Electronic 12V DC Capacitive Discharge Ignition (CDI) Circuits

The following post describes a simple yet enhanced 12V capacitive discharge ignition system which derives its operating voltage from the battery instead of the alternator for generating the igniting sparks.

Since it works independently from the alternator voltage, without depending on a pickup coil signal, it is able to function more efficiently and consistently, enabling a much smoother ride of the vehicle even at lower speeds.

Contact Breaker Vs CDI

A capacitive discharge ignition unit also called the CDI unit is the modern alternative for the age old contact breakers, which were quite crude with their functions and reliability.

The modern CDI is an electronic version of the contact breaker which uses sophisticated electronic parts for generating the required arching across the spark plug terminals.

The concept is not complicated at all, the section of the alternator provides the required 100 to 200V AC to the CDI circuit, where the voltage is intermittently stored and discharged by a high voltage capacitor through a few rectifying diodes.

These rapid bursts of high voltage discharges are dumped into the primary winding of an ignition coil where its appropriately stepped up to many thousands of volts for acquiring the required arcing, which ultimately functions as the igniting sparks across the connected spark plug contacts.

I have already discussed the basic electronic CDI circuit in one of my previous posts, though the circuit is extremely versatile, it depends and derives its operating voltage from the alternator. Since the alternator voltage depends on the engine speeds, the generated voltages tend to get affected with varying speeds.

At higher speeds it works fine, but at lower speeds, the alternator voltage also lowers, this results in an inconsistent sparking forcing the alternator and the engine to stutter.

This inconsistency ultimately affects the functioning of the CDI and the whole system starts getting hampered,  sometimes even causing the engine to halt.

The circuit of an enhanced capacitive discharge ignition circuit which is discussed here, eliminates the use of the alternator voltage for functioning, instead it utilizes the battery voltage for generating the required actions.

The Circuit Concept

The whole concept for this electronic CDI can be understood by studying the shown circuit diagram below:

The diodes, the SCR and the associated components form a standard CDI circuit.

The high voltage of around 200V which needs to be fed to the above circuit is generated through an ordinary step down transformer connected the other way round.

The secondary winding of the transformer now becomes the primary and vice versa.

The low voltage primary winding is fed with high current pulsating DC generated by a standard IC555 circuit via a power transistor.

This pulsating voltage is stepped up to the required 200V and becomes the operating voltage for the attached CDI circuit.

The CDI circuit converts this 200V into bursts of high current for feeding the input winding of the ignition coil.

These rapid high current bursts are further amplified to many thousands of volts by the ignition coil and finally fed to the connected spark plug for the required arcing and the initiating the ignition of the vehicle.

As can be seen the input voltage is acquired from a 12V DC source which is actually the battery of the vehicle.

Due to this the generated sparks are very consistent without interruptions providing the vehicle a constant supply of the required ignition sparks irrespective of the vehicle situation.

The consistent sparking also makes the fuel consumption efficient, makes the engine less prone to wear and tear and enhances the overall mileage of the vehicle.

Use a 1K resistor at the base of TIP122...... 100 ohm is incorrectly shown

Synchronizing with Wheel RPM

If you want the above circuit to be triggered by the alternator so that the combustion is ideally efficient and synchronized with the wheel RPM, the above design may be modified in the following way:

A 1K resistor is used at the base of TIP122...... since 100 ohm is incorrectly shown.

The above configuration may be further modified as shown in the following diagram, which appears to be the most appropriate way of implementing the proposed enhanced CDI circuit for all 2 and 3 wheelers.

How it Works

As we know, the reset pin#4 of IC 555 requires a positive potential to allow the normal functioning of the IC 555 as an astable or as monostable. If the pin#4 is not associated with the positive line, the IC remains dormant and disabled.

Here the pin#4 of the IC can be seen connected with the alternator voltage. This voltage can be of any level from the alternator, it doesn't matter, since it is appropriately stabilized by the 33 k resistor and the following zener diode, capacitor network.

The alternator will generate a positive and negative cycle pulses, in response to each rotation of the vehicle wheel.

The positive pulse will be converted into a 12 V positive feed at the pin#4 which will cause the circuit to initiate and stay activated during the entire positive pulse duration cycle of the waveform.

During these periods, the IC 555 will operate and fire the SCR multiple number if times in short bursts, causing the ignition to fire with higher efficiency and for a sustained period of time during the firing angle of the combustion and the piston.

This will also enable the CDI to work in tandem with the wheel rotation generating an ideally synchronized combustion of the engine and with an optimal efficiency.

Finalized Enhanced CDI Design with PWM Control

CDI PCB Circuit

Parts List

All resistors are 1/4w unless stated

1K - 1
10K- 1
POT 10K - 1
100 Ohms 1/2 watt - 1
56 Ohms 1/2 watt - 1
Diodes 1N4007 - 9

Capacitors

1uF/25V - 1
0.01uF/50V Ceramic - 1
105/400V PPC - 1

Semiconductors

IC 555 - 1

Mosfet IRF540 - 1
SCR - BT151

Transformer 0-12V/220V/1amp - 1

CDI ignition coil - 1

Video Clip showing the Test Result of the above shown electronic capacitive discharge circuit system

Another Version of Electronic Ignition

The following diagram provides another version of a IC 555 based electronic ignition system, which I got from an old magazine page:

Here, the left side stage which includes the IC 555, 4 transistors and the X1 transformer, form a 12 V to 500 V step up push pull inverter.

The right side section using the SCR BT151, and the associated circuitry forms the capacitive discharge ignition stage.

The design works with the old contact breaker type of mechanism, for timing the ignition and triggering the SCR.

While the contact beaker remains closed, the SCR remains disabled, and this allows the capacitor C2 to charge via the 500 V DC from the inverter output. Next, as soon as the contact breaker opens, the SCR gets its gate trigger via R9, 10, 11, 12, and C4, and it fires causing C2 to discharge across the attached ignition coil primary winding, which in turn causes the secondary of the ignition coil to produce the required high voltage pulse into the spark plug for the required ignition.

With Pickup Trigger

In modern vehicles we find the contact breaker being replaced with a pickup coil, which ensures a solid state working of the system without any wear and tear. The above electronic CDI design could be also used with pickup coil set up, with the following given modifications.