Toro 51643 Service Manual page 37

lgnition Operation Trigger Module (cont'd)
PRIMARY VOLTAGE WAVEFORM
Figure 53
As the magnets rotate past the coil, voltage is produced.
This voltage, when uninterrupted, is first positive, then
negative as the magnet passes by the coil. This effect is
caused by the two opposing poles of the magnet.
Explanation of the trigger module also requires an
understanding of the NPN transistor. See Figure 54.
I
C
(Collector)
I
E
(Emitter)
NPN-type
Figure 54
A transistor has a certain minimum voltage that
it
requires across the base and emitter (points B and E in
Figure 54 above) before it will "turn on". Once
it
has
turned on,
it
allows a small current, l1, to flow as shown
above. At the same time, the transistor allows a large
current, 12, to flow from point C to
E.
The magnitude of
current l2 will vary in proportion to the smaller current,
l1.
Thus, the transistor functions
as
an amplifier in that
it
allows a small current to control a large one.
26
The following is the process the trigger module uses to
break the primary circuit to produce spark: See Figure
55
Figure 55
1.
The magnet passes by the coil and induces an
alternating voltage.
2.
As the voltage begins to increase, (approximately
point "a" in Figure 53) transistor Tr2 is turned on
and current flows from point
" C '
to point "D"
through R3, R4, and Tr2. See Figure
56.
Figure 56
3. Current l1 flowing through Tr2 induces a larger
current
1 2 .
Note that current l1 is
very
small and
that l2 is much larger. See Figure 57.
MTI unit Ignition coil
Ground
Spark plug
4.
When the voltage is at the point "a" level as denoted
in "Figure
53,
Tr1 is still in the off mode