Primary Ignition Using the 20:1 Attenuator

How to connect the oscilloscope
Example waveform and notes
Technical information

How to connect the oscilloscope when testing:-
a primary ignition circuit

Plug the 20:1 Attenuator into channel A on the PicoScope and plug a BNC test lead into the attenuator. Placing a large black crocodile clip on the test lead with the black moulding (negative) and a small red crocodile clip onto the test lead with the red moulding (positive). Place the black crocodile clip onto the battery negative terminal and probe the coil's negative (or number 1) terminal with the small red crocodile clip as illustrated in figure 44.1.

Fig. 44.1

The example waveform shows that the voltage seen during this test is relatively high and the scaling of the oscilloscope is therefore adjusted to suit. It is important that the 20:1 Attenuator is used in all situations when a voltage exceeding 20 volts is to be measured.

With the example waveform displayed on the screen you can now hit the space bar to start looking at live readings.

Example primary (single cylinder) waveform

Ignition primary waveform notes

The ignition primary waveform is looking at and measuring the readings seen on the negative side of the ignition coil. The earth path of the coil can produce over 350 volts.
Within the primary picture there are several sections that need closer examination. In the waveform shown, the horizontal voltage line at the centre of the oscilloscope is at fairly constant voltage of approximately 40 volts, which then drops sharply into what is referred to as the coil oscillation, these can also be seen in Figure 44.2.

Fig. 44.2
Fig. 44.3

The length of the aforementioned horizontal voltage line is the 'spark duration' or 'burn time', which in this particular case is 1.036 ms, this can again be seen in figure 44.3. The coil oscillation period should display a minimum number of 4 to 5 peaks (both upper and lower). A loss of peaks would indicate that the coil needs substituting for another of comparable values.

There is no current in the coil's primary circuit until the dwell period (figure 44.4), this is when the coil is earthed and the voltage seen drops to zero. The dwell period is controlled by the ignition amplifier and the length of the dwell is determined by the time it takes to build up approximately 8 amps. When this predetermined current has been reached, the amplifier stops increasing the primary current and it is maintained until the earth is removed from the coil, at the precise moment of ignition.






Fig. 44.4
Fig. 44.5

The vertical line at the centre of the trace is in excess of 200 volts, this is called the 'induced voltage'. The induced voltage is produced by a process called magnetic inductance. At the point of ignition, the coil's earth circuit is removed and the magnetic field or flux collapses across the coil's windings, this in turn induces an average voltage between 150 to 350 volts (Figure 44.5). The coil's High Tension (HT) output will be proportional to the induced voltage. The height of the induced voltage is sometimes referred to as the primary peak volts.

Technical information - primary ignition circuits

The primary ignition is so called as it forms the first part of the ignition circuit. This circuit is used to provide the initial stage towards the secondary High Tension (HT) output. The primary circuit has, over the past few years, evolved from the basic contact breaker points and condenser to the distributorless and coil per cylinder systems in common use today. The basic origin of all of these ignition systems evolves around the magnetic inductive principle.

Magnetic Inductance

This principle is based around a magnetic field (or flux) being produced, as the coil's earth circuit is completed by either the contacts or the amplifier providing the coil negative terminal with a path to earth. When this circuit is complete, a magnetic field is produced and builds until the coil's magnetic field becomes maximised or saturated. At the predetermined point of ignition, the coil's earth is removed and the magnetic field or flux collapses across the coils 250 to 350 primary windings, which in turn induces a voltage of 150 to 350 volts.

This induced voltage will be determined by :-

  • The number of turns in the primary winding.
  • The strength of the magnetic flux, which is proportionate to the current in the primary circuit.
  • The rate of collapse, which is determined by the speed of the switching of the earth path.

Dwell period

Dwell is measured as an angle: with contact ignition, the points gap determines the dwell angle. The definition of contact ignition dwell is: 'the number of degrees of distributor rotation with the contacts closed'.
As an example, a 4 cylinder engine will have a dwell of approximately 45 degrees, which is 50% of one cylinders complete primary cycle. The dwell period on an engine with electronic ignition is controlled by the current limiting circuit within the amplifier or Electronic Control Module (ECM).
The dwell on a constant energy system will be seen to expand as the engine speed increases, to compensate for a shorter time period. The term 'constant energy' refers to the available voltage produced by the coil. This, regardless of engine speed, will remain constant, as opposed to contact ignition where an increase in engine speed means the contacts are closed for a shorter time period. This reduces the effective time that the coil has to fully saturate and maximise the strength of the magnetic flux.

The induced voltage on a variable dwell system will remain constant regardless of engine speed, while this voltage will reduce on contact systems. This induced voltage can be seen on a primary waveform.