Function(s) of condenser in points-type ignition system?

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Bill Crowell

Bill Crowell

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A friend and I are having a discussion on another forum, and I can't figure out if he's right or if I am, so I was hoping that maybe one of you smart guys could straighten me out.

In a "Kettering" points-type ignition, does the condenser (by charging) serve only the purpose of preventing spark at the points, or does it also (by discharging) create a sudden negative voltage spike on the coil primary, and thus amplify the coil's voltage transformation?

If you believe the condenser functions only on its charge cycle, by suppressing spark at the points, and that its discharge is not necessary to the operation of the Kettering system, how do you explain the rather modest turns ratio of an automotive coil? If only a max of battery voltage were ever present on the primary, you would need a higher turns ratio in order to get enough spark from the secondary.

But if you believe that the condenser also functions upon discharge by creating a negative voltage bucking the forward current flow in the coil primary, how do you explain the fact that (I think) such an engine will run with the condenser disconnected?
 
It's used to prevent point arcing. Engines will run like garbage if at all with a bad or no condenser. Here's a quote:

The Primary Circuit
With the contact breaker points closed, a 12v current flowing from the car's battery induces an electro-magnetic charge in the primary coil (1). At the same time, a small capacitor, or 'condenser', placed in parallel to the circuit is also charged for an auxiliary purpose.
As the engine-driven distributor drive shaft rotates, a corner lobe of its campushes the contact breaker points open, breaking the primary circuit and preventing the primary coil from charging any further. The condenser acts as dampener at this point, preventing current from arcing (jumping) across the open gap in the breaker points, ensuring that the circuit is broken cleanly.
 
I'm with you as far as you went, Toolman, but what do you think about the theory that when the condenser discharges (as all condensers do), it assists the coil by creating a negative voltage spike on the primary?
 
Bill Crowell said:
I'm with you as far as you went, Toolman, but what do you think about the theory that when the condenser discharges (as all condensers do), it assists the coil by creating a negative voltage spike on the primary?
Click to expand...
I'm far from a expert but I don't see a negative voltage spike on a scope pattern when the points open.
scope.jpg
 
The condenser in older points systems FORMS THE SPARK, by trading the inductive charge in the coil primary for energy in the condenser, the "flywheel effect" in a tank / pulse circuit.

Toolman even said it.........older systems with a no or bad condenser "run not at all."

In the waveform posted above, the reducing AC wave IS the condenser / coil trading energy, effectively stretching out the coil energy
 
Bill Crowell said:
A friend and I are having a discussion on another forum, and I can't figure out if he's right or if I am, so I was hoping that maybe one of you smart guys could straighten me out.

In a "Kettering" points-type ignition, does the condenser (by charging) serve only the purpose of preventing spark at the points, or does it also (by discharging) create a sudden negative voltage spike on the coil primary, and thus amplify the coil's voltage transformation?

If you believe the condenser functions only on its charge cycle, by suppressing spark at the points, and that its discharge is not necessary to the operation of the Kettering system, how do you explain the rather modest turns ratio of an automotive coil? If only a max of battery voltage were ever present on the primary, you would need a higher turns ratio in order to get enough spark from the secondary.

But if you believe that the condenser also functions upon discharge by creating a negative voltage bucking the forward current flow in the coil primary, how do you explain the fact that (I think) such an engine will run with the condenser disconnected?
Click to expand...
The answer is kind of both. The condenser prevents or limits arcing of the ignition points when the points open and it allows the current oscillations in the primary circuit that occur when the points open to dissipate faster, which allows a faster collapse of the magnetic field. The current oscillations act as AC current and pass through the condenser. The faster the magnetic field in the coil collapses the stronger the induced current in the secondary, that being the purpose of the exercise.

ignition waveforms tutorial 1Automotive Test Solutions
 
Rustedwrench said:
it allows the current oscillations in the primary circuit that occur when the points open to dissipate faster, which allows a faster collapse of the magnetic field. The current oscillations act as AC current and pass through the condenser.
Click to expand...

No this is not true. The magnetic field of the coil collapses when the points open, inducing voltage/ current which CHARGES the cap, which is an effective "dead short" at the moment the points open.

When the cap reaches it's peak of charge, it DISCHARGES into the coil, causing the coil to again absorb some amount of energy. This IS IN FACT WHAT YOU SEE with the tapering oscillations in the waveform. If you REMOVE the cap, there will be FEW if only maybe one or two oscillations, as the coil discharges into a very tiny capacitance,.........which would be the capacitance between the turns of the coil and other "stray circuit" capacitance caused by wiring, etc.
 
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In modern transistor ignition systems, where the transistor replaces the points, a capacitor on coil (-) is not used. A modern IGBT transistor has only a few hundred pF of capacitance, a value much less than a points capacitor. The use of a large capacitor, charged by prior ignition event, would result in a transient current as transistor turns on. The IGBT transistors have built in voltage limit, collector to emitter, that clamps over voltage, for protection. The limit varies by specs, but typically 360 to 500V.
The capacitor on the coil (+) is desirable, it provides an AC path, with lower impedance, than the lengthy wired path back to battery. Improved spark, and reduction in RF emmisions.
 
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67Dart273 said:
No this is not true. The magnetic field of the coil collapses when the points open, inducing voltage/ current which CHARGES the cap, which is an effective "dead short" at the moment the points open.

When the cap reaches it's peak of charge, it DISCHARGES into the coil, causing the coil to again absorb some amount of energy. This IS IN FACT WHAT YOU SEE with the tapering oscillations in the waveform. If you REMOVE the cap, there will be FEW if only maybe one or two oscillations, as the coil discharges into a very tiny capacitance,.........which would be the capacitance between the turns of the coil and other "stray circuit" capacitance caused by wiring, etc.
Click to expand...
You are obviously a very smart guy and I am not going to debate with you. I would encourage anyone who wishes to know more on the subject to go to the link I posted above. On the page that displays scroll to the bottom and click on the link to download the pdf of the article, which contains illustrations of the ignition waveforms that are marked and are referred to in the article. The author, Bernie Thompson, is one of the top guys in the automotive diagnostics field. Similar articles have been written by Mac Vandenbrink who worked for Allen test products for over 30 years and was primary in the design of the old Allen Smart Scope ignition analyzer.
 
My old auto instructor in high school simply explained that the condenser is a shock absorber for the points. That cleared it up for us.
 
RustyRatRod said:
My old auto instructor in high school simply explained that the condenser is a shock absorber for the points. That cleared it up for us.
Click to expand...

There is mathematical similarity between electrical system involving resistance, inductance and capacitance, and a mechanical system with a spring, damper and mass. The mechanical chattering or bounce of points, is independent of the capacitor. The capacitor, slows down the rate of voltage change, reducing the peak transients, and arcing.

In a mechanical system, the damper has similarity to a resistor, so not sure what the instructor had in mind.
 
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