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Replacement
By JOHN CLARKE
CDI Module
for small petrol motors
If the CDI module in your motorbike, outboard, ride-on mower
or other small petrol motor fails, you could be in for a shock.
Depending on the brand or model, they can cost up to $400. You
can build this one for less than $50 and it will do the same job
for most engines.
R
EADERS HAVE BEEN asking us
for years to design a drop-in CDI
module for motorbikes, outboards and
other small petrol motors. You can
understand why. It can be a real shock
to front up to your local dealer and
find out the price for such a module.
It is even harder to justify the prices
charged when you see the circuit
components involved.
Those days, a great many small petrol engines use a Capacitor Discharge
Ignition (CDI) module. The high-voltage capacitor is charged directly from
a generator located on the flywheel.
A battery may still be included and
32 Silicon Chip
used to drive lights and ancillaries
but this is used independently of the
ignition.
CDI is a great improvement on the
old magneto ignition systems. Not
only does the CDI deliver higher spark
energy but it also dispenses with the
points which were inevitably subject
to wear and required periodic cleaning, adjustment and replacement.
The one drawback is that CDI systems don’t last forever – they can fail.
While the failure can be within the
flywheel generating coils or the ignition coil, it is most likely to be the
CDI module itself and then you will
find that the replacement can be very
expensive.
The CDI Module described here
may be used to replace a failed factory
unit for an engine that incorporates a
generator and trigger coil to provide
the high-voltage and the firing point.
Most of these CDI systems operate in
a similar way but there are variations
in design that use the opposite polarity
for voltage generation and are therefore
unsuitable for our module.
While some tests can be performed
to check for suitability, we cannot
guarantee that the module will work
for every engine. Even so, because this
siliconchip.com.au
CDI Module uses cheap and readily
available parts, it may be worth a try
if you are unwilling to fork out lots of
hard cash for a genuine replacement
module.
S1
GENERATOR
COIL
How CDI works
Fig.1 shows the connections required for a typical CDI module. The
generator (magneto) coil provides the
high voltage to charge a capacitor (in
the CDI module), while the trigger
coil provides the signal to dump the
capacitor’s high voltage charge into the
ignition coil. A kill switch shunts the
high-voltage supply from the generator
to prevent ignition.
Fig.2 shows how CDI works. It
comprises three main components:
the ignition coil, a capacitor (C1) and a
Silicon Controlled Rectifier (SCR). The
SCR behaves as a switch. It is normally
a high impedance until a small trigger
voltage is applied between its gate and
cathode. It then conducts and behaves
like a diode. After triggering, the SCR
switches off when the current through
it falls close to zero.
Initially, the SCR is off and capacitor C1 is discharged. Positive voltage
from the generator then charges C1 via
diode D1 and the primary winding of
the ignition coil. The current path is
shown in red as “IC”.
C1 is discharged when the SCR
is subsequently triggered, allowing
current to flow back through the ignition coil primary. This current path
is shown in green as “ID”. The fast
discharge of C1 and resulting current
through the ignition coil causes a high
voltage to be developed across the
secondary winding of the ignition coil,
to fire the spark plug(s).
Once the spark plug is extinguished,
the collapsing field of the ignition
coil develops a reverse current flow
via diode D2 to partially recharge
capacitor C1.
Typically, the generator coil delivers
about 1A in charging the capacitor up
to about 350V. If C1 is 1mF, then it will
charge in about 350ms – much quicker
than the time between sparks, even in
a high-revving engine.
No RPM advance
Note that the CDI Module does not
incorporate RPM advance and so it
provides a fixed timing from the trigger coil – most common with small
engines.
Some engines do incorporate RPM
siliconchip.com.au
IGNITION
COIL
KILL SWITCH
SPARK
PLUG
+
CDI
MODULE
TRIGGER
COIL
TYPICAL MODULE FOR CDI WITH EXTERNAL CONNECTIONS SHOWN
Fig.1: how a typical CDI module is connected. The generator (magneto)
coil provides a high voltage to charge a capacitor in the CDI module, while
the trigger coil provides the timing signal to dump the capacitor’s high
voltage charge into the ignition coil.
IGNITION
COIL
CHARGE CURRENT
GENERATOR
COIL
C1
SCR
D1
TRIGGER
COIL
IC
K
A
SPARK
PLUG
+
A
K
TRIGGER
CONDITIONING
G
D2
K
A
DISCHARGE
CURRENT ID
BASIC CDI OPERATION
Fig.2: how the CDI module works. Initially, the generator coil charges C1
to a high voltage (via diode D1). A trigger pulse (from the trigger coil) then
turns on the SCR and this quickly discharges C1 by allowing current to
flow back through the coil primary.
advance using a special trigger coil and
magnetic core design that advances
the firing edge with increasing RPM.
This is achieved by having a stepped
or shaped coil core that has a larger
gap at its leading edge compared to
the trailing edge – see Fig.3.
At low speeds the coil voltage required for triggering is developed at
the trailing edge of the magnet but as
revs increase, the leading edge of the
magnet is able to induce more voltage
in the coil and so firing occurs earlier.
This is shown in Fig.4.
Other designs use electronic advance but these require extra power
for the circuitry and tend to be used
only with battery-powered systems.
Circuit details
The simplest circuit arrangement
for the CDI module is shown in Fig.5.
Voltage from the generator coil charges
capacitor C1 (and C2) via diode D1
and the ignition coil primary. As
previously mentioned, D2 is there to
conduct the reverse current flow from
the ignition coil after the capacitor has
discharged.
The two in-series 1MW resistors
across capacitor C1 are there to discharge the capacitor if the SCR does
not fire. This is a safety feature that
prevents a nasty electric shock if you
happen to connect yourself across the
capacitor. It takes about two seconds
for the capacitor to discharge to a safe
value.
Provision has been made on the
PC board for two discharge capacitors, C1 & C2. This allows the use of
either two 0.47mF capacitors or two
1mF capacitors. A higher capacitance
will produce greater spark energy,
May 2008 33
reverse voltage on the gate while the
51W resistor limits the gate current to a
safe value. A 1kW resistor ties the gate
to ground to prevent false triggering,
while the 100nF capacitor filters noise
and transients that may cause the SCR
to trigger at the wrong time.
A kill switch connection has also
been provided to shunt the generator
current to ground and stop the motor.
(FLYWHEEL)
N
MAGN ET
S
SMALL GAP
LARGER GAP
Circuit refinements
TRIGGER
COIL
CORE
ADVANCE TRIGGER HEAD DESIGN
Fig.3: some engines achieve RPM advance using a special trigger coil with
a stepped magnetic core that has a larger gap at its leading edge compared
to the trailing edge. This advances the firing edge with increasing RPM.
+
FIRING POINT
TDC (TOP DEAD CENTRE)
1.5V
TRIGGER
COIL
VOLTAGE
TIME
ADVANCE
–
50ms
A AT LOWER RPM
+
FIRING POINT
TDC
1.5V
TRIGGER
COIL
VOLTAGE
TIME
ADVANCE
10ms
–
B AT HIGHER RPM
Fig.4: the effect of a stepped trigger core design is shown in these timing
advance waveforms. At low speeds, the coil voltage required for triggering
is developed only at the trailing edge of the magnet (waveform A).
However, at higher revs, the leading edge of the magnet induce a greater
voltage into the coil and so firing occurs earlier (waveform B).
provided the generator coil can charge
the capacitors to the full voltage in the
required time.
The trigger coil provides the neces34 Silicon Chip
sary signal to trigger the SCR. When
the coil voltage goes positive, it feeds
current to the gate of the SCR via a 51W
resistor and diode D3. D3 prevents
The simple circuit of Fig.5 works
well but additional circuitry can improve reliability and provide for more
consistent triggering. The extended
circuit is shown in Fig.6.
First, diode D4 has been added
across the generator and thus shunts
negative excursions across the coil
to less than -0.7V. Without D4, the
anode of diode D1 can be subject to
-350V from the negative swings of the
generator. This means that diode D1
could have over 700V across it if the
capacitor is charged to +350V.
While D1 is rated at 1000V, D4
reduces the maximum likely voltage
across it to around 350V or so and
thereby reduces the possibility of reverse breakdown of the diode.
Triggering in this version of the
circuit has also been improved in two
ways. First, we have added a series
10mF capacitor to the gate of the SCR.
This capacitor prevents false triggering due to any DC offset from the
trigger coil that may be more positive
than it should be because of remnant
magnetism in the coil’s core. The 1kW
resistor across the capacitor is there
to discharge the capacitor and is high
enough in value to prevent it triggering
the SCR on its own. Diode D5 prevents
the 10mF capacitor from being charged
with reverse polarity when the trigger
coil output swings negative.
The second improvement involves
the use of a negative temperature coefficient (NTC) thermistor across the gate
of the SCR. This thermistor reduces its
resistance with increasing temperature
and is used to compensate for the
lowered triggering requirement of the
SCR (for both voltage and current) at
higher temperatures.
Effectively, the NTC thermistor
forms a voltage divider with the 51W
resistor. At 25°C, the thermistor is
500W and so it attenuates the signal
from the trigger coil to 91%. However,
at 100°C, the NTC thermistor resistance is around 35W and the trigger
siliconchip.com.au
IGNITION
COIL
KILL SWITCH
1M
1M
S1
TRIGGER
COIL
C2
K
A
D3
1N4004
51
A
K
A
G
SCR1
BT151
100nF
+
K
D2
1N5408
K
1k
A
D1-D3
A
SC
2008
CDI MODULE
1 PC board, code 05105081, 64
x 45mm
1 potting box, 70 x 50 x 20mm
(Jaycar HB-5204 or equivalent)
1 500W NTC thermistor (Jaycar
RN-3434)
1 M3 x 10mm screw
1 M3 nut
C1
D1
1N5408
GENERATOR
COIL
Parts List
SPARK
PLUG
Semiconductors
1 C122E, BT151 500V SCR
(SCR1)
3 1N5408 3A 1000V diodes
(D1,D2,D4)
1 1N4004 1A 400V diode (D3
for Basic Version; D5 for Extra
Features Version)
BT151
K
K
(BASIC VERSION)
A
G
Fig.5: this is the circuit for the Basic Version. The kill switch is there to
stop the motor by shunting the generator coil’s output to ground, while the
1kW resistor on SCR1’s gate prevents false triggering due to noise.
signal is divided down to 41% of the
trigger coil value.
This attenuation in signal level attempts to match the SCR’s reduced
trigger level requirement at higher temperature. So as the temperature rises,
the signal is increasingly attenuated
and as a consequence, the SCR fires
at the same trigger coil voltage over a
wide temperature range.
Without the thermistor, the SCR
would be subject to timing changes
with temperature.
Construction
A small PC board coded 05105081
and measuring 64 x 45mm caters for
Capacitors
1 10mF 25V PC electrolytic
1 1mF 275VAC or 280VAC metallised polypropylene; or
2 0.47mF 275VAC or 280VAC
metallised polypropylene; or
2 1mF 275VAC or 280VAC metallised polypropylene – see
text
1 100nF MKT polyester
1 10nF MKT polyester
both versions of the circuit. This can fit
into a plastic utility box that measures
70 x 50 x 20mm and this box allows
the whole module to be subsequently
potted.
Begin by checking the PC board for
the correct hole sizes. The four corner
mounting holes should be drilled to
3mm, as should the hole for the SCR
mounting tab. That done, check the PC
board for breaks in the copper tracks
or for shorts between tracks. Make any
repairs before assembly.
Fig.7 shows the simple version of
the circuit, while Fig.8 shows the
more complex version. The choice is
yours but we recommend the version
Resistors (0.25W 1%)
2 1MW
1 51W
1 1kW
Miscellaneous
Automotive wire, crimp connectors,
neutral-cure silicone sealant.
IGNITION
COIL
KILL SWITCH
C1, C2: 2 x 470nF 275V AC
OR C1: 1 x 1 F 275V AC
OR C1, C2: 2 x 1 F 275V AC
S1
D1
1N5408
GENERATOR
COIL
A
A
TRIGGER
COIL
51
D5
1N4004
SCR1
BT151
A
1k
D2
1N5408
K
NTC1*
+
K
G
K
100nF
SPARK
PLUG
C1
C2
D4
1N5408
1M
K
10 F 25V
K
1M
A
10nF
A
BT151
* 500 AT 25°C
D1-D4
SC
2008
CDI MODULE
K
(EXTRA FEATURES VERSION)
A
K
A
G
Fig.6: the Extra Features Version includes diode D4 to shunt negative excursions across the generator coil to less
than -0.7V and thus limit the voltage across D1 to around 350V. It also features an improved trigger circuit, to ensure
consistent firing of the SCR with variations in temperature.
siliconchip.com.au
May 2008 35
TRIGGER
COIL
D3
K
100nF
51
1k
A
G
NTC1
C2
K
100nF
1M
D2
SCR1
BT151
A
CHASSIS
TRIGGER
COIL
ELUD O M ID C
K
K
1M
A
A
18050150
K
2©
8 0 0C1
G
1k
51
A
A
1M
K
C2
2©
8 0 0C1
5408
A
CHASSIS
K
KILL
SWITCH
5408
SCR1
BT151
D1
5408
D4
A
D5
K
K
KILL
SWITCH
GENERATOR
COIL
18050150
5408
D1
1M
A
5408
D2
GENERATOR
COIL
10nF
10 FELUD O M ID C
TO IGNITION
COIL +
TO IGNITION
COIL +
'BASIC' CDI VERSION
'EXTRA FEATURES' CDI VERSION
Fig.7: follow this parts layout diagram to build the
“Basic Version” of the CDI Module. It can be used for
non-critical applications.
Fig.8: the “Extra Features” version is the one that we
recommend you build. Take care with the orientation
of the diodes and the 10mF electrolytic capacitor.
in Fig.8. In fact, the following assembly procedure assumes that you are
building the “Extra Features” version.
Start by installing the diodes, taking care to orient each one correctly.
The resistors can then go in – their
values can be checked against the accompanying table and with a digital
multimeter.
Next, install the thermistor, the
smaller capacitors and the 10mF electrolytic, making sure it is oriented correctly. The discharge capacitor(s) can
then be installed. As noted above, we
have provided for two capacitors and
also for two different lead spacing on
the PC board.
The SCR is mounted horizontally
with its leads bent down by 90° so
that they pass through their holes in
the PC board. Secure its tab using an
M3 x 10mm screw and M3 nut before
soldering the leads.
The wiring from the PC board to
the generator coil, kill switch and to
the ignition coil must all be rated at
250VAC and 7.5A. Automotive wire
should be suitable or you can use
240VAC mains wire salvaged from a
mains extension cord. The wiring for
the chassis connection should also be
rated at 7.5A or more.
By contrast, the trigger lead does not
have to be heavy duty but should have
suitable insulation for automotive use.
Sheath the wires in some flexible tubing to prevent possible chaffing of the
wiring insulation. Better still, you may
be able to use the existing wiring for
the original CDI module.
If you want the best spark possible,
you can try adding a second 1mF capacitor in parallel with the first. This
may improve the “fatness” (intensity)
of the spark. In some cases though,
a 1mF capacitance will give the best
spark because 2mF may load the generator coil too much and lower the
charge voltage.
Once the board is complete, run
the external connections and test the
CDI for correct operation. Adjust the
ignition timing according to the manufacturer’s instructions.
as this will corrode the wires and copper pattern on the PC board.
Note that the capacitor(s) will protrude a little from the top of the potting
box. The box can be mounted on the
engine frame using suitable brackets.
It should be placed away from the
exhaust side of the engine.
Make sure that any mounting screws
for the box do not penetrate and make
contact with the circuit.
Testing the generator coil
Sometimes the generator coil can
fail due to either a shorted turn or a
broken wire. You can test for a break
in the coil by measuring its resistance
– ie, between its output and ground.
If the coil is OK, its resistance will
probably be less than 200W.
A shorted turn is not easily checked
except using a special shorted turns
Potting the circuit
Table 2: Capacitor Codes
As previously indicated, we used
a potting box (Jaycar Cat. HB-5204)
to house the CDI unit. Potting allows
the components to be protected from
vibration, water and dust. You must
use a “neutral-cure” silicone sealant
for this job.
Do not use an “acid-cure” silicone,
Value mF Value
1mF
1mF
470nF 0.47mF
100nF 0.1mF
10nF
.01mF
IEC Code EIA Code
1u0
105
470n
474
100n
104
10n
103
Table 1: Resistor Colour Codes
o
o
o
o
No.
2
1
1
36 Silicon Chip
Value
1MW
1kW
51W
4-Band Code (1%)
brown black green brown
brown black red brown
green brown black brown
5-Band Code (1%)
brown black black yellow brown
brown black black brown brown
green brown black gold brown
siliconchip.com.au
This completed CDI module is the “Extra Features” version.
You may have to experiment with the number of discharge
capacitors to get the best spark – see text.
tester. However, you can get some idea
if the coil is delivering sufficient voltage by measuring it with a multimeter
set to read AC volts up to 300V. The
voltage is measured when the engine
is turned over.
Take care if making this measurement, since the generated voltage can
give you an electric shock. DO NOT
touch any of the wiring when turning
the motor over.
Note that the voltage measured
across the generator coil will not be
anywhere near the voltage that it develops when running. That’s because
the multimeter does not respond well
to the low-frequency voltage fluctuations that occur when kicking the
engine over. In addition, most multimeters do not respond to the peak of
the waveform but to the average of a
sinewave.
In practice, you should get a reading
of about 50V AC from the coil.
Another way of testing the coil voltage is to connect the CDI module and
measure the DC voltage between the
cathode of D1 and the chassis while
kicking the motor over. The reading
The board should be installed in a plastic case and potted
using neutral-cure silicone sealant to ensure reliability (ie,
to protect against vibration, moisture and dust).
should at least get to 200V DC if you
can kick the motor over fast enough.
Alternatively, if an oscilloscope is
available, the voltage waveform can be
measured with the probe set to 10:1.
One point we have not mentioned is
the polarity of the voltage. The capacitor needs to charge to a positive voltage
before the trigger signal occurs. If the
voltage from the generator coil is negative before triggering occurs, it will
mean that the CDI module described
here is not suitable for replacing the
module in your engine.
You can check the polarity using a
multimeter set to DC volts – it’s just
a matter of checking that the voltage
on SCR1’s anode goes positive before
the SCR is triggered and negative after
the trigger.
Trigger coil testing
The trigger coil can be tested in the
same way as the generator coil (ie,
measure the voltage between D3 or
D5’s cathode and chassis as the motor is kicked over). This voltage will
be quite small compared to that from
the generator coil and only occurs
Looking for real performance?
•
•
•
•
Warning
This CDI module is not intended
for use as a replacement for CDI
units that generate their own high
voltage from an inverter requiring a
12V battery supply.
To replace one of these units,
you could adapt one of our previous
designs, such as the High Energy
Ignition (SILICON CHIP December
1995 and January 2006) or the MultiSpark CDI (September 1997). Alternatively, you could consider using
the Programmable Ignition System
from March, April & May 2007.
over a short portion of each engine
revolution.
Typically, you might measure a
trigger voltage of less than 1V using
a multimeter set to read AC volts.
The trigger coil voltage can also be
observed on an oscilloscope.
Of course, the real test is when it is
used with the CDI module itself, as it
SC
must be able to trigger the SCR.
160 PAGES
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$19.80 (inc GST) NZ
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