# The capacitor in a points ignition

• CHICAGO
In summary, the capacitor in a car's old points ignition system serves two main purposes. The first is to protect the points from getting burnt. The second purpose is to create a resonant circuit with the ignition coil, which prevents high frequency radiation and ensures that all energy is used to ignite the spark. This capacitor also helps the primary coil to generate around 200 volts when the points open and close. This can be seen in a simulation by Bob S, where the capacitor charges up to 2 amps and creates a voltage of approximately 300 volts. The coil secondary voltage is then boosted to around 30,000 volts due to the coil's turns ratio.
CHICAGO
Hi all

In a car old points ignition system, I understand the capacitor (condenser) main purpose is that of protecting the points from getting burnt.

The second purpose for this capacitor, could it be to make a resonant circuit among the ignition coil in order to prevent it from radiation?.

I mean the coil-condenser would make the system to oscillate at a lower frequency in the way that all the energy would be used to ignite the spark and there would be no loss in the case of a high frecuency radiation.

Am I wrong?

Another question. In what way does this capacitor help the primary coil to get those some 200 volts?

When the points close, the capacitor is shorted and the coil primary charges up to about 2 amps (limited by coil internal resistance of ~6 ohms). When the points open, there is a very large V = L dI/dt voltage, ~300 volts, across the capacitor that resonates back and forth with current in the coil, for ~ 10 cycles. The coil secondary:primary turns ratio is ~100:1 so the coil secondary voltage is ~30,000 volts. See my LTSPICE simulation
https://www.physicsforums.com/attachment.php?attachmentid=20126&d=1250393723
The points close at 0 and 4 milliseconds, and they open at 2 and 6 milliseconds.
Bob S

I can confirm that the capacitor in a points ignition system has two main purposes. The first is to protect the points from getting burnt by absorbing the excess energy that is generated when the points open and close. This helps to prolong the life of the points and ensure the ignition system continues to function properly.

The second purpose of the capacitor is to create a resonant circuit with the ignition coil. This helps to prevent high frequency radiation, which can interfere with other electronic components in the vehicle. By creating a resonant circuit, the energy is used more efficiently to ignite the spark and there is less loss in the form of radiation.

In terms of the capacitor helping the primary coil to reach 200 volts, it plays a crucial role in the overall function of the ignition system. The capacitor stores and releases energy at specific intervals, which helps to create the high voltage needed to produce a spark. Without the capacitor, the ignition system would not be able to generate enough voltage to ignite the fuel mixture in the engine.

I hope this answers your questions and provides a better understanding of the importance of the capacitor in a points ignition system.

## What is the purpose of the capacitor in a points ignition?

The capacitor, also known as a condenser, in a points ignition system helps to reduce arcing and prolong the life of the points by absorbing excess voltage when they open.

## How does the capacitor work in a points ignition?

When the points are closed, the capacitor charges up to the same voltage as the battery. When the points open, the capacitor releases its stored energy, reducing the voltage spike that would normally occur.

## What happens if the capacitor in a points ignition fails?

If the capacitor fails, the points may experience excessive arcing and burn out more quickly. This can also cause issues with the timing and overall performance of the ignition system.

## How do I know if the capacitor in my points ignition needs to be replaced?

If you are experiencing issues with your ignition, such as inconsistent spark or difficulty starting the engine, it is possible that the capacitor may need to be replaced. However, it is always best to consult a professional for proper diagnosis.

## Can I replace the capacitor in a points ignition with a different type of capacitor?

It is not recommended to replace the capacitor in a points ignition with a different type, as it may not have the same specifications and could potentially cause damage to the ignition system. It is best to use the same type of capacitor that was originally installed.

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