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Primary Ignition Waveform Notes
The ignition primary is looking at and measuring the readings seen on the negative side of the coil. The earth path of the ignition coil can produce over 350 volts. Within the picture there are several sections that need closer examination and is therefore important to select the correct voltage scale.
In the illustration shown, the horizontal voltage line (primary Sparkline) in 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 Oscillations’. The length of the afore mentioned line is the ‘Spark Duration’ or ‘Burn Time’, which in this particular case is 1 ms.
The coil oscillation should display a minimum number of 4 - 5 peaks (both upper and lower) should be seen. A loss of peaks on this oscillation shows that the coil needs substituting for another of comparable values.
There is no current in the coils primary's circuit until the dwell period, when the coil is earthed and the voltage seen is drops to zero volts. 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 to approximately 8 amps.
When this pre-determined current has been reached, the amplifier stops increasing the primary current building the current and this is maintained until the earth is removed from the coil, at the precise moment of ignition.
The high vertical line at the centre of the trace is over 200 volts, this is called the 'induced voltage'. Further information is available on the example ‘Induced Voltage’ waveform.
All these sections of the primary trace are also illustration in individual waveforms listed in the menu. 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.
A low (0 - 50) voltage scale is required to observe the sparkline and the coil oscillation, while a higher voltage of 0 - 400 volts will require the to check the induced voltage.
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