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This is only a preview of the May 1988 issue of Silicon Chip. You can view 39 of the 96 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Articles in this series:
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Manufacturer's data
on the Motorola MC3334P
high energy ignition IC
Motorola's MC3334P is designed specifically to suit
five terminal ignition systems made by Delco (USA)
for the aftermarket (ie, for fitting to older cars).
The MC3334P, driving a high voltage Darlington
transistor, provides an ignition system which
optimises spark energy at minimum power
dissipation.
By LEO SIMPSON
a transient voltage of up to 90 volts
for up to 300 milliseconds. Output
sink current (into pin 7) is quoted as
300 milliamps (maximum, steady
state) but it can take up to one amp
for as long as 300 milliseconds.
Operating temperature range is
from - 40° to + 125°.
Fig.1 shows Motorola's suggested
application circuit for the
MC3334P. This shows input pins 4
and 5 connected to a reluctor
pickup with an inductance of 1.35
Henries. Reluctor or variable reluctance pickups are used in many
modern vehicles and consist of a
toothed wheel (one tooth for each
Motorola quote the following
features for the MC3334P:
• Very low peripheral component
count
• No critical system resistors
• Wide supply voltage operating
range (4-24V)
• Overvoltage shutdown (30V)
• Dwell automatically adjusts to
produce optimum stored energy
without waste
• Externally adjustable peak
current
• Transient protected outputs
Maximum supply voltage is
quoted as 24V DC (steady state)
although the device will withstand
cylinder) which rotates near a permanent magnet which has a coil of
wire wound around it.
The signal generated by the
reluctor is shown on Fig.1 as a
spiky waveform which is processed
by the IC to give correct turn-off
time for the Darlington transistor
(MJ10012). The capacitor at pin 3
controls the dwell time which
means that the Darlington is turned
on after about one millisecond.
A sensing resistor, Rs, is connected in series with the Darlington
transistor to monitor the coil curVBATT
4-24 Vdc
RBATT
300
CFILTER :I
0.1
-::-
6 Vee
r-,
Reluctor :
Pickup
1.35 H
:
,,;•.o--......------.--------..N\,--+--.---1
:
C1 •
Power
Output
and
OVP
RL
: "<:<>--<1----47_0_ _,_o_k....,_"VV\,-C--l---l----,----~
L-'
Ignition
Coil
Primary
8.0 mH
RoRIVE
100
OUT
c2•
=!;:0.01
*Optional Parts for
Extended Transient
Protection
.. A 350 V zener clamp is requ ired
when using the standard MJ10012 .
This clamp is not required if a
selected version with V(BR)CEO(sus) "" 550 V
is used.
Dwell
Reference
Buffer
Current
Liniit
Sense
8
MC3334
Sense
2
1
Power Ground
Ground~- - - - - - - '
-::-
Fig.1: Motorola's suggested circuit for the MC3334P ignition IC. It uses a reluctor pickup which is
typical of electronic ignition fitted as original equipment on many new cars.
42
SILICON CHIP
VBATT = 13 2 Vdc
6 .0
5.5A
5.0
}40
~
.2 30
.
2.0
1.0
bj3 260
I
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I
133
100
70
50
I I
I I I
I I
10
0
I I
I
I
I
I
15
I
I I
I
33 Freq (Hz)
I
I
20
I
I I
I
25
I
I I I I I
I I
30
ms
Fig.2: the primary coil current diagram for spark rates up to 333Hz. The "flat-topped"
waveform shows the effect of the inbuilt current limiting associated with pin 8.
rent. Pin 8 of the IC is used as a current sense input to limit the coil current to a suggested value of 5.5
amps. The idea behind this is to
limit the coil dissipation at low
engine revs.
(Our circuit does not have the
current limit feature because it will
be used mostly with cars that have
ballast resistors in series with their
coils. The second reason for not using the current limit feature is that
it will automatically increase the
power dissipation on the Darlington
transistor at low engine revs. Finally, the third reason for not including
current limit is that we did not want
any trimpots or user adjustments in
the circuit.)
Fig.2 is a rather strange diagram
depicting the ignition coil current
and transistor conduction times for
spark rates from 33Hz to 333Hz
(lefthand side). It also shows the
coil current being limited to 5.5
amps at low engine revs.
333Hz is equivalent to 5000 RPM
for an 8-cylinder motor, 6700 RPM
r - - - - - - - - - - - - -,
for a 6-cylinder motor, and 10,000
RPM for a 4-cylinder motor.
While it may not be immediately
obvious from Fig.2, the MC3334P
does provide a variable dwell angle
feature. It does this by measuring
the slope of the reluctor waveform,
with the dwell reference buffer,
and stores the value on the dwell
capacitor at pin 3.
Our circuit for points-equipped
distributors does not use this
feature and the spark duration is
fixed at close to one millisecond. ~
.s Vee
-- - - - ---7, - ----------7
I
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:
I
I
14V
I I
I
I
5.2 V 2.0 '1>
I
I
I
I I 5 .2 V
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I
~---~
5 I
X
S2 I
I
I
I
I
I
I Input
I Comparator
I
l with
1
Hysteresis
I
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Simplified
Internal
I I
Power
Supply
I I
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1I
I
1
I 1
I I
I I
I I
L _ _ _ _ _ _-=-_ ___ ;::,7 _
I I
L ___ __ _ __ _ _
_J
4
, - - -- 7,------ --- -- 7
Sl
I
I
Output
Stage
with OVP
and
-=- : :
I
_J
I
L_
2
Current
Lim iting
_
-
-
_ _ _ _ _ _ _ _j
Sense
Ground
1
r
Powe r
Ground
I
I
Charge
Up
Fig.3: internal schematic of the MC3334P. Not~
that the output at pin 7 is open-collector so it
will "sink" current but cannot "source" any
current without an external pull-up resistor.
I
I I
3
C
Charge
Oown
I I
I I
I
1
I I
1
X
I
I -::L _____
I I
I I
I I
_
_
_j L __ - ___ -
Dwell
Referen ce
Buffer
_ _ _ _ _ _J
MAY1988
43
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