Primary - Ignition
(scroll for Primary - Ignition Current)
ignition primary is measured on the negative side of the ignition coil. The
earth path of the coil can produce over 350 volts.
primary the horizontal voltage line 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 here
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
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.
is no current in the coil's primary circuit until the dwell period, this is when
the coil is earthed and the voltage seen drops to zero.
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.
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.
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
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.
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.
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.
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.
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.
Primary - Ignition (Current) Diagnostics
switches on as the dwell period starts and rises until the requisite 8 amps is
achieved within the primary circuit, at which point the current is maintained
until it is released at the point of ignition.
dwell will expand as the engine revs are increased, this is to maintain a
constant coil saturation time, hence the term 'constant energy'. If the 'X'
cursor was placed at the beginning of the dwell period and the 'O' cursor
placed on the induced voltage line, the coil saturation time can be measured.
This will remain exactly the same regardless of engine speed.
of the ignition amplifier is to switch the relatively high primary current of
approximately 8 to 10 amps to earth when the component receives a signal from
either the pick-up or Electronic Control Module (ECM).
from the coil is determined by the speed of the switching: the faster this
occurs the greater the induced voltage into the coil's primary winding.
can best be described as a solid state relay, allowing a small current to
switch a much higher current. Due to this high amperage, the component will
get hot, and a liberal application of heat transfer grease should be applied
to the mating surface to dissipate the heat: without this precaution the
component will overheat and fail. The earth connection is crucial to the
correct operation of the amplifier, an earth path waveform and topic can be
found in the main menu.
can also be referred to as a 'module' or an 'igniter'.