[AMCA Forum Moderator merging two comments offered by member BoschZEH related to electricity as it related to the forum thread]

Condenser keeps burning up


Keeping this as non-technical as possible, AC is easy to switch off. You do it every time you flip a light switch and disconnect 110V AC from the bulb. I'll come back to the reasons why in a moment.

DC is another matter since it takes very little voltage to create an arc. As the contacts of the switch in a DC circuit separate an arc is created and that arc grows in length, conducting current, and keeping the circuit connected.

When the points of an ignition circuit start to open a few things happen. The current running through the coil and from there through the points tries to instantaneously collapse to zero. However, as a result of that one of the laws of physics generates a voltage that tries to keep the current from collapsing. This creates a voltage across the points as they start to open that is several hundred volts, creating an arc. If something didn't happen to suppress this arc the current would continue to flow in the circuit through that arc and the plug wouldn't fire.

A property of condensers (capacitors) is they have a very low "resistance" to high frequencies. Since the current in the coil is rapidly collapsing, that's another way of saying it's at a high frequency. What this means in practice is since the condenser is in parallel with the points, the current "sees" two possible paths as the points start to open: 1) through the arc that has started to develop across the points, continuing the circuit as it was, or 2) into the condenser. Since the "resistance" of the condenser is lower than that of the arc, the current takes that route and the arc across the points dies.

However, after a very short time determined by the electrical properties of the circuit (~millisecond), the current reverses itself and the charge comes back out of the condenser. Then, after another very short time it again reverses itself. However, this time the current only "sees" one path, into the condenser, because the arc across the now-opening points already had been quenched.

To rephrase this, what the condenser does is provide a very low resistance path for the current that had been flowing through the points. It diverts the current long enough for the points to separate far enough that, despite the large voltage now across the points, an arc can't be re-established.

Given the above, it can be seen why a bad condenser causes problems with running. The engine needs a spark with a certain minimum energy to come at a well-determined time BTDC. With a bad condenser an arc is maintained across the points for random amounts of time that both causes the plug to fire at random times as well as uses up some of the power needed to ignite the fuel.

Looking at the points in subdued light when the engine is running is an excellent diagnostic. If you see more than pinpricks of light at the points, the condenser is bad.

p.s. back to the 110V light switch. Although no condenser is involved, the frequency (60 Hz) is low. When the contacts first start to open an arc develops. However, 1/60 of a sec. later the AC has reversed sign and the arc is quenched. Then another 1/60 of a sec. after that the voltage and polarity is back to the original 110 V, but by this time the contacts are too far apart for an arc to develop and the light goes out.

[Merged from a second post by BoschZEH]

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.