for that statement, let me explain.

If you look at the courses offered for an electrical engineering degree you'll find 'digital systems design', 'signal processing', 'computer architecture', 'antenna theory', 'microwave engineering', etc., but none on 'obsolete automotive electronics'. Although all EE specialties are built on the same foundation, the practical applications of the fundamentals are considerably different in each of them.

As an example, all EEs learn about transformers, which are AC components where the output of the secondary varies linearly with the input to the primary. However, although an automotive coil might appear at first glance to be a transformer, it is used in the "unusual" (for an EE) way of having a DC, not AC, current in the primary, and aside from the turns ratio (and resistances of the coils, and inductances) what is important for its function is the non-linear behavior in the few milliseconds after having that DC primary current abruptly removed.

An EE certainly can learn about automotive electronics, but he (like I did) will have to learn it on his own because it's not taught in engineering curricula and it's different enough from what is taught that the practical aspects are not obvious (e.g. how many mJ of energy need to be transferred from the coil to the spark plug for an engine to run?). Maybe it's just the crowds I run with, but none of the EEs I've discussed this subject with over the past 30+ years have understood it beyond a superficial level, although at the start of our conversations most assumed that because of their training they would understand. EEs who understand obsolete automotive electronics at a practical level certainly exist, but they are self-taught as well as the exception.

Anyway, most aspects of automotive electronics don't require anything like a B.S. degree in EE to understand so the necessary knowledge can be obtained by anyone who isn't intimidated by the subject and puts in the effort to learn.