Almost every car built in Britain before 1980 used the traditional Points/Condenser ignition system shown below. Although prone to unreliability due to the number of components needed, in operating principle it is a relatively simple affair.
The ignition system is comprised of two separate circuits:
1. The low-tension (primary) circuit, which generates the spark.
2. The high-tension (secondary) circuit, which delivers the spark to the relevant spark plug.
The Ignition Coil
The heart of the ignition system is the ignition coil. It is in this sealed unit that the high-voltage electrical pulse necessary to create an ignition spark is generated. The ignition coil contains two separate coils of wire arranged concentrically (one inside the other). The outer, primary coil (1) is made up of around 200 turns of thick wire, whilst the inner, secondary coil (2) comprises of around 20,000 turns of much thinner wire. Using the transformer effect, charge can be transferred from the primary to the secondary coil, converting 12v of electricity from the car's battery into the 10,000v that creates the spark.
The Primary Circuit
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 cam pushes 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.
The Secondary Circuit
Once the primary circuit is broken, the electro-magnetic field in the primary coil (1) becomes unstable and begins to collapse. This charge is then transferred to the inner, secondary coil (2), at which point the transformer effect drastically increases the voltage from 12v to around 10,000v, whilst proportionally decreasing the amperage. This pulse of high-voltage electricity is then taken away from the secondary coil through a high-tension lead to the rotor arm, which is mounted on top of the rotating distributor drive shaft. At this point, the rotor arm is aligned with one of its contacts. The pulse passes through this contact and along a second high-tension lead to the spark plug. At the end of the spark plug is a small gap, the other side of which is earthed to the negative side of the battery through the vehicle's engine block. As the pulse jumps across this gap, it effectively short-circuits and a large spark results, igniting the air/fuel mixture in the cylinder and beginning the engine's power stroke.
As the engine-driven distributor drive shaft rotates, a corner lobe of its cam pushes 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.
The Secondary Circuit
Once the primary circuit is broken, the electro-magnetic field in the primary coil (1) becomes unstable and begins to collapse. This charge is then transferred to the inner, secondary coil (2), at which point the transformer effect drastically increases the voltage from 12v to around 10,000v, whilst proportionally decreasing the amperage. This pulse of high-voltage electricity is then taken away from the secondary coil through a high-tension lead to the rotor arm, which is mounted on top of the rotating distributor drive shaft. At this point, the rotor arm is aligned with one of its contacts. The pulse passes through this contact and along a second high-tension lead to the spark plug. At the end of the spark plug is a small gap, the other side of which is earthed to the negative side of the battery through the vehicle's engine block. As the pulse jumps across this gap, it effectively short-circuits and a large spark results, igniting the air/fuel mixture in the cylinder and beginning the engine's power stroke.
The Cycle
As the distributor drive shaft continues to rotate, and the corner lobe of the cam moves away, the contact breaker points will spring shut, completing the primary circuit and beginning the process of charging the primary coil once again.
As the rotor arm is connected to the same drive shaft as the cam, the two rotate at the same rate. Therefore, at the point that the next corner lobe of the cam is opening the contact breaker points, the rotor arm will be aligned with the next contact ready to distribute the impending pulse to the next spark plug in the engine's cycle.
The cycle continues in this manner of charging and discharging the coils indefinitely. In a 4-stroke, 4-cylinder engine (as shown in the diagram), for every 1000 revolutions of the engine, the ignition cycle will be completed 2000 times.
For more information on the individual components in a points/condenser ignition system, see the separate articles in the Components Explained section.
This diagram is not correct. It shows both sides of the points grounded/shorted all the time and the coil primary disconnected when the points open..
ReplyDeleteThis comment has been removed by the author.
DeleteCapacitor shorted all the time does nothing
DeleteShould be in parallel across points gap to absorb spark...charge and protect points as they open and close
DeleteAlso started motor appears to be in series with coil primary when cranking .... Rest seems ok
DeleteShould show wire to tacho too
Deletethat is true the diagram shows the coil wire on the wrong side of the points
ReplyDeletesorry but you are both wrong the fixed side of the points are grounded and the movable side of the points are connected to the negative side of the coil .
ReplyDeleteThe moveable side of the contact point, although connected to the negative side of the ignition coil is always positive. The fixed side of the contact point is the one grounded to the negative. The illustration also show the wrong wiring for the condenser. It should be body ground while the single wire is connected to the positive terminal of the contact point.
Deletethat depends if the system is negative or positive earth, yes the low tension lead in this diagram is wrong, it should be fitted to the sprung loaded and isolated side of the points along with the condenser.
DeleteWhat's incorrect is that the wire from the condensor is supposed to be on the non stationarey terminal of the points.
ReplyDeleteBingo! The condenser is connected across the points. When the points closs, the condenser voltage is clamped at zero, and current from the battery "charges" (via a building magnetic field), the primary side of the coil. When this is complete, the current flowing through the coil is now constant. When the points close, the condenser has two functions. One is to shunt the current which because of the collapsing primary coil sides magnetic field, wants to continue to flow, and would otherwise arc and quickly ruin the points. This probably wouldn't be a danger, because without the condenser, the ignition won't work. The condenser is charged up (voltage wise), by the current flowing from the coil because of the energy stored in the now collapsing magnetic field. This current flow quickly charges the capacitor, and then the capacitor then sends current back to the coil. This continues with each back and forth current swing being smaller. This is called wringing. The first cycle or longer (at a certain frequency), is what induces a corresponding (but higher) voltage and current that creates the spark at the spark plus. So the condenser protects the points from arcing, and allows a rapid and oscillating discharge of the primary coil, and that is necessary for generating a strong spark. Without the condenser, the spark would be weak or not even there. F on the schematic, F on the explanation.
DeleteWhat I see wrong with the diagram is that the cam lobe is driving the neg side of the points. Should be the pos side. This is a decent article. Writer has more knowledge he could have shared.Hard to find someone to really explain it well!
Deleteits correct
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ReplyDeletethe secondary circuit is in fact the primary circuit with a third circuit hanging off it for sparks to swing of with no limitations, there are little mechanical monkey pulses with electrical naras ready to implode the interior of the combustions chamber with a tyre from the left creating some insane torque which puts wholes in roads
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Yes almost every car built in Britain before 1980 used the traditional Condenser ignition system and Compressor.
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