Readers 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
used to drive lights and ancillaries but this is used independently of the
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 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
How CDI works
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.
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
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.
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
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
Some engines do incorporate RPM 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.
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
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
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,
provided the generator coil can charge the capacitors to the full voltage in the
The trigger coil provides the necessary 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 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
A kill switch connection has also been provided to shunt the
generator current to ground and stop the motor.
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.
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.
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.
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).
A 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
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 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 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.
A small PC board coded 05105081 and measuring 64 x 45mm caters
for both versions of the circuit. This can fit into a plastic box that measures
70 x 50 x 20mm and this box allows the whole module to be subsequently
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.
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
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.
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.
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
Once the board is complete, run the external connections and
test the CDI for correct operation. Adjust the ignition timing according to the
Potting the circuit
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, 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.
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 10μF electrolytic capacitor.
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 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
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 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.
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.
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).
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 over a short portion of each engine
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
must be able to trigger the SCR.
Table 1: Resistor Colour Codes
|No.|| Value ||
4-Band Code (1%) ||5-Band Code (1%)|
|2|| 1MW ||brown black
green brown|| brown black black yellow brown |
|1 ||1kW|| brown black
red brown|| brown black black brown brown|
|1|| 51W ||green brown
black brown||green brown black gold brown|
Table 2: Capacitor Codes
Value|| IEC Code|| EIA Code|
|470nF ||0.47μF ||470n ||474|
|10nF ||.01μF ||10n ||103|
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 Multi-Spark CDI (September 1997). Alternatively, you could consider using the Programmable Ignition System from March, April & May 2007.