Contact breaker ignition systems provide a simple means of establishing and interrupting the current flowing in the primary ignition circuit.
A basic system consists of:
When these components are connected together, they form two distinct circuits, Primary and Secondary.
In the primary circuit, the positive terminal of the battery is connected to the ignition switch, then to the positive terminal of the ignition coil. A connection is then made from the negative terminal to the contact breaker moving contact point. The fixed contact point is connected to ground, at the vehicle frame, as is the negative battery terminal. This allows the circuit to be completed through the frame, when current is flowing. A capacitor is connected in parallel with the moving contact and also has a ground connection through its retaining screw.
In the secondary circuit, the secondary winding, with many thousands of turns of fine enameled copper wire is wound around a central soft iron core. It is connected internally to the primary winding at one end, and at the other to the central output terminal, also called the H.T. or high tension terminal. The central coil terminal is then connected by a high tension lead or cable to the center terminal of the distributor cap. A spring loaded carbon brush connects the terminal to the center of the rotor arm or button. This ensures contact is made with the rotor arm at all times, even when the engine is running. As the rotor arm rotates, it lines up with the segment inside the distributor cap which leads to the spark plug for that cylinder.
Contact breaker
A contact breaker (or "points") is a type of electrical switch, and the term typically refers to the switching device found in the distributor of the ignition systems of non diesel-powered internal combustion engines. The purpose of the contact breaker is to interrupt the current flowing in the primary circuit of the ignition coil. When this occurs, the collapsing current induces a current in the secondary winding of the coil, which has very many more turns. This causes a very large voltage to appear at the coil output for a short period - enough to arc across the electrodes of a spark plug.
The contact breaker is operated by an engine-driven cam, and the position of the contact breaker is set so that they open (and hence generate a spark) at the exactly correct moment needed to ignite the fuel at the top of the piston's compression stroke. The contact breaker is usually mounted on a plate that is able to rotate relative to the camshaft operating it. The plate is rotated by a centrifugal mechanism, thus advancing the timing (making the spark occur earlier) at higher revolutions. This gives the fuel time to burn so that the resulting gases reach their maximum pressure at the same time as the piston reaches the top of the cylinder. The plate's position can also be moved a small distance using a small vacuum-operated servomechanism, providing advanced timing when the engine is required to speed up on demand. This helps to prevent pre-ignition.
Contact breaker points suffer from wear - both mechanical (since they open and close several times every turn of the engine) and caused by arcing across the contacts. This latter effect is largely prevented by placing a capacitor across the contact breaker - this is usually referred to by the more old fashioned term condenser by mechanics. As well as suppressing arcing, it helps boost the coil output by creating a resonant LC circuit with the coil windings. A drawback of using a mechanical switch as part of the ignition timing is that it is not very precise, needs regular adjustment, and at higher revolutions, its mass becomes significant, leading to poor operation at higher engine speeds. These effects can largely be overcome using electronic ignition systems, where the contact breakers are retrofitted by a massless sensor device.
Source: CDX Global & Wikipedia - en.wikipedia.org
